LinuxLists.cc - [PATCH v7 0/7] Make alloc_contig

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Subject: [PATCH v7 4/7] mm,hugetlb: Split prep_new_huge_page functionality

Currently, prep_new_huge_page() performs two functions.
It sets the right state for a new hugetlb, and increases the hstate's
counters to account for the new page.

Let us split its functionality into two separate functions, decoupling
the handling of the counters from initializing a hugepage.
The outcome is having __prep_new_huge_page(), which only
initializes the page , and __prep_account_new_huge_page(), which adds
the new page to the hstate's counters.

This allows us to be able to set a hugetlb without having to worry
about the counter/locking. It will prove useful in the next patch.
prep_new_huge_page() still calls both functions.

Signed-off-by: Oscar Salvador <[email protected]>
---
mm/hugetlb.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e40d5fe5c63c..0607b2b71ac6 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1483,7 +1483,16 @@ void free_huge_page(struct page *page)
}
}

-static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
+/*
+ * Must be called with the hugetlb lock held
+ */
+static void __prep_account_new_huge_page(struct hstate *h, int nid)
+{
+ h->nr_huge_pages++;
+ h->nr_huge_pages_node[nid]++;
+}
+
+static void __prep_new_huge_page(struct page *page)
{
INIT_LIST_HEAD(&page->lru);
set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
@@ -1491,9 +1500,13 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
set_hugetlb_cgroup(page, NULL);
set_hugetlb_cgroup_rsvd(page, NULL);
ClearHPageFreed(page);
+}
+
+static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
+{
+ __prep_new_huge_page(page);
spin_lock_irq(&hugetlb_lock);
- h->nr_huge_pages++;
- h->nr_huge_pages_node[nid]++;
+ __prep_account_new_huge_page(h, nid);
spin_unlock_irq(&hugetlb_lock);
}

--
2.16.3

2021-04-13 14:33:03

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Subject: [PATCH v7 5/7] mm: Make alloc_contig_range handle free hugetlb pages

alloc_contig_range will fail if it ever sees a HugeTLB page within the
range we are trying to allocate, even when that page is free and can be
easily reallocated.
This has proved to be problematic for some users of alloc_contic_range,
e.g: CMA and virtio-mem, where those would fail the call even when those
pages lay in ZONE_MOVABLE and are free.

We can do better by trying to replace such page.

Free hugepages are tricky to handle so as to no userspace application
notices disruption, we need to replace the current free hugepage with
a new one.

In order to do that, a new function called alloc_and_dissolve_huge_page
is introduced.
This function will first try to get a new fresh hugepage, and if it
succeeds, it will replace the old one in the free hugepage pool.

The free page replacement is done under hugetlb_lock, so no external
users of hugetlb will notice the change.
To allocate the new huge page, we use alloc_buddy_huge_page(), so we
do not have to deal with any counters, and prep_new_huge_page() is not
called. This is valulable because in case we need to free the new page,
we only need to call __free_pages().

Once we know that the page to be replaced is a genuine 0-refcounted
huge page, we remove the old page from the freelist by remove_hugetlb_page().
Then, we can call __prep_new_huge_page() and __prep_account_new_huge_page()
for the new huge page to properly initialize it and increment the
hstate->nr_huge_pages counter (previously decremented by
remove_hugetlb_page()).
Once done, the page is enqueued by enqueue_huge_page() and it is ready
to be used.

There is one tricky case when
page's refcount is 0 because it is in the process of being released.
A missing PageHugeFreed bit will tell us that freeing is in flight so
we retry after dropping the hugetlb_lock. The race window should be
small and the next retry should make a forward progress.

E.g:

CPU0 CPU1
free_huge_page() isolate_or_dissolve_huge_page
PageHuge() == T
alloc_and_dissolve_huge_page
alloc_buddy_huge_page()
spin_lock_irq(hugetlb_lock)
// PageHuge() && !PageHugeFreed &&
// !PageCount()
spin_unlock_irq(hugetlb_lock)
spin_lock_irq(hugetlb_lock)
1) update_and_free_page
PageHuge() == F
__free_pages()
2) enqueue_huge_page
SetPageHugeFreed()
spin_unlock(&hugetlb_lock)
spin_lock_irq(hugetlb_lock)
1) PageHuge() == F (freed by case#1 from CPU0)
2) PageHuge() == T
PageHugeFreed() == T
- proceed with replacing the page

In the case above we retry as the window race is quite small and we have high
chances to succeed next time.

With regard to the allocation, we restrict it to the node the page belongs
to with __GFP_THISNODE, meaning we do not fallback on other node's zones.

Note that gigantic hugetlb pages are fenced off since there is a cyclic
dependency between them and alloc_contig_range.

Signed-off-by: Oscar Salvador <[email protected]>
---
include/linux/hugetlb.h | 6 +++
mm/compaction.c | 37 ++++++++++++++--
mm/hugetlb.c | 115 ++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 155 insertions(+), 3 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 09f1fd12a6fa..b2d2118bfd1a 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -595,6 +595,7 @@ struct huge_bootmem_page {
struct hstate *hstate;
};

+int isolate_or_dissolve_huge_page(struct page *page);
struct page *alloc_huge_page(struct vm_area_struct *vma,
unsigned long addr, int avoid_reserve);
struct page *alloc_huge_page_nodemask(struct hstate *h, int preferred_nid,
@@ -877,6 +878,11 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
#else /* CONFIG_HUGETLB_PAGE */
struct hstate {};

+static inline int isolate_or_dissolve_huge_page(struct page *page)
+{
+ return -ENOMEM;
+}
+
static inline struct page *alloc_huge_page(struct vm_area_struct *vma,
unsigned long addr,
int avoid_reserve)
diff --git a/mm/compaction.c b/mm/compaction.c
index eeba4668c22c..89426b6d1ea3 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -788,7 +788,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
* Isolate all pages that can be migrated from the range specified by
* [low_pfn, end_pfn). The range is expected to be within same pageblock.
* Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
- * or 0.
+ * -ENOMEM in case we could not allocate a page, or 0.
* cc->migrate_pfn will contain the next pfn to scan (which may be both less,
* equal to or more that end_pfn).
*
@@ -809,6 +809,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
bool skip_on_failure = false;
unsigned long next_skip_pfn = 0;
bool skip_updated = false;
+ bool fatal_error = false;
int ret = 0;

cc->migrate_pfn = low_pfn;
@@ -907,6 +908,33 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
valid_page = page;
}

+ if (PageHuge(page) && cc->alloc_contig) {
+ ret = isolate_or_dissolve_huge_page(page);
+
+ /*
+ * Fail isolation in case isolate_or_dissolve_huge_page
+ * reports an error. In case of -ENOMEM, abort right away.
+ */
+ if (ret < 0) {
+ /*
+ * Do not report -EBUSY down the chain.
+ */
+ if (ret == -ENOMEM)
+ fatal_error = true;
+ else
+ ret = 0;
+ low_pfn += (1UL << compound_order(page)) - 1;
+ goto isolate_fail;
+ }
+
+ /*
+ * Ok, the hugepage was dissolved. Now these pages are
+ * Buddy and cannot be re-allocated because they are
+ * isolated. Fall-through as the check below handles
+ * Buddy pages.
+ */
+ }
+
/*
* Skip if free. We read page order here without zone lock
* which is generally unsafe, but the race window is small and
@@ -1066,7 +1094,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
put_page(page);

isolate_fail:
- if (!skip_on_failure)
+ if (!skip_on_failure && !fatal_error)
continue;

/*
@@ -1092,6 +1120,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
*/
next_skip_pfn += 1UL << cc->order;
}
+
+ if (fatal_error)
+ break;
}

/*
@@ -1145,7 +1176,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
* @end_pfn: The one-past-last PFN.
*
* Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
- * or 0.
+ * -ENOMEM in case we could not allocate a page, or 0.
*/
int
isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0607b2b71ac6..4a664d6e82c1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2266,6 +2266,121 @@ static void restore_reserve_on_error(struct hstate *h,
}
}

