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 proofed 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 dissolve such pages.
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 dissolve the old one.
If the old hugepage cannot be be dissolved, we have to dissolve the new
hugepage we just got.
Should that fail as well, we count is as a surplus, so the pool will be
re-balanced when a hugepage gets free instead of enqueues again.
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 | 12 ++++++++
mm/hugetlb.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 93 insertions(+)
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index b5807f23caf8..72352d718829 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -505,6 +505,7 @@ struct huge_bootmem_page {
struct hstate *hstate;
};
+bool 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,
@@ -775,6 +776,11 @@ void set_page_huge_active(struct page *page);
#else /* CONFIG_HUGETLB_PAGE */
struct hstate {};
+static inline bool isolate_or_dissolve_huge_page(struct page *page)
+{
+ return false;
+}
+
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 190ccdaa6c19..d52506ed9db7 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -905,6 +905,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
valid_page = page;
}
+ if (PageHuge(page) && cc->alloc_contig) {
+ if (!isolate_or_dissolve_huge_page(page))
+ 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
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4bdb58ab14cb..a4fbbe924a55 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2294,6 +2294,81 @@ static void restore_reserve_on_error(struct hstate *h,
}
}
+static bool alloc_and_dissolve_huge_page(struct hstate *h, struct page *page)
+{
+ gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
+ int nid = page_to_nid(page);
+ struct page *new_page;
+ bool ret = false;
+
+ /*
+ * Before dissolving the page, we need to allocate a new one,
+ * so the pool remains stable.
+ */
+ new_page = alloc_fresh_huge_page(h, gfp_mask, nid, NULL, NULL);
+ if (new_page) {
+ /*
+ * Free it into the hugepage allocator
+ */
+ put_page(new_page);
+
+ /*
+ * Ok, we got a new free hugepage to replace this one. Try to
+ * dissolve the old page.
+ */
+ if (!dissolve_free_huge_page(page)) {
+ ret = true;
+ } else if (dissolve_free_huge_page(new_page)) {
+ /*
+ * Seems the old page could not be dissolved, so try to
+ * dissolve the freshly allocated page. If that fails
+ * too, let us count the new page as a surplus. Doing so
+ * allows the pool to be re-balanced when pages are freed
+ * instead of enqueued again.
+ */
+ spin_lock(&hugetlb_lock);
+ h->surplus_huge_pages++;
+ h->surplus_huge_pages_node[nid]++;
+ spin_unlock(&hugetlb_lock);
+ }
+ }
+
+ return ret;
+}
+
+bool isolate_or_dissolve_huge_page(struct page *page)
+{
+ struct hstate *h = NULL;
+ struct page *head;
+ bool ret = false;
+
+ spin_lock(&hugetlb_lock);
+ if (PageHuge(page)) {
+ head = compound_head(page);
+ h = page_hstate(head);
+ }
+ spin_unlock(&hugetlb_lock);
+
+ /*
+ * The page might have been dissolved from under our feet.
+ * If that is the case, return success as if we dissolved it ourselves.
+ */
+ if (!h)
+ return true;
+
+ /*
+ * Fence off gigantic pages as there is a cyclic dependency
+ * between alloc_contig_range and them.
+ */
+ if (hstate_is_gigantic(h))
+ return ret;
+
+ if(!page_count(head) && alloc_and_dissolve_huge_page(h, head))
+ ret = true;
+
+ return ret;
+}
+
struct page *alloc_huge_page(struct vm_area_struct *vma,
unsigned long addr, int avoid_reserve)
{
--
2.16.3
On 2/18/21 4:00 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 proofed 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 dissolve such pages.
>
> 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 dissolve the old one.
>
> If the old hugepage cannot be be dissolved, we have to dissolve the new
> hugepage we just got.
> Should that fail as well, we count is as a surplus, so the pool will be
> re-balanced when a hugepage gets free instead of enqueues again.
>
> 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 | 12 ++++++++
> mm/hugetlb.c | 75 +++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 93 insertions(+)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index b5807f23caf8..72352d718829 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -505,6 +505,7 @@ struct huge_bootmem_page {
> struct hstate *hstate;
> };
>
> +bool 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,
> @@ -775,6 +776,11 @@ void set_page_huge_active(struct page *page);
> #else /* CONFIG_HUGETLB_PAGE */
> struct hstate {};
>
> +static inline bool isolate_or_dissolve_huge_page(struct page *page)
> +{
> + return false;
> +}
> +
> 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 190ccdaa6c19..d52506ed9db7 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -905,6 +905,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> valid_page = page;
> }
>
> + if (PageHuge(page) && cc->alloc_contig) {
> + if (!isolate_or_dissolve_huge_page(page))
> + 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
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4bdb58ab14cb..a4fbbe924a55 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2294,6 +2294,81 @@ static void restore_reserve_on_error(struct hstate *h,
> }
> }
>
> +static bool alloc_and_dissolve_huge_page(struct hstate *h, struct page *page)
> +{
> + gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
> + int nid = page_to_nid(page);
> + struct page *new_page;
> + bool ret = false;
> +
> + /*
> + * Before dissolving the page, we need to allocate a new one,
> + * so the pool remains stable.
