Freeing a hugetlb page and releasing base pages back to the underlying
allocator such as buddy or cma is performed in two steps:
- remove_hugetlb_folio() is called to remove the folio from hugetlb
lists, get a ref on the page and remove hugetlb destructor. This
all must be done under the hugetlb lock. After this call, the page
can be treated as a normal compound page or a collection of base
size pages.
- update_and_free_hugetlb_folio() is called to allocate vmemmap if
needed and the free routine of the underlying allocator is called
on the resulting page. We can not hold the hugetlb lock here.
One issue with this scheme is that a memory error could occur between
these two steps. In this case, the memory error handling code treats
the old hugetlb page as a normal compound page or collection of base
pages. It will then try to SetPageHWPoison(page) on the page with an
error. If the page with error is a tail page without vmemmap, a write
error will occur when trying to set the flag.
Address this issue by modifying remove_hugetlb_folio() and
update_and_free_hugetlb_folio() such that the hugetlb destructor is not
cleared until after allocating vmemmap. Since clearing the destructor
requires holding the hugetlb lock, the clearing is done in
remove_hugetlb_folio() if the vmemmap is present. This saves a
lock/unlock cycle. Otherwise, destructor is cleared in
update_and_free_hugetlb_folio() after allocating vmemmap.
Note that this will leave hugetlb pages in a state where they are marked
free (by hugetlb specific page flag) and have a ref count. This is not
a normal state. The only code that would notice is the memory error
code, and it is set up to retry in such a case.
A subsequent patch will create a routine to do bulk processing of
vmemmap allocation. This will eliminate a lock/unlock cycle for each
hugetlb page in the case where we are freeing a large number of pages.
Fixes: ad2fa3717b74 ("mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page")
Cc: <[email protected]>
Signed-off-by: Mike Kravetz <[email protected]>
---
mm/hugetlb.c | 75 +++++++++++++++++++++++++++++++++++-----------------
1 file changed, 51 insertions(+), 24 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e4a28ce0667f..1b67bf341c32 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1580,9 +1580,37 @@ static inline void destroy_compound_gigantic_folio(struct folio *folio,
unsigned int order) { }
#endif
+static inline void __clear_hugetlb_destructor(struct hstate *h,
+ struct folio *folio)
+{
+ lockdep_assert_held(&hugetlb_lock);
+
+ /*
+ * Very subtle
+ *
+ * For non-gigantic pages set the destructor to the normal compound
+ * page dtor. This is needed in case someone takes an additional
+ * temporary ref to the page, and freeing is delayed until they drop
+ * their reference.
+ *
+ * For gigantic pages set the destructor to the null dtor. This
+ * destructor will never be called. Before freeing the gigantic
+ * page destroy_compound_gigantic_folio will turn the folio into a
+ * simple group of pages. After this the destructor does not
+ * apply.
+ *
+ */
+ if (hstate_is_gigantic(h))
+ folio_set_compound_dtor(folio, NULL_COMPOUND_DTOR);
+ else
+ folio_set_compound_dtor(folio, COMPOUND_PAGE_DTOR);
+}
+
/*
- * Remove hugetlb folio from lists, and update dtor so that the folio appears
- * as just a compound page.
+ * Remove hugetlb folio from lists.
+ * If vmemmap exists for the folio, update dtor so that the folio appears
+ * as just a compound page. Otherwise, wait until after allocating vmemmap
+ * to update dtor.
*
* A reference is held on the folio, except in the case of demote.
*
@@ -1613,31 +1641,19 @@ static void __remove_hugetlb_folio(struct hstate *h, struct folio *folio,
}
/*
- * Very subtle
- *
- * For non-gigantic pages set the destructor to the normal compound
- * page dtor. This is needed in case someone takes an additional
- * temporary ref to the page, and freeing is delayed until they drop
- * their reference.
- *
- * For gigantic pages set the destructor to the null dtor. This
- * destructor will never be called. Before freeing the gigantic
- * page destroy_compound_gigantic_folio will turn the folio into a
- * simple group of pages. After this the destructor does not
- * apply.
- *
- * This handles the case where more than one ref is held when and
- * after update_and_free_hugetlb_folio is called.
- *
- * In the case of demote we do not ref count the page as it will soon
- * be turned into a page of smaller size.