+/*
+ * alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
+ * @h: struct hstate old page belongs to
+ * @old_page: Old page to dissolve
+ * Returns 0 on success, otherwise negated error.
+ */
+
+static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
+{
+ gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
+ int nid = page_to_nid(old_page);
+ struct page *new_page;
+ int ret = 0;
+
+ /*
+ * Before dissolving the page, we need to allocate a new one for the
+ * pool to remain stable. Using alloc_buddy_huge_page() allows us to
+ * not having to deal with prep_new_page() and avoids dealing of any
+ * counters. This simplifies and let us do the whole thing under the
+ * lock.
+ */
+ new_page = alloc_buddy_huge_page(h, gfp_mask, nid, NULL, NULL);
+ if (!new_page)
+ return -ENOMEM;
+
+retry:
+ spin_lock_irq(&hugetlb_lock);
+ if (!PageHuge(old_page)) {
+ /*
+ * Freed from under us. Drop new_page too.
+ */
+ goto free_new;
+ } else if (page_count(old_page)) {
+ /*
+ * Someone has grabbed the page, fail for now.
+ */
+ ret = -EBUSY;
+ goto free_new;
+ } else if (!HPageFreed(old_page)) {
+ /*
+ * Page's refcount is 0 but it has not been enqueued in the
+ * freelist yet. Race window is small, so we can succeed here if
+ * we retry.
+ */
+ spin_unlock_irq(&hugetlb_lock);
+ cond_resched();
+ goto retry;
+ } else {
+ /*
+ * Ok, old_page is still a genuine free hugepage. Remove it from
+ * the freelist and decrease the counters. These will be
+ * incremented again when calling __prep_account_new_huge_page()
+ * and enqueue_huge_page() for new_page. The counters will remain
+ * stable since this happens under the lock.
+ */
+ remove_hugetlb_page(h, old_page, false);
+
+ /*
+ * Call __prep_new_huge_page() to construct the hugetlb page, and
+ * enqueue it then to place it in the freelists. After this,
+ * counters are back on track. Free hugepages have a refcount of 0,
+ * so we need to decrease new_page's count as well.
+ */
+ __prep_new_huge_page(new_page);
+ __prep_account_new_huge_page(h, nid);
+ page_ref_dec(new_page);
+ enqueue_huge_page(h, new_page);
+
+ /*
+ * Pages have been replaced, we can safely free the old one.
+ */
+ spin_unlock_irq(&hugetlb_lock);
+ update_and_free_page(h, old_page);
+ }
+
+ return ret;
+
+free_new:
+ spin_unlock_irq(&hugetlb_lock);
+ __free_pages(new_page, huge_page_order(h));
+
+ return ret;
+}
+
+int isolate_or_dissolve_huge_page(struct page *page)
+{
+ struct hstate *h;
+ struct page *head;
+
+ /*
+ * The page might have been dissolved from under our feet, so make sure
+ * to carefully check the state under the lock.
+ * Return success when racing as if we dissolved the page ourselves.
+ */
+ spin_lock_irq(&hugetlb_lock);
+ if (PageHuge(page)) {
+ head = compound_head(page);
+ h = page_hstate(head);
+ } else {
+ spin_unlock(&hugetlb_lock);
+ return 0;
+ }
+ spin_unlock_irq(&hugetlb_lock);
+
+ /*
+ * Fence off gigantic pages as there is a cyclic dependency between
+ * alloc_contig_range and them. Return -ENOME as this has the effect
+ * of bailing out right away without further retrying.
+ */
+ if (hstate_is_gigantic(h))
+ return -ENOMEM;
+
+ return alloc_and_dissolve_huge_page(h, head);
+}
+
struct page *alloc_huge_page(struct vm_area_struct *vma,
unsigned long addr, int avoid_reserve)
{
--
2.16.3

2021-04-13 14:33:43

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Subject: [PATCH v7 7/7] mm,page_alloc: Drop unnecessary checks from pfn_range_valid_contig

pfn_range_valid_contig() bails out when it finds an in-use page or a
hugetlb page, among other things.
We can drop the in-use page check since __alloc_contig_pages can migrate
away those pages, and the hugetlb page check can go too since
isolate_migratepages_range is now capable of dealing with hugetlb pages.
Either way, those checks are racy so let the end function handle it
when the time comes.

Signed-off-by: Oscar Salvador <[email protected]>
Suggested-by: David Hildenbrand <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
---
mm/page_alloc.c | 6 ------
1 file changed, 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b5a94de3cdde..c5338e912ace 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8901,12 +8901,6 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,

if (PageReserved(page))
return false;
-
- if (page_count(page) > 0)
- return false;
-
- if (PageHuge(page))
- return false;
}
return true;
}
--
2.16.3

2021-04-13 15:28:11

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Subject: Re: [PATCH v7 4/7] mm,hugetlb: Split prep_new_huge_page functionality

On Tue 13-04-21 12:47:44, Oscar Salvador wrote:
[...]
> +static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> +{
> + __prep_new_huge_page(page);
> spin_lock_irq(&hugetlb_lock);
> - h->nr_huge_pages++;
> - h->nr_huge_pages_node[nid]++;
> + __prep_account_new_huge_page(h, nid);
> spin_unlock_irq(&hugetlb_lock);
> }

Any reason to decouple the locking from the accounting?
>
> --
> 2.16.3

--
Michal Hocko
SUSE Labs

2021-04-13 15:29:58

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Subject: Re: [PATCH v7 5/7] mm: Make alloc_contig_range handle free hugetlb pages

On Tue 13-04-21 12:47:45, Oscar Salvador wrote:
> alloc_contig_range will fail if it ever sees a HugeTLB page within the
> range we are trying to allocate, even when that page is free and can be
> easily reallocated.
> This has proved to be problematic for some users of alloc_contic_range,
> e.g: CMA and virtio-mem, where those would fail the call even when those
> pages lay in ZONE_MOVABLE and are free.
>
> We can do better by trying to replace such page.
>
> Free hugepages are tricky to handle so as to no userspace application
> notices disruption, we need to replace the current free hugepage with
> a new one.
>
> In order to do that, a new function called alloc_and_dissolve_huge_page
> is introduced.
> This function will first try to get a new fresh hugepage, and if it
> succeeds, it will replace the old one in the free hugepage pool.
>
> The free page replacement is done under hugetlb_lock, so no external
> users of hugetlb will notice the change.
> To allocate the new huge page, we use alloc_buddy_huge_page(), so we
> do not have to deal with any counters, and prep_new_huge_page() is not
> called. This is valulable because in case we need to free the new page,
> we only need to call __free_pages().
>
> Once we know that the page to be replaced is a genuine 0-refcounted
> huge page, we remove the old page from the freelist by remove_hugetlb_page().
> Then, we can call __prep_new_huge_page() and __prep_account_new_huge_page()
> for the new huge page to properly initialize it and increment the
> hstate->nr_huge_pages counter (previously decremented by
> remove_hugetlb_page()).
> Once done, the page is enqueued by enqueue_huge_page() and it is ready
> to be used.
>
> There is one tricky case when
> page's refcount is 0 because it is in the process of being released.
> A missing PageHugeFreed bit will tell us that freeing is in flight so
> we retry after dropping the hugetlb_lock. The race window should be
> small and the next retry should make a forward progress.
>
> E.g:
>
> CPU0 CPU1
> free_huge_page() isolate_or_dissolve_huge_page
> PageHuge() == T
> alloc_and_dissolve_huge_page
> alloc_buddy_huge_page()
> spin_lock_irq(hugetlb_lock)
> // PageHuge() && !PageHugeFreed &&
> // !PageCount()
> spin_unlock_irq(hugetlb_lock)
> spin_lock_irq(hugetlb_lock)
> 1) update_and_free_page
> PageHuge() == F
> __free_pages()
> 2) enqueue_huge_page
> SetPageHugeFreed()
> spin_unlock(&hugetlb_lock)
> spin_lock_irq(hugetlb_lock)
> 1) PageHuge() == F (freed by case#1 from CPU0)
> 2) PageHuge() == T
> PageHugeFreed() == T
> - proceed with replacing the page
>
> In the case above we retry as the window race is quite small and we have high
> chances to succeed next time.
>
> With regard to the allocation, we restrict it to the node the page belongs
> to with __GFP_THISNODE, meaning we do not fallback on other node's zones.
>
> Note that gigantic hugetlb pages are fenced off since there is a cyclic
> dependency between them and alloc_contig_range.
>
> Signed-off-by: Oscar Salvador <[email protected]>

Acked-by: Michal Hocko <[email protected]>

One minor nit below
[...]

> + /*
> + * Ok, old_page is still a genuine free hugepage. Remove it from
> + * the freelist and decrease the counters. These will be
> + * incremented again when calling __prep_account_new_huge_page()
> + * and enqueue_huge_page() for new_page. The counters will remain
> + * stable since this happens under the lock.
> + */
> + remove_hugetlb_page(h, old_page, false);
> +
> + /*
> + * Call __prep_new_huge_page() to construct the hugetlb page, and
> + * enqueue it then to place it in the freelists. After this,
> + * counters are back on track. Free hugepages have a refcount of 0,
> + * so we need to decrease new_page's count as well.
> + */
> + __prep_new_huge_page(new_page);
> + __prep_account_new_huge_page(h, nid);

I think it would help to put something like the following into the
comment above this really strange construct.

/*
* new_page needs to be initialized with the standard
* hugetlb state. This is normally done by
* prep_new_huge_page but that takes hugetlb_lock which
* is already held so we need to open code it here.
* Reference count trick is needed because allocator
* gives us referenced page but the pool requires pages
* with 0 refcount.
*/

> + page_ref_dec(new_page);
> + enqueue_huge_page(h, new_page);
> +
> + /*
> + * Pages have been replaced, we can safely free the old one.
> + */
> + spin_unlock_irq(&hugetlb_lock);
> + update_and_free_page(h, old_page);

--
Michal Hocko
SUSE Labs

2021-04-13 17:29:00

[permalink] [raw]

Subject: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

Currently, the clearing of the flag is done under the lock, but this
is unnecessary as we just allocated the page and we did not give it
away yet, so no one should be messing with it.

Also, this helps making clear that here the lock is only protecting the
counter.