> + */
> + new_page = alloc_fresh_huge_page(h, gfp_mask, nid, NULL, NULL);
> + if (new_page) {
> + /*
> + * Free it into the hugepage allocator
> + */
> + put_page(new_page);
> +
Suppose an admin does
echo 0 > \
/sys/devices/system/node/node<nid>/hugepages/hugepages-2048kB/nr_hugepages
right now and dissolves both the original and new page.
> + /*
> + * Ok, we got a new free hugepage to replace this one. Try to
> + * dissolve the old page.
> + */
> + if (!dissolve_free_huge_page(page)) {
> + ret = true;
dissolve_free_huge_page will fail for the original page
> + } else if (dissolve_free_huge_page(new_page)) {
and, will fail for the new page
> + /*
> + * Seems the old page could not be dissolved, so try to
> + * dissolve the freshly allocated page. If that fails
> + * too, let us count the new page as a surplus. Doing so
> + * allows the pool to be re-balanced when pages are freed
> + * instead of enqueued again.
> + */
> + spin_lock(&hugetlb_lock);
> + h->surplus_huge_pages++;
> + h->surplus_huge_pages_node[nid]++;
> + spin_unlock(&hugetlb_lock);
Those counts will be wrong as there are no huge pages on the node.
I'll think about this more tomorrow.
Pretty sure this is an issue, but I could be wrong. Just wanted to give
a heads up.
--
Mike Kravetz
> + }
> + }
> +
> + return ret;
> +}
> +
> +bool isolate_or_dissolve_huge_page(struct page *page)
> +{
> + struct hstate *h = NULL;
> + struct page *head;
> + bool ret = false;
> +
> + spin_lock(&hugetlb_lock);
> + if (PageHuge(page)) {
> + head = compound_head(page);
> + h = page_hstate(head);
> + }
> + spin_unlock(&hugetlb_lock);
> +
> + /*
> + * The page might have been dissolved from under our feet.
> + * If that is the case, return success as if we dissolved it ourselves.
> + */
> + if (!h)
> + return true;
> +
> + /*
> + * Fence off gigantic pages as there is a cyclic dependency
> + * between alloc_contig_range and them.
> + */
> + if (hstate_is_gigantic(h))
> + return ret;
> +
> + if(!page_count(head) && alloc_and_dissolve_huge_page(h, head))
> + ret = true;
> +
> + return ret;
> +}
> +
> struct page *alloc_huge_page(struct vm_area_struct *vma,
> unsigned long addr, int avoid_reserve)
> {
>
On 2021-02-19 03:10, Mike Kravetz wrote:
> Those counts will be wrong as there are no huge pages on the node.
>
> I'll think about this more tomorrow.
> Pretty sure this is an issue, but I could be wrong. Just wanted to
> give
> a heads up.
Yes, this is a problem, although the fixup would be to check whether we
have any hugepages.
Nevertheless, I think we should not be touching surplus at all but
rather make the page temporary.
I am exploring making migrate_pages() handle free hugepages as Michal
suggested, so the approach is cleaner and we do not need extra
functions. I yet have to see if that is feasible, as some issues come to
my mind like the page needs to be in a list to go to migrate_pages, but
if it is in that list, it is not in hugepages_freelist, and that could
disrupt userspace as it could not dequeue hugepages if it demands it.
I have to check. Shoult not be possible, we can always make the page
temporary here.
> --
> Mike Kravetz
>
>> + }
>> + }
>> +
>> + return ret;
>> +}
>> +
>> +bool isolate_or_dissolve_huge_page(struct page *page)
>> +{
>> + struct hstate *h = NULL;
>> + struct page *head;
>> + bool ret = false;
>> +
>> + spin_lock(&hugetlb_lock);
>> + if (PageHuge(page)) {
>> + head = compound_head(page);
>> + h = page_hstate(head);
>> + }
>> + spin_unlock(&hugetlb_lock);
>> +
>> + /*
>> + * The page might have been dissolved from under our feet.
>> + * If that is the case, return success as if we dissolved it
>> ourselves.
>> + */
>> + if (!h)
>> + return true;
>> +
>> + /*
>> + * Fence off gigantic pages as there is a cyclic dependency
>> + * between alloc_contig_range and them.
>> + */
>> + if (hstate_is_gigantic(h))
>> + return ret;
>> +
>> + if(!page_count(head) && alloc_and_dissolve_huge_page(h, head))
>> + ret = true;
>> +
>> + return ret;
>> +}
>> +
>> struct page *alloc_huge_page(struct vm_area_struct *vma,
>> unsigned long addr, int avoid_reserve)
>> {
>>
--
Oscar Salvador
SUSE L3