+ * We can only clear the hugetlb destructor after allocating vmemmap
+ * pages. Otherwise, someone (memory error handling) may try to write
+ * to tail struct pages.
+ */
+ if (!folio_test_hugetlb_vmemmap_optimized(folio))
+ __clear_hugetlb_destructor(h, folio);
+
+ /*
+ * In the case of demote we do not ref count the page as it will soon
+ * be turned into a page of smaller size.
*/
if (!demote)
folio_ref_unfreeze(folio, 1);
- if (hstate_is_gigantic(h))
- folio_set_compound_dtor(folio, NULL_COMPOUND_DTOR);
- else
- folio_set_compound_dtor(folio, COMPOUND_PAGE_DTOR);
h->nr_huge_pages--;
h->nr_huge_pages_node[nid]--;
@@ -1706,6 +1722,7 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
{
int i;
struct page *subpage;
+ bool clear_dtor = folio_test_hugetlb_vmemmap_optimized(folio);
if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
return;
@@ -1736,6 +1753,16 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
if (unlikely(folio_test_hwpoison(folio)))
folio_clear_hugetlb_hwpoison(folio);
+ /*
+ * If vmemmap pages were allocated above, then we need to clear the
+ * hugetlb destructor under the hugetlb lock.
+ */
+ if (clear_dtor) {
+ spin_lock_irq(&hugetlb_lock);
+ __clear_hugetlb_destructor(h, folio);
+ spin_unlock_irq(&hugetlb_lock);
+ }
+
for (i = 0; i < pages_per_huge_page(h); i++) {
subpage = folio_page(folio, i);
subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
--
2.41.0
> On Jul 12, 2023, at 06:09, Mike Kravetz <[email protected]> wrote:
>
> Freeing a hugetlb page and releasing base pages back to the underlying
> allocator such as buddy or cma is performed in two steps:
> - remove_hugetlb_folio() is called to remove the folio from hugetlb
> lists, get a ref on the page and remove hugetlb destructor. This
> all must be done under the hugetlb lock. After this call, the page
> can be treated as a normal compound page or a collection of base
> size pages.
> - update_and_free_hugetlb_folio() is called to allocate vmemmap if
> needed and the free routine of the underlying allocator is called
> on the resulting page. We can not hold the hugetlb lock here.
>
> One issue with this scheme is that a memory error could occur between
> these two steps. In this case, the memory error handling code treats
> the old hugetlb page as a normal compound page or collection of base
> pages. It will then try to SetPageHWPoison(page) on the page with an
> error. If the page with error is a tail page without vmemmap, a write
> error will occur when trying to set the flag.
>
> Address this issue by modifying remove_hugetlb_folio() and
> update_and_free_hugetlb_folio() such that the hugetlb destructor is not
> cleared until after allocating vmemmap. Since clearing the destructor
> requires holding the hugetlb lock, the clearing is done in
> remove_hugetlb_folio() if the vmemmap is present. This saves a
> lock/unlock cycle. Otherwise, destructor is cleared in
> update_and_free_hugetlb_folio() after allocating vmemmap.
>
> Note that this will leave hugetlb pages in a state where they are marked
> free (by hugetlb specific page flag) and have a ref count. This is not
> a normal state. The only code that would notice is the memory error
> code, and it is set up to retry in such a case.
>
> A subsequent patch will create a routine to do bulk processing of
> vmemmap allocation. This will eliminate a lock/unlock cycle for each
> hugetlb page in the case where we are freeing a large number of pages.
>
> Fixes: ad2fa3717b74 ("mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page")
> Cc: <[email protected]>
> Signed-off-by: Mike Kravetz <[email protected]>
Hi Mike,
I have seen an issue proposed by Jiaqi Yan in [1]. I didn't see any
resolution for it. Am I missing something with this fix?
[1] https://lore.kernel.org/linux-mm/CACw3F53iPiLrJt4pyaX2aaZ5BVg9tj8x_k6-v7=9Xn1nrh=UCw@mail.gmail.com/
Thanks.