Signed-off-by: Oscar Salvador <[email protected]>
---
mm/hugetlb.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 54d81d5947ed..e40d5fe5c63c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1490,10 +1490,10 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
hugetlb_set_page_subpool(page, NULL);
set_hugetlb_cgroup(page, NULL);
set_hugetlb_cgroup_rsvd(page, NULL);
+ ClearHPageFreed(page);
spin_lock_irq(&hugetlb_lock);
h->nr_huge_pages++;
h->nr_huge_pages_node[nid]++;
- ClearHPageFreed(page);
spin_unlock_irq(&hugetlb_lock);
}

--
2.16.3

2021-04-13 17:29:00

[permalink] [raw]

Subject: [PATCH v7 6/7] mm: Make alloc_contig_range handle in-use hugetlb pages

alloc_contig_range() will fail if it finds a HugeTLB page within the range,
without a chance to handle them. Since HugeTLB pages can be migrated as any
LRU or Movable page, it does not make sense to bail out without trying.
Enable the interface to recognize in-use HugeTLB pages so we can migrate
them, and have much better chances to succeed the call.

Signed-off-by: Oscar Salvador <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Acked-by: Michal Hocko <[email protected]>
---
include/linux/hugetlb.h | 5 +++--
mm/compaction.c | 12 +++++++++++-
mm/hugetlb.c | 22 +++++++++++++++++-----
mm/vmscan.c | 5 +++--
4 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index b2d2118bfd1a..b92f25ccef58 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -595,7 +595,7 @@ struct huge_bootmem_page {
struct hstate *hstate;
};

-int isolate_or_dissolve_huge_page(struct page *page);
+int isolate_or_dissolve_huge_page(struct page *page, struct list_head *list);
struct page *alloc_huge_page(struct vm_area_struct *vma,
unsigned long addr, int avoid_reserve);
struct page *alloc_huge_page_nodemask(struct hstate *h, int preferred_nid,
@@ -878,7 +878,8 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
#else /* CONFIG_HUGETLB_PAGE */
struct hstate {};

-static inline int isolate_or_dissolve_huge_page(struct page *page)
+static inline int isolate_or_dissolve_huge_page(struct page *page,
+ struct list_head *list)
{
return -ENOMEM;
}
diff --git a/mm/compaction.c b/mm/compaction.c
index 89426b6d1ea3..bb8ff3543972 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -909,7 +909,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
}

if (PageHuge(page) && cc->alloc_contig) {
- ret = isolate_or_dissolve_huge_page(page);
+ ret = isolate_or_dissolve_huge_page(page, &cc->migratepages);

/*
* Fail isolation in case isolate_or_dissolve_huge_page
@@ -927,6 +927,15 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
goto isolate_fail;
}

+ if (PageHuge(page)) {
+ /*
+ * Hugepage was successfully isolated and placed
+ * on the cc->migratepages list.
+ */
+ low_pfn += compound_nr(page) - 1;
+ goto isolate_success_no_list;
+ }
+
/*
* Ok, the hugepage was dissolved. Now these pages are
* Buddy and cannot be re-allocated because they are
@@ -1068,6 +1077,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,

isolate_success:
list_add(&page->lru, &cc->migratepages);
+isolate_success_no_list:
cc->nr_migratepages += compound_nr(page);
nr_isolated += compound_nr(page);

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4a664d6e82c1..24a453ff47f2 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2270,10 +2270,12 @@ static void restore_reserve_on_error(struct hstate *h,
* alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
* @h: struct hstate old page belongs to
* @old_page: Old page to dissolve
+ * @list: List to isolate the page in case we need to
* Returns 0 on success, otherwise negated error.
*/

-static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
+static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
+ struct list_head *list)
{
gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
int nid = page_to_nid(old_page);
@@ -2300,9 +2302,13 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
goto free_new;
} else if (page_count(old_page)) {
/*
- * Someone has grabbed the page, fail for now.
+ * Someone has grabbed the page, try to isolate it here.
+ * Fail with -EBUSY if not possible.
*/
- ret = -EBUSY;
+ spin_unlock_irq(&hugetlb_lock);
+ if (!isolate_huge_page(old_page, list))
+ ret = -EBUSY;
+ spin_lock_irq(&hugetlb_lock);
goto free_new;
} else if (!HPageFreed(old_page)) {
/*
@@ -2350,10 +2356,11 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
return ret;
}

-int isolate_or_dissolve_huge_page(struct page *page)
+int isolate_or_dissolve_huge_page(struct page *page, struct list_head *list)
{
struct hstate *h;
struct page *head;
+ int ret = -EBUSY;

/*
* The page might have been dissolved from under our feet, so make sure
@@ -2378,7 +2385,12 @@ int isolate_or_dissolve_huge_page(struct page *page)
if (hstate_is_gigantic(h))
return -ENOMEM;

- return alloc_and_dissolve_huge_page(h, head);
+ if (page_count(head) && isolate_huge_page(head, list))
+ ret = 0;
+ else if (!page_count(head))
+ ret = alloc_and_dissolve_huge_page(h, head, list);
+
+ return ret;
}

struct page *alloc_huge_page(struct vm_area_struct *vma,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index bb8321026c0c..5199b9696bab 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1703,8 +1703,9 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
LIST_HEAD(clean_pages);

list_for_each_entry_safe(page, next, page_list, lru) {
- if (page_is_file_lru(page) && !PageDirty(page) &&
- !__PageMovable(page) && !PageUnevictable(page)) {
+ if (!PageHuge(page) && page_is_file_lru(page) &&
+ !PageDirty(page) && !__PageMovable(page) &&
+ !PageUnevictable(page)) {
ClearPageActive(page);
list_move(&page->lru, &clean_pages);
}
--
2.16.3

2021-04-13 17:29:00

[permalink] [raw]

Subject: [PATCH v7 2/7] mm,compaction: Let isolate_migratepages_{range,block} return error codes

Currently, isolate_migratepages_{range,block} and their callers use
a pfn == 0 vs pfn != 0 scheme to let the caller know whether there was
any error during isolation.
This does not work as soon as we need to start reporting different error
codes and make sure we pass them down the chain, so they are properly
interpreted by functions like e.g: alloc_contig_range.

Let us rework isolate_migratepages_{range,block} so we can report error
codes.
Since isolate_migratepages_block will stop returning the next pfn to be
scanned, we reuse the cc->migrate_pfn field to keep track of that.

Signed-off-by: Oscar Salvador <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
mm/compaction.c | 54 +++++++++++++++++++++++++++---------------------------
mm/internal.h | 10 ++++++++--
mm/page_alloc.c | 7 +++----
3 files changed, 38 insertions(+), 33 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 8c5028bfbd56..eeba4668c22c 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -787,15 +787,15 @@ static bool too_many_isolated(pg_data_t *pgdat)
*
* Isolate all pages that can be migrated from the range specified by
* [low_pfn, end_pfn). The range is expected to be within same pageblock.
- * Returns zero if there is a fatal signal pending, otherwise PFN of the
- * first page that was not scanned (which may be both less, equal to or more
- * than end_pfn).
+ * Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
+ * or 0.
+ * cc->migrate_pfn will contain the next pfn to scan (which may be both less,
+ * equal to or more that end_pfn).
*
* The pages are isolated on cc->migratepages list (not required to be empty),
- * and cc->nr_migratepages is updated accordingly. The cc->migrate_pfn field
- * is neither read nor updated.
+ * and cc->nr_migratepages is updated accordingly.
*/
-static unsigned long
+static int
isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
unsigned long end_pfn, isolate_mode_t isolate_mode)
{
@@ -809,6 +809,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
bool skip_on_failure = false;
unsigned long next_skip_pfn = 0;
bool skip_updated = false;
+ int ret = 0;
+
+ cc->migrate_pfn = low_pfn;

/*
* Ensure that there are not too many pages isolated from the LRU
@@ -818,16 +821,16 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
while (unlikely(too_many_isolated(pgdat))) {
/* stop isolation if there are still pages not migrated */
if (cc->nr_migratepages)
- return 0;
+ return -EAGAIN;

/* async migration should just abort */
if (cc->mode == MIGRATE_ASYNC)
- return 0;
+ return -EAGAIN;

congestion_wait(BLK_RW_ASYNC, HZ/10);

if (fatal_signal_pending(current))
- return 0;
+ return -EINTR;
}

cond_resched();
@@ -875,8 +878,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,

if (fatal_signal_pending(current)) {
cc->contended = true;
+ ret = -EINTR;

- low_pfn = 0;
goto fatal_pending;
}

@@ -1130,7 +1133,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
if (nr_isolated)
count_compact_events(COMPACTISOLATED, nr_isolated);

- return low_pfn;
+ cc->migrate_pfn = low_pfn;
+
+ return ret;
}

/**
@@ -1139,15 +1144,15 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
* @start_pfn: The first PFN to start isolating.
* @end_pfn: The one-past-last PFN.
*
- * Returns zero if isolation fails fatally due to e.g. pending signal.
- * Otherwise, function returns one-past-the-last PFN of isolated page
- * (which may be greater than end_pfn if end fell in a middle of a THP page).
+ * Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
+ * or 0.
*/
-unsigned long
+int
isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
unsigned long end_pfn)
{
unsigned long pfn, block_start_pfn, block_end_pfn;
+ int ret = 0;

/* Scan block by block. First and last block may be incomplete */
pfn = start_pfn;
@@ -1166,17 +1171,17 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
block_end_pfn, cc->zone))
continue;

- pfn = isolate_migratepages_block(cc, pfn, block_end_pfn,
- ISOLATE_UNEVICTABLE);
+ ret = isolate_migratepages_block(cc, pfn, block_end_pfn,
+ ISOLATE_UNEVICTABLE);

- if (!pfn)
+ if (ret)
break;

if (cc->nr_migratepages >= COMPACT_CLUSTER_MAX)
break;
}

- return pfn;
+ return ret;
}

#endif /* CONFIG_COMPACTION || CONFIG_CMA */
@@ -1847,7 +1852,7 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
*/
for (; block_end_pfn <= cc->free_pfn;
fast_find_block = false,
- low_pfn = block_end_pfn,
+ cc->migrate_pfn = low_pfn = block_end_pfn,
block_start_pfn = block_end_pfn,
block_end_pfn += pageblock_nr_pages) {

@@ -1889,10 +1894,8 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
}

/* Perform the isolation */
- low_pfn = isolate_migratepages_block(cc, low_pfn,
- block_end_pfn, isolate_mode);
-
- if (!low_pfn)
+ if (isolate_migratepages_block(cc, low_pfn, block_end_pfn,
+ isolate_mode))
return ISOLATE_ABORT;

/*
@@ -1903,9 +1906,6 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
break;
}

- /* Record where migration scanner will be restarted. */
- cc->migrate_pfn = low_pfn;
-
return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
}

diff --git a/mm/internal.h b/mm/internal.h
index f469f69309de..46eb82eaa195 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -244,7 +244,13 @@ struct compact_control {
unsigned int nr_freepages; /* Number of isolated free pages */
unsigned int nr_migratepages; /* Number of pages to migrate */
unsigned long free_pfn; /* isolate_freepages search base */
- unsigned long migrate_pfn; /* isolate_migratepages search base */
+ /*
+ * Acts as an in/out parameter to page isolation for migration.
+ * isolate_migratepages uses it as a search base.
+ * isolate_migratepages_block will update the value to the next pfn
+ * after the last isolated one.
+ */
+ unsigned long migrate_pfn;
unsigned long fast_start_pfn; /* a pfn to start linear scan from */
struct zone *zone;
unsigned long total_migrate_scanned;
@@ -280,7 +286,7 @@ struct capture_control {
unsigned long
isolate_freepages_range(struct compact_control *cc,
unsigned long start_pfn, unsigned long end_pfn);
-unsigned long
+int
isolate_migratepages_range(struct compact_control *cc,
unsigned long low_pfn, unsigned long end_pfn);
int find_suitable_fallback(struct free_area *area, unsigned int order,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 689454692de1..b5a94de3cdde 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8690,11 +8690,10 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,

if (list_empty(&cc->migratepages)) {
cc->nr_migratepages = 0;
- pfn = isolate_migratepages_range(cc, pfn, end);
- if (!pfn) {
- ret = -EINTR;
+ ret = isolate_migratepages_range(cc, pfn, end);
+ if (ret && ret != -EAGAIN)
break;
- }
+ pfn = cc->migrate_pfn;
tries = 0;
} else if (++tries == 5) {
ret = -EBUSY;
--
2.16.3

2021-04-13 17:54:33

[permalink] [raw]

Subject: Re: [PATCH v7 4/7] mm,hugetlb: Split prep_new_huge_page functionality

On Tue 13-04-21 15:24:32, Michal Hocko wrote:
> On Tue 13-04-21 12:47:44, Oscar Salvador wrote:
> [...]
> > +static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> > +{
> > + __prep_new_huge_page(page);
> > spin_lock_irq(&hugetlb_lock);
> > - h->nr_huge_pages++;
> > - h->nr_huge_pages_node[nid]++;
> > + __prep_account_new_huge_page(h, nid);
> > spin_unlock_irq(&hugetlb_lock);
> > }
>
> Any reason to decouple the locking from the accounting?

OK, I spoke too soon. The next patch already has the locking around when
calling this. Please add a lockdep assert into the helper to make the
locking expectations more obvious.

With that
Acked-by: Michal Hocko <[email protected]>

--
Michal Hocko
SUSE Labs

2021-04-13 22:23:12

[permalink] [raw]

Subject: Re: [PATCH v7 2/7] mm,compaction: Let isolate_migratepages_{range,block} return error codes

On 4/13/21 3:47 AM, Oscar Salvador wrote:
> Currently, isolate_migratepages_{range,block} and their callers use
> a pfn == 0 vs pfn != 0 scheme to let the caller know whether there was
> any error during isolation.
> This does not work as soon as we need to start reporting different error
> codes and make sure we pass them down the chain, so they are properly
> interpreted by functions like e.g: alloc_contig_range.
>
> Let us rework isolate_migratepages_{range,block} so we can report error
> codes.
> Since isolate_migratepages_block will stop returning the next pfn to be
> scanned, we reuse the cc->migrate_pfn field to keep track of that.
>
> Signed-off-by: Oscar Salvador <[email protected]>
> Acked-by: Vlastimil Babka <[email protected]>

Acked-by: Mike Kravetz <[email protected]>

--
Mike Kravetz

2021-04-14 07:19:37

[permalink] [raw]

Subject: Re: [PATCH v7 5/7] mm: Make alloc_contig_range handle free hugetlb pages

On 4/13/21 3:47 AM, Oscar Salvador wrote:
> alloc_contig_range will fail if it ever sees a HugeTLB page within the
> range we are trying to allocate, even when that page is free and can be
> easily reallocated.
> This has proved to be problematic for some users of alloc_contic_range,
> e.g: CMA and virtio-mem, where those would fail the call even when those
> pages lay in ZONE_MOVABLE and are free.
>
> We can do better by trying to replace such page.
>
> Free hugepages are tricky to handle so as to no userspace application
> notices disruption, we need to replace the current free hugepage with
> a new one.
>
> In order to do that, a new function called alloc_and_dissolve_huge_page
> is introduced.
> This function will first try to get a new fresh hugepage, and if it
> succeeds, it will replace the old one in the free hugepage pool.
>
> The free page replacement is done under hugetlb_lock, so no external
> users of hugetlb will notice the change.
> To allocate the new huge page, we use alloc_buddy_huge_page(), so we
> do not have to deal with any counters, and prep_new_huge_page() is not
> called. This is valulable because in case we need to free the new page,
> we only need to call __free_pages().
>
> Once we know that the page to be replaced is a genuine 0-refcounted
> huge page, we remove the old page from the freelist by remove_hugetlb_page().
> Then, we can call __prep_new_huge_page() and __prep_account_new_huge_page()
> for the new huge page to properly initialize it and increment the
> hstate->nr_huge_pages counter (previously decremented by
> remove_hugetlb_page()).
> Once done, the page is enqueued by enqueue_huge_page() and it is ready
> to be used.
>
> There is one tricky case when
> page's refcount is 0 because it is in the process of being released.
> A missing PageHugeFreed bit will tell us that freeing is in flight so
> we retry after dropping the hugetlb_lock. The race window should be
> small and the next retry should make a forward progress.
>
> E.g:
>
> CPU0 CPU1
> free_huge_page() isolate_or_dissolve_huge_page
> PageHuge() == T
> alloc_and_dissolve_huge_page
> alloc_buddy_huge_page()
> spin_lock_irq(hugetlb_lock)
> // PageHuge() && !PageHugeFreed &&
> // !PageCount()
> spin_unlock_irq(hugetlb_lock)
> spin_lock_irq(hugetlb_lock)
> 1) update_and_free_page
> PageHuge() == F
> __free_pages()
> 2) enqueue_huge_page
> SetPageHugeFreed()
> spin_unlock(&hugetlb_lock)

Very small nit, the above should be spin_unlock_irq(&hugetlb_lock)