On 07/12/23 16:03, Muchun Song wrote:
>
>
> > On Jul 12, 2023, at 06:09, Mike Kravetz <[email protected]> wrote:
> >
> > Freeing a hugetlb page and releasing base pages back to the underlying
> > allocator such as buddy or cma is performed in two steps:
> > - remove_hugetlb_folio() is called to remove the folio from hugetlb
> > lists, get a ref on the page and remove hugetlb destructor. This
> > all must be done under the hugetlb lock. After this call, the page
> > can be treated as a normal compound page or a collection of base
> > size pages.
> > - update_and_free_hugetlb_folio() is called to allocate vmemmap if
> > needed and the free routine of the underlying allocator is called
> > on the resulting page. We can not hold the hugetlb lock here.
> >
> > One issue with this scheme is that a memory error could occur between
> > these two steps. In this case, the memory error handling code treats
> > the old hugetlb page as a normal compound page or collection of base
> > pages. It will then try to SetPageHWPoison(page) on the page with an
> > error. If the page with error is a tail page without vmemmap, a write
> > error will occur when trying to set the flag.
> >
> > Address this issue by modifying remove_hugetlb_folio() and
> > update_and_free_hugetlb_folio() such that the hugetlb destructor is not
> > cleared until after allocating vmemmap. Since clearing the destructor
> > requires holding the hugetlb lock, the clearing is done in
> > remove_hugetlb_folio() if the vmemmap is present. This saves a
> > lock/unlock cycle. Otherwise, destructor is cleared in
> > update_and_free_hugetlb_folio() after allocating vmemmap.
> >
> > Note that this will leave hugetlb pages in a state where they are marked
> > free (by hugetlb specific page flag) and have a ref count. This is not
> > a normal state. The only code that would notice is the memory error
> > code, and it is set up to retry in such a case.
> >
> > A subsequent patch will create a routine to do bulk processing of
> > vmemmap allocation. This will eliminate a lock/unlock cycle for each
> > hugetlb page in the case where we are freeing a large number of pages.
> >
> > Fixes: ad2fa3717b74 ("mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page")
> > Cc: <[email protected]>
> > Signed-off-by: Mike Kravetz <[email protected]>
>
> Hi Mike,
>
> I have seen an issue proposed by Jiaqi Yan in [1]. I didn't see any
> resolution for it. Am I missing something with this fix?
>
> [1] https://lore.kernel.org/linux-mm/CACw3F53iPiLrJt4pyaX2aaZ5BVg9tj8x_k6-v7=9Xn1nrh=UCw@mail.gmail.com/
>
My mistake! I sent the old version of the patch.
The new version was modified to simply check the destructor via
folio_test_hugetlb() in order to decide if it should be cleared.
I will send V2. Sorry!
--
Mike Kravetz
On 07/12/23 11:14, Mike Kravetz wrote:
> On 07/12/23 16:03, Muchun Song wrote:
> > > On Jul 12, 2023, at 06:09, Mike Kravetz <[email protected]> wrote:
> > >
> > > Note that this will leave hugetlb pages in a state where they are marked
> > > free (by hugetlb specific page flag) and have a ref count. This is not
> > > a normal state. The only code that would notice is the memory error
> > > code, and it is set up to retry in such a case.
> > >
> > > A subsequent patch will create a routine to do bulk processing of
> > > vmemmap allocation. This will eliminate a lock/unlock cycle for each
> > > hugetlb page in the case where we are freeing a large number of pages.
> > >
> > > Fixes: ad2fa3717b74 ("mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page")
> > > Cc: <[email protected]>
> > > Signed-off-by: Mike Kravetz <[email protected]>
> >
> > Hi Mike,
> >
> > I have seen an issue proposed by Jiaqi Yan in [1]. I didn't see any
> > resolution for it. Am I missing something with this fix?
> >
> > [1] https://lore.kernel.org/linux-mm/CACw3F53iPiLrJt4pyaX2aaZ5BVg9tj8x_k6-v7=9Xn1nrh=UCw@mail.gmail.com/
> >
>
> My mistake! I sent the old version of the patch.
>
> The new version was modified to simply check the destructor via
> folio_test_hugetlb() in order to decide if it should be cleared.
>
> I will send V2. Sorry!
I was about to send v2 when I noticed that this approach opened another
race window. :( Closing the window should be just a matter of
reordering code. I will take a day or two to make sure I did not miss
something else.
--
Mike Kravetz