> spin_lock_irq(hugetlb_lock)
> 1) PageHuge() == F (freed by case#1 from CPU0)
> 2) PageHuge() == T
> PageHugeFreed() == T
> - proceed with replacing the page
>
> In the case above we retry as the window race is quite small and we have high
> chances to succeed next time.
>
> With regard to the allocation, we restrict it to the node the page belongs
> to with __GFP_THISNODE, meaning we do not fallback on other node's zones.
>
> Note that gigantic hugetlb pages are fenced off since there is a cyclic
> dependency between them and alloc_contig_range.
>
> Signed-off-by: Oscar Salvador <[email protected]>
> ---
...
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 0607b2b71ac6..4a664d6e82c1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2266,6 +2266,121 @@ static void restore_reserve_on_error(struct hstate *h,
> }
> }
>
> +/*
> + * alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
> + * @h: struct hstate old page belongs to
> + * @old_page: Old page to dissolve
> + * Returns 0 on success, otherwise negated error.
> + */
> +
> +static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
> +{
> + gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
> + int nid = page_to_nid(old_page);
> + struct page *new_page;
> + int ret = 0;
> +
> + /*
> + * Before dissolving the page, we need to allocate a new one for the
> + * pool to remain stable. Using alloc_buddy_huge_page() allows us to
> + * not having to deal with prep_new_page() and avoids dealing of any
> + * counters. This simplifies and let us do the whole thing under the
> + * lock.
> + */
> + new_page = alloc_buddy_huge_page(h, gfp_mask, nid, NULL, NULL);
> + if (!new_page)
> + return -ENOMEM;
> +
> +retry:
> + spin_lock_irq(&hugetlb_lock);
> + if (!PageHuge(old_page)) {
> + /*
> + * Freed from under us. Drop new_page too.
> + */
> + goto free_new;
> + } else if (page_count(old_page)) {
> + /*
> + * Someone has grabbed the page, fail for now.
> + */
> + ret = -EBUSY;
> + goto free_new;
> + } else if (!HPageFreed(old_page)) {
> + /*
> + * Page's refcount is 0 but it has not been enqueued in the
> + * freelist yet. Race window is small, so we can succeed here if
> + * we retry.
> + */
> + spin_unlock_irq(&hugetlb_lock);
> + cond_resched();
> + goto retry;
> + } else {
> + /*
> + * Ok, old_page is still a genuine free hugepage. Remove it from
> + * the freelist and decrease the counters. These will be
> + * incremented again when calling __prep_account_new_huge_page()
> + * and enqueue_huge_page() for new_page. The counters will remain
> + * stable since this happens under the lock.
> + */
> + remove_hugetlb_page(h, old_page, false);
> +
> + /*
> + * Call __prep_new_huge_page() to construct the hugetlb page, and
> + * enqueue it then to place it in the freelists. After this,
> + * counters are back on track. Free hugepages have a refcount of 0,
> + * so we need to decrease new_page's count as well.
> + */
> + __prep_new_huge_page(new_page);
> + __prep_account_new_huge_page(h, nid);
> + page_ref_dec(new_page);
> + enqueue_huge_page(h, new_page);
> +
> + /*
> + * Pages have been replaced, we can safely free the old one.
> + */
> + spin_unlock_irq(&hugetlb_lock);
> + update_and_free_page(h, old_page);
> + }
> +
> + return ret;
> +
> +free_new:
> + spin_unlock_irq(&hugetlb_lock);
> + __free_pages(new_page, huge_page_order(h));
> +
> + return ret;
> +}
> +
> +int isolate_or_dissolve_huge_page(struct page *page)
> +{
> + struct hstate *h;
> + struct page *head;
> +
> + /*
> + * The page might have been dissolved from under our feet, so make sure
> + * to carefully check the state under the lock.
> + * Return success when racing as if we dissolved the page ourselves.
> + */
> + spin_lock_irq(&hugetlb_lock);
> + if (PageHuge(page)) {
> + head = compound_head(page);
> + h = page_hstate(head);
> + } else {
> + spin_unlock(&hugetlb_lock);

Should be be spin_unlock_irq(&hugetlb_lock);

Other than that, it looks good.
--
Mike Kravetz

2021-04-14 07:28:30

[permalink] [raw]

Subject: Re: [PATCH v7 6/7] mm: Make alloc_contig_range handle in-use hugetlb pages

On 4/13/21 3:47 AM, Oscar Salvador wrote:
> alloc_contig_range() will fail if it finds a HugeTLB page within the range,
> without a chance to handle them. Since HugeTLB pages can be migrated as any
> LRU or Movable page, it does not make sense to bail out without trying.
> Enable the interface to recognize in-use HugeTLB pages so we can migrate
> them, and have much better chances to succeed the call.
>
> Signed-off-by: Oscar Salvador <[email protected]>
> Reviewed-by: Mike Kravetz <[email protected]>
> Acked-by: Michal Hocko <[email protected]>

One small issue/question/request below.

> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4a664d6e82c1..24a453ff47f2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2270,10 +2270,12 @@ static void restore_reserve_on_error(struct hstate *h,
> * alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
> * @h: struct hstate old page belongs to
> * @old_page: Old page to dissolve
> + * @list: List to isolate the page in case we need to
> * Returns 0 on success, otherwise negated error.
> */
>
> -static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
> +static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page,
> + struct list_head *list)
> {
> gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
> int nid = page_to_nid(old_page);
> @@ -2300,9 +2302,13 @@ static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
> goto free_new;
> } else if (page_count(old_page)) {
> /*
> - * Someone has grabbed the page, fail for now.
> + * Someone has grabbed the page, try to isolate it here.
> + * Fail with -EBUSY if not possible.
> */
> - ret = -EBUSY;
> + spin_unlock_irq(&hugetlb_lock);
> + if (!isolate_huge_page(old_page, list))
> + ret = -EBUSY;
> + spin_lock_irq(&hugetlb_lock);
> goto free_new;

The label free_new is:

free_new:
spin_unlock_irq(&hugetlb_lock);
__free_pages(new_page, huge_page_order(h));

return ret;

So, we are locking and immediately unlocking without any code in
between. Usually, I don't like like multiple labels before return.
However, perhaps we should add another to avoid this unnecessary
cycle. On the other hand, this is an uncommon race condition so the
simple code may be acceptable.
--
Mike Kravetz

2021-04-14 08:38:36

[permalink] [raw]

Subject: Re: [PATCH v7 4/7] mm,hugetlb: Split prep_new_huge_page functionality

On 4/13/21 3:47 AM, Oscar Salvador wrote:
> Currently, prep_new_huge_page() performs two functions.
> It sets the right state for a new hugetlb, and increases the hstate's
> counters to account for the new page.
>
> Let us split its functionality into two separate functions, decoupling
> the handling of the counters from initializing a hugepage.
> The outcome is having __prep_new_huge_page(), which only
> initializes the page , and __prep_account_new_huge_page(), which adds
> the new page to the hstate's counters.
>
> This allows us to be able to set a hugetlb without having to worry
> about the counter/locking. It will prove useful in the next patch.
> prep_new_huge_page() still calls both functions.
>
> Signed-off-by: Oscar Salvador <[email protected]>
> ---
> mm/hugetlb.c | 19 ++++++++++++++++---
> 1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e40d5fe5c63c..0607b2b71ac6 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1483,7 +1483,16 @@ void free_huge_page(struct page *page)
> }
> }
>
> -static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> +/*
> + * Must be called with the hugetlb lock held
> + */
> +static void __prep_account_new_huge_page(struct hstate *h, int nid)
> +{
> + h->nr_huge_pages++;
> + h->nr_huge_pages_node[nid]++;

I would prefer if we also move setting the destructor to this routine.
set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);

That way, PageHuge() will be false until it 'really' is a huge page.
If not, we could potentially go into that retry loop in
dissolve_free_huge_page or alloc_and_dissolve_huge_page in patch 5.
--
Mike Kravetz

> +}
> +
> +static void __prep_new_huge_page(struct page *page)
> {
> INIT_LIST_HEAD(&page->lru);
> set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> @@ -1491,9 +1500,13 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> set_hugetlb_cgroup(page, NULL);
> set_hugetlb_cgroup_rsvd(page, NULL);
> ClearHPageFreed(page);
> +}
> +
> +static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> +{
> + __prep_new_huge_page(page);
> spin_lock_irq(&hugetlb_lock);
> - h->nr_huge_pages++;
> - h->nr_huge_pages_node[nid]++;
> + __prep_account_new_huge_page(h, nid);
> spin_unlock_irq(&hugetlb_lock);
> }
>
>

2021-04-14 09:05:49

[permalink] [raw]

Subject: Re: [PATCH v7 7/7] mm,page_alloc: Drop unnecessary checks from pfn_range_valid_contig

On 4/13/21 3:47 AM, Oscar Salvador wrote:
> pfn_range_valid_contig() bails out when it finds an in-use page or a
> hugetlb page, among other things.
> We can drop the in-use page check since __alloc_contig_pages can migrate
> away those pages, and the hugetlb page check can go too since
> isolate_migratepages_range is now capable of dealing with hugetlb pages.
> Either way, those checks are racy so let the end function handle it
> when the time comes.
>
> Signed-off-by: Oscar Salvador <[email protected]>
> Suggested-by: David Hildenbrand <[email protected]>
> Reviewed-by: David Hildenbrand <[email protected]>
> ---
> mm/page_alloc.c | 6 ------
> 1 file changed, 6 deletions(-)

Acked-by: Mike Kravetz <[email protected]>
--
Mike Kravetz

2021-04-14 13:41:23

[permalink] [raw]

Subject: Re: [PATCH v7 6/7] mm: Make alloc_contig_range handle in-use hugetlb pages

On Tue, Apr 13, 2021 at 03:48:53PM -0700, Mike Kravetz wrote:
> The label free_new is:
>
> free_new:
> spin_unlock_irq(&hugetlb_lock);
> __free_pages(new_page, huge_page_order(h));
>
> return ret;
>
> So, we are locking and immediately unlocking without any code in
> between. Usually, I don't like like multiple labels before return.
> However, perhaps we should add another to avoid this unnecessary
> cycle. On the other hand, this is an uncommon race condition so the
> simple code may be acceptable.

I guess we could have something like:

free_new:
spin_unlock_irq(&hugetlb_lock);
free_new_nolock:
__free_pages(new_page, huge_page_order(h));

return ret;

And let the retry go to there without locking. But as you said, the
racecondition is rare enough, so I am not sure if this buys us much.
But I can certainly add it if you feel strong about it.

--
Oscar Salvador
SUSE L3

2021-04-14 13:41:23

[permalink] [raw]

Subject: Re: [PATCH v7 5/7] mm: Make alloc_contig_range handle free hugetlb pages

On Tue, Apr 13, 2021 at 03:29:02PM -0700, Mike Kravetz wrote:
> > spin_lock_irq(hugetlb_lock)
> > 1) update_and_free_page
> > PageHuge() == F
> > __free_pages()
> > 2) enqueue_huge_page
> > SetPageHugeFreed()
> > spin_unlock(&hugetlb_lock)
>
> Very small nit, the above should be spin_unlock_irq(&hugetlb_lock)

Right, I missed it somehow.

> > + /*
> > + * The page might have been dissolved from under our feet, so make sure
> > + * to carefully check the state under the lock.
> > + * Return success when racing as if we dissolved the page ourselves.
> > + */
> > + spin_lock_irq(&hugetlb_lock);
> > + if (PageHuge(page)) {
> > + head = compound_head(page);
> > + h = page_hstate(head);
> > + } else {
> > + spin_unlock(&hugetlb_lock);
>
> Should be be spin_unlock_irq(&hugetlb_lock);
>
> Other than that, it looks good.

Yeah, I will amend it in the next version.

Thanks Mike!

--
Oscar Salvador
SUSE L3

2021-04-14 13:41:31

[permalink] [raw]

Subject: Re: [PATCH v7 4/7] mm,hugetlb: Split prep_new_huge_page functionality

On Tue, Apr 13, 2021 at 02:33:41PM -0700, Mike Kravetz wrote:
> > -static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> > +/*
> > + * Must be called with the hugetlb lock held
> > + */
> > +static void __prep_account_new_huge_page(struct hstate *h, int nid)
> > +{
> > + h->nr_huge_pages++;
> > + h->nr_huge_pages_node[nid]++;
>
> I would prefer if we also move setting the destructor to this routine.
> set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);

Uhm, but that is the routine that does the accounting, it feels wrong
here, plus...

>
> That way, PageHuge() will be false until it 'really' is a huge page.
> If not, we could potentially go into that retry loop in
> dissolve_free_huge_page or alloc_and_dissolve_huge_page in patch 5.

...I do not follow here, could you please elaborate some more?
Unless I am missing something, behaviour should not be any different with this
patch.

Thanks

--
Oscar Salvador
SUSE L3

2021-04-14 15:18:13

[permalink] [raw]

Subject: Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

On Wed 14-04-21 12:01:47, Oscar Salvador wrote:
> On Wed, Apr 14, 2021 at 10:28:33AM +0200, Michal Hocko wrote:
> > You are right it doesn't do it there. But all struct pages, even those
> > that are allocated by the bootmem allocator should initialize its struct
> > pages. They would be poisoned otherwise, right? I would have to look at
> > the exact code path but IIRC this should be around the time bootmem
> > allocator state transitions to the page allocator.
>
> Ok, you are right.
> struct pages are initialized a bit earlier through:
>
> start_kernel
> setup_arch
> paging_init
> zone_sizes_init
> free_area_init
> free_area_init_node
> free_area_init_core
> memmap_init_zone
> memmap_init_range
> __init_single_page
>
> While the allocation of bootmem hugetlb happens
>
> start_kernel
> parse_args
> ...
> hugepages_setup
> ...
> hugetlb_hstate_alloc_pages
> __alloc_bootmem_huge_page
>
> which is after the setup_arch() call.

Thanks for pulling those paths. It is always painful to crawl that code.

> So by the time we get the page from __alloc_bootmem_huge_page(), fields are
> zeroed.
> I thought we might get in trouble because memblock_alloc_try_nid_raw() calls
> page_init_poison() which poisons the chunk with 0xff,e.g:
>
> [ 1.955471] boot: ffffffffffffffff ffffffffffffffff ffffffffffffffff ffffffffffffffff
> [ 1.955476] boot: ffffffffffffffff ffffffffffffffff ffffffffffffffff ffffffffffffffff
>
> but it seems that does not the memmap struct page.

Well, to be precise it does the very same thing with memamp struct pages
but that is before the initialization code you have pointed out above.
In this context it just poisons the allocated content which is the GB
page storage.

> I checked, and when we get there in __alloc_bootmem_huge_page, page->private is
> still zeroed, so I guess it should be safe to assume that we do not really need
> to clear the flag in __prep_new_huge_page() routine?

It would be quite nasty if the struct pages content would be undefined.
Maybe that is possible but then I would rather stick the initialization
into __alloc_bootmem_huge_page.

Thanks!
--
Michal Hocko
SUSE Labs

2021-04-14 15:32:28

[permalink] [raw]

Subject: Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

On Wed 14-04-21 12:49:53, Oscar Salvador wrote:
> On Wed, Apr 14, 2021 at 12:32:58PM +0200, Michal Hocko wrote:
[...]
> > > I checked, and when we get there in __alloc_bootmem_huge_page, page->private is
> > > still zeroed, so I guess it should be safe to assume that we do not really need
> > > to clear the flag in __prep_new_huge_page() routine?
> >
> > It would be quite nasty if the struct pages content would be undefined.
> > Maybe that is possible but then I would rather stick the initialization
> > into __alloc_bootmem_huge_page.
>
> Yes, but I do not think that is really possible unless I missed something.

Yeah, it should be fine. I was thinking of a alloc, modify struct pages,
free back to the bootmem allocator sequence. But I do not remember ever
seeing sequence like that. Bootmem allocator users tend to be simple,
allocate storage and either retain it for the life time. Other than
PageReserved bit they do not touch metadata. If we want to be paranoid
then we can add VM_WARN_ON for unexpected state when allocating from the
bootmem. But I am not sure this is really worth it.
--
Michal Hocko
SUSE Labs

2021-04-14 15:37:38

[permalink] [raw]

Subject: Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

On 14.04.21 13:09, Michal Hocko wrote:
> On Wed 14-04-21 12:49:53, Oscar Salvador wrote:
>> On Wed, Apr 14, 2021 at 12:32:58PM +0200, Michal Hocko wrote:
> [...]
>>>> I checked, and when we get there in __alloc_bootmem_huge_page, page->private is
>>>> still zeroed, so I guess it should be safe to assume that we do not really need
>>>> to clear the flag in __prep_new_huge_page() routine?
>>>
>>> It would be quite nasty if the struct pages content would be undefined.
>>> Maybe that is possible but then I would rather stick the initialization
>>> into __alloc_bootmem_huge_page.
>>
>> Yes, but I do not think that is really possible unless I missed something.
>
> Yeah, it should be fine. I was thinking of a alloc, modify struct pages,
> free back to the bootmem allocator sequence. But I do not remember ever
> seeing sequence like that.

We do have code like that, though.

Take a look at arch/x86/mm/init_64.c:free_pagetable() as one example.

But in general, we assume freeing code (buddy, but also memblock)
restores the state of the memmap to the original state it obtained the
memmap. So if it's properly initialized when coming from the allocator
the first time, it should remain properly initialized when re-entering
and re-leaving the allocator.

--
Thanks,

David / dhildenb

2021-04-14 15:38:54

[permalink] [raw]

Subject: Re: [PATCH v7 2/7] mm,compaction: Let isolate_migratepages_{range,block} return error codes

On 13.04.21 12:47, Oscar Salvador wrote:
> Currently, isolate_migratepages_{range,block} and their callers use
> a pfn == 0 vs pfn != 0 scheme to let the caller know whether there was
> any error during isolation.
> This does not work as soon as we need to start reporting different error
> codes and make sure we pass them down the chain, so they are properly
> interpreted by functions like e.g: alloc_contig_range.
>
> Let us rework isolate_migratepages_{range,block} so we can report error
> codes.
> Since isolate_migratepages_block will stop returning the next pfn to be
> scanned, we reuse the cc->migrate_pfn field to keep track of that.
>
> Signed-off-by: Oscar Salvador <[email protected]>
> Acked-by: Vlastimil Babka <[email protected]>
> ---
> mm/compaction.c | 54 +++++++++++++++++++++++++++---------------------------
> mm/internal.h | 10 ++++++++--
> mm/page_alloc.c | 7 +++----
> 3 files changed, 38 insertions(+), 33 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 8c5028bfbd56..eeba4668c22c 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -787,15 +787,15 @@ static bool too_many_isolated(pg_data_t *pgdat)
> *
> * Isolate all pages that can be migrated from the range specified by
> * [low_pfn, end_pfn). The range is expected to be within same pageblock.
> - * Returns zero if there is a fatal signal pending, otherwise PFN of the
> - * first page that was not scanned (which may be both less, equal to or more
> - * than end_pfn).
> + * Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
> + * or 0.
> + * cc->migrate_pfn will contain the next pfn to scan (which may be both less,
> + * equal to or more that end_pfn).

I failed to parse the last sentence -- e.g., using "both" and then
listing three options. Also, s/than/than/? Can we simplify to

"cc->migrate_pfn will contain the next pfn to scan"

> *
> * The pages are isolated on cc->migratepages list (not required to be empty),
> - * and cc->nr_migratepages is updated accordingly. The cc->migrate_pfn field
> - * is neither read nor updated.
> + * and cc->nr_migratepages is updated accordingly.
> */
> -static unsigned long
> +static int
> isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> unsigned long end_pfn, isolate_mode_t isolate_mode)
> {
> @@ -809,6 +809,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> bool skip_on_failure = false;
> unsigned long next_skip_pfn = 0;
> bool skip_updated = false;
> + int ret = 0;
> +
> + cc->migrate_pfn = low_pfn;
>
> /*
> * Ensure that there are not too many pages isolated from the LRU
> @@ -818,16 +821,16 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> while (unlikely(too_many_isolated(pgdat))) {
> /* stop isolation if there are still pages not migrated */
> if (cc->nr_migratepages)
> - return 0;
> + return -EAGAIN;
>
> /* async migration should just abort */
> if (cc->mode == MIGRATE_ASYNC)
> - return 0;
> + return -EAGAIN;
>
> congestion_wait(BLK_RW_ASYNC, HZ/10);
>
> if (fatal_signal_pending(current))
> - return 0;
> + return -EINTR;
> }
>
> cond_resched();
> @@ -875,8 +878,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>
> if (fatal_signal_pending(current)) {
> cc->contended = true;
> + ret = -EINTR;
>
> - low_pfn = 0;
> goto fatal_pending;
> }
>
> @@ -1130,7 +1133,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> if (nr_isolated)
> count_compact_events(COMPACTISOLATED, nr_isolated);
>
> - return low_pfn;
> + cc->migrate_pfn = low_pfn;
> +
> + return ret;
> }
>
> /**
> @@ -1139,15 +1144,15 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> * @start_pfn: The first PFN to start isolating.
> * @end_pfn: The one-past-last PFN.
> *
> - * Returns zero if isolation fails fatally due to e.g. pending signal.
> - * Otherwise, function returns one-past-the-last PFN of isolated page
> - * (which may be greater than end_pfn if end fell in a middle of a THP page).
> + * Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,

errno is usually positive.

> + * or 0.

I'd be more specific here. Something like

"
Returns 0 if isolation succeeded. Returns -EINTR if a fatal signal is
pending. Returns -EAGAIN when contended and the caller should retry.

In any case, cc->migrate_pfn is set to one-past-the-last PFN that has
been isolated.
"

--
Thanks,

David / dhildenb

2021-04-14 15:48:14

[permalink] [raw]

Subject: Re: [PATCH v7 5/7] mm: Make alloc_contig_range handle free hugetlb pages

> +static inline int isolate_or_dissolve_huge_page(struct page *page)
> +{
> + return -ENOMEM;

Without CONFIG_HUGETLB_PAGE, there is no way someone could possible pass
in something valid. Although it doesn't matter too much, -EINVAL or
similar sounds a bit better.

> +}
> +
> static inline struct page *alloc_huge_page(struct vm_area_struct *vma,
> unsigned long addr,
> int avoid_reserve)
> diff --git a/mm/compaction.c b/mm/compaction.c
> index eeba4668c22c..89426b6d1ea3 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -788,7 +788,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
> * Isolate all pages that can be migrated from the range specified by
> * [low_pfn, end_pfn). The range is expected to be within same pageblock.
> * Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
> - * or 0.
> + * -ENOMEM in case we could not allocate a page, or 0.
> * cc->migrate_pfn will contain the next pfn to scan (which may be both less,
> * equal to or more that end_pfn).
> *
> @@ -809,6 +809,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> bool skip_on_failure = false;
> unsigned long next_skip_pfn = 0;
> bool skip_updated = false;
> + bool fatal_error = false;

Can't we use "ret == -ENOMEM" instead of fatal_error?

> int ret = 0;
>
> cc->migrate_pfn = low_pfn;
> @@ -907,6 +908,33 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> valid_page = page;
> }
>
> + if (PageHuge(page) && cc->alloc_contig) {
> + ret = isolate_or_dissolve_huge_page(page);
> +
> + /*
> + * Fail isolation in case isolate_or_dissolve_huge_page
> + * reports an error. In case of -ENOMEM, abort right away.
> + */
> + if (ret < 0) {
> + /*
> + * Do not report -EBUSY down the chain.
> + */
> + if (ret == -ENOMEM)
> + fatal_error = true;
> + else
> + ret = 0;
> + low_pfn += (1UL << compound_order(page)) - 1;
> + goto isolate_fail;
> + }
> +
> + /*
> + * Ok, the hugepage was dissolved. Now these pages are
> + * Buddy and cannot be re-allocated because they are
> + * isolated. Fall-through as the check below handles
> + * Buddy pages.
> + */
> + }
> +
> /*
> * Skip if free. We read page order here without zone lock
> * which is generally unsafe, but the race window is small and
> @@ -1066,7 +1094,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> put_page(page);
>
> isolate_fail:
> - if (!skip_on_failure)
> + if (!skip_on_failure && !fatal_error)
> continue;
>
> /*
> @@ -1092,6 +1120,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> */
> next_skip_pfn += 1UL << cc->order;
> }
> +
> + if (fatal_error)
> + break;
> }
>
> /*
> @@ -1145,7 +1176,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> * @end_pfn: The one-past-last PFN.
> *
> * Returns errno, like -EAGAIN or -EINTR in case e.g signal pending or congestion,
> - * or 0.
> + * -ENOMEM in case we could not allocate a page, or 0.
> */
> int
> isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 0607b2b71ac6..4a664d6e82c1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2266,6 +2266,121 @@ static void restore_reserve_on_error(struct hstate *h,
> }
> }
>
> +/*
> + * alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
> + * @h: struct hstate old page belongs to
> + * @old_page: Old page to dissolve
> + * Returns 0 on success, otherwise negated error.
> + */
> +
> +static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
> +{
> + gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
> + int nid = page_to_nid(old_page);
> + struct page *new_page;
> + int ret = 0;
> +
> + /*
> + * Before dissolving the page, we need to allocate a new one for the
> + * pool to remain stable. Using alloc_buddy_huge_page() allows us to
> + * not having to deal with prep_new_page() and avoids dealing of any
> + * counters. This simplifies and let us do the whole thing under the
> + * lock.
> + */
> + new_page = alloc_buddy_huge_page(h, gfp_mask, nid, NULL, NULL);
> + if (!new_page)
> + return -ENOMEM;
> +
> +retry:
> + spin_lock_irq(&hugetlb_lock);
> + if (!PageHuge(old_page)) {
> + /*
> + * Freed from under us. Drop new_page too.
> + */
> + goto free_new;
> + } else if (page_count(old_page)) {
> + /*
> + * Someone has grabbed the page, fail for now.
> + */
> + ret = -EBUSY;
> + goto free_new;
> + } else if (!HPageFreed(old_page)) {
> + /*
> + * Page's refcount is 0 but it has not been enqueued in the
> + * freelist yet. Race window is small, so we can succeed here if
> + * we retry.
> + */
> + spin_unlock_irq(&hugetlb_lock);
> + cond_resched();
> + goto retry;
> + } else {
> + /*
> + * Ok, old_page is still a genuine free hugepage. Remove it from
> + * the freelist and decrease the counters. These will be
> + * incremented again when calling __prep_account_new_huge_page()
> + * and enqueue_huge_page() for new_page. The counters will remain
> + * stable since this happens under the lock.
> + */
> + remove_hugetlb_page(h, old_page, false);
> +
> + /*
> + * Call __prep_new_huge_page() to construct the hugetlb page, and
> + * enqueue it then to place it in the freelists. After this,
> + * counters are back on track. Free hugepages have a refcount of 0,
> + * so we need to decrease new_page's count as well.
> + */
> + __prep_new_huge_page(new_page);
> + __prep_account_new_huge_page(h, nid);
> + page_ref_dec(new_page);
> + enqueue_huge_page(h, new_page);
> +
> + /*
> + * Pages have been replaced, we can safely free the old one.
> + */
> + spin_unlock_irq(&hugetlb_lock);
> + update_and_free_page(h, old_page);
> + }
> +
> + return ret;
> +
> +free_new:
> + spin_unlock_irq(&hugetlb_lock);
> + __free_pages(new_page, huge_page_order(h));
> +
> + return ret;
> +}
> +
> +int isolate_or_dissolve_huge_page(struct page *page)
> +{
> + struct hstate *h;
> + struct page *head;
> +
> + /*
> + * The page might have been dissolved from under our feet, so make sure
> + * to carefully check the state under the lock.
> + * Return success when racing as if we dissolved the page ourselves.
> + */
> + spin_lock_irq(&hugetlb_lock);
> + if (PageHuge(page)) {
> + head = compound_head(page);
> + h = page_hstate(head);
> + } else {
> + spin_unlock(&hugetlb_lock);
> + return 0;
> + }
> + spin_unlock_irq(&hugetlb_lock);
> +
> + /*
> + * Fence off gigantic pages as there is a cyclic dependency between
> + * alloc_contig_range and them. Return -ENOME as this has the effect

s/-ENOME/-ENOMEM/

> + * of bailing out right away without further retrying.
> + */
> + if (hstate_is_gigantic(h))
> + return -ENOMEM;
> +
> + return alloc_and_dissolve_huge_page(h, head);
> +}
> +
> struct page *alloc_huge_page(struct vm_area_struct *vma,
> unsigned long addr, int avoid_reserve)
> {
>

Complicated stuff, but looks good to me.

--
Thanks,

David / dhildenb

2021-04-14 18:10:15

[permalink] [raw]

Subject: Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

On 4/14/21 3:49 AM, Oscar Salvador wrote:
> On Wed, Apr 14, 2021 at 12:32:58PM +0200, Michal Hocko wrote:
>> Well, to be precise it does the very same thing with memamp struct pages
>> but that is before the initialization code you have pointed out above.
>> In this context it just poisons the allocated content which is the GB
>> page storage.
>
> Right.
>
>>> I checked, and when we get there in __alloc_bootmem_huge_page, page->private is
>>> still zeroed, so I guess it should be safe to assume that we do not really need
>>> to clear the flag in __prep_new_huge_page() routine?
>>
>> It would be quite nasty if the struct pages content would be undefined.
>> Maybe that is possible but then I would rather stick the initialization
>> into __alloc_bootmem_huge_page.
>
> Yes, but I do not think that is really possible unless I missed something.
> Let us see what Mike thinks of it, if there are no objections, we can
> get rid of the clearing flag right there.
>

Thanks for crawling through that code Oscar!

I do not think you missed anything. Let's just get rid of the flag
clearing.
--
Mike Kravetz

2021-04-14 18:12:47

[permalink] [raw]

Subject: Re: [PATCH v7 4/7] mm,hugetlb: Split prep_new_huge_page functionality

On 4/13/21 9:59 PM, Oscar Salvador wrote:
> On Tue, Apr 13, 2021 at 02:33:41PM -0700, Mike Kravetz wrote:
>>> -static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>>> +/*
>>> + * Must be called with the hugetlb lock held
>>> + */
>>> +static void __prep_account_new_huge_page(struct hstate *h, int nid)
>>> +{
>>> + h->nr_huge_pages++;
>>> + h->nr_huge_pages_node[nid]++;
>>
>> I would prefer if we also move setting the destructor to this routine.
>> set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
>
> Uhm, but that is the routine that does the accounting, it feels wrong
> here, plus...
>
>>
>> That way, PageHuge() will be false until it 'really' is a huge page.
>> If not, we could potentially go into that retry loop in
>> dissolve_free_huge_page or alloc_and_dissolve_huge_page in patch 5.
>
> ...I do not follow here, could you please elaborate some more?
> Unless I am missing something, behaviour should not be any different with this
> patch.
>

I was thinking of the time between the call to __prep_new_huge_page and
__prep_account_new_huge_page. In that time, PageHuge() will be true but
the page is not yet fully being managed as a hugetlb page. My thought
was that isolation, migration, offline or any code that does pfn
scanning might the page as PageHuge() (after taking lock) and start to
process it.

Now I see that in patch 5 you call both __prep_new_huge_page and
__prep_account_new_huge_page with the lock held. So, no issue. Sorry.

I 'think' there may still be a potential race with the prep_new_huge_page
routine, but that existed before any of your changes. It may also be
that 'synchronization' at the pageblock level which prevents some of
these pfn scanning operations to operate on the same pageblocks may
prevent this from ever happening.

Mostly thinking out loud. Upon further thought, I have no objections to
this change. Sorry for the noise.

Reviewed-by: Mike Kravetz <[email protected]>
--
Mike Kravetz

2021-04-14 22:52:25

[permalink] [raw]

Subject: Re: [PATCH v7 3/7] mm,hugetlb: Clear HPageFreed outside of the lock

On Wed, Apr 14, 2021 at 12:32:58PM +0200, Michal Hocko wrote:
> Well, to be precise it does the very same thing with memamp struct pages
> but that is before the initialization code you have pointed out above.
> In this context it just poisons the allocated content which is the GB
> page storage.

Right.

> > I checked, and when we get there in __alloc_bootmem_huge_page, page->private is
> > still zeroed, so I guess it should be safe to assume that we do not really need
> > to clear the flag in __prep_new_huge_page() routine?
>
> It would be quite nasty if the struct pages content would be undefined.
> Maybe that is possible but then I would rather stick the initialization
> into __alloc_bootmem_huge_page.

Yes, but I do not think that is really possible unless I missed something.
Let us see what Mike thinks of it, if there are no objections, we can
get rid of the clearing flag right there.

--
Oscar Salvador
SUSE L3

2021-04-15 00:17:13

[permalink] [raw]

Subject: Re: [PATCH v7 4/7] mm,hugetlb: Split prep_new_huge_page functionality

On 14.04.21 06:59, Oscar Salvador wrote:
> On Tue, Apr 13, 2021 at 02:33:41PM -0700, Mike Kravetz wrote:
>>> -static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>>> +/*
>>> + * Must be called with the hugetlb lock held
>>> + */
>>> +static void __prep_account_new_huge_page(struct hstate *h, int nid)
>>> +{
>>> + h->nr_huge_pages++;
>>> + h->nr_huge_pages_node[nid]++;
>>
>> I would prefer if we also move setting the destructor to this routine.
>> set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
>
> Uhm, but that is the routine that does the accounting, it feels wrong
> here, plus...

I agree. If we want the final activation separately, it might be better
to have it as a separate step/function like __active_new_huge_page().

--
Thanks,

David / dhildenb

2021-04-15 00:36:47

[permalink] [raw]

Subject: Re: [PATCH v7 6/7] mm: Make alloc_contig_range handle in-use hugetlb pages

On 4/13/21 9:52 PM, Oscar Salvador wrote:
> On Tue, Apr 13, 2021 at 03:48:53PM -0700, Mike Kravetz wrote:
>> The label free_new is:
>>
>> free_new:
>> spin_unlock_irq(&hugetlb_lock);
>> __free_pages(new_page, huge_page_order(h));
>>
>> return ret;
>>
>> So, we are locking and immediately unlocking without any code in
>> between. Usually, I don't like like multiple labels before return.
>> However, perhaps we should add another to avoid this unnecessary
>> cycle. On the other hand, this is an uncommon race condition so the
>> simple code may be acceptable.
>
> I guess we could have something like:
>
> free_new:
> spin_unlock_irq(&hugetlb_lock);
> free_new_nolock:
> __free_pages(new_page, huge_page_order(h));
>
> return ret;
>
> And let the retry go to there without locking. But as you said, the
> racecondition is rare enough, so I am not sure if this buys us much.
> But I can certainly add it if you feel strong about it.

No strong feelings. I am fine with it as is.

--
Mike Kravetz

2021-04-15 08:30:43

[permalink] [raw]

Subject: Re: [PATCH v7 5/7] mm: Make alloc_contig_range handle free hugetlb pages

On Wed, Apr 14, 2021 at 02:26:21PM +0200, David Hildenbrand wrote:
> > +static inline int isolate_or_dissolve_huge_page(struct page *page)
> > +{
> > + return -ENOMEM;
>
> Without CONFIG_HUGETLB_PAGE, there is no way someone could possible pass in
> something valid. Although it doesn't matter too much, -EINVAL or similar
> sounds a bit better.

I guess we could, was just to make it consistent with the kind of error
we return when we have it enabled.

> > @@ -809,6 +809,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> > bool skip_on_failure = false;
> > unsigned long next_skip_pfn = 0;
> > bool skip_updated = false;
> > + bool fatal_error = false;
>
> Can't we use "ret == -ENOMEM" instead of fatal_error?

Yes, we can, I will see how it looks.

[...]

> > + /*
> > + * Fence off gigantic pages as there is a cyclic dependency between
> > + * alloc_contig_range and them. Return -ENOME as this has the effect
>
> s/-ENOME/-ENOMEM/

thanks, I missed that one.

>
> > + * of bailing out right away without further retrying.
> > + */
> > + if (hstate_is_gigantic(h))
> > + return -ENOMEM;
> > +
> > + return alloc_and_dissolve_huge_page(h, head);
> > +}
> > +
> > struct page *alloc_huge_page(struct vm_area_struct *vma,
> > unsigned long addr, int avoid_reserve)
> > {
> >
>
> Complicated stuff, but looks good to me.

Thanks for having a look!

--
Oscar Salvador
SUSE L3

2021-04-15 08:30:54

[permalink] [raw]

Subject: Re: [PATCH v7 5/7] mm: Make alloc_contig_range handle free hugetlb pages

On Tue, Apr 13, 2021 at 03:40:18PM +0200, Michal Hocko wrote:
> > + /*
> > + * Call __prep_new_huge_page() to construct the hugetlb page, and
> > + * enqueue it then to place it in the freelists. After this,
> > + * counters are back on track. Free hugepages have a refcount of 0,
> > + * so we need to decrease new_page's count as well.
> > + */
> > + __prep_new_huge_page(new_page);
> > + __prep_account_new_huge_page(h, nid);
>
> I think it would help to put something like the following into the
> comment above this really strange construct.
>
> /*
> * new_page needs to be initialized with the standard
> * hugetlb state. This is normally done by
> * prep_new_huge_page but that takes hugetlb_lock which
> * is already held so we need to open code it here.
> * Reference count trick is needed because allocator
> * gives us referenced page but the pool requires pages
> * with 0 refcount.
> */

Ok, I will try to add more info, thanks Michal!

--
Oscar Salvador
SUSE L3

2021-04-15 08:43:36