Re: [PATCH 6/8] mm/memory-failure: Convert memory_failure() to use a folio

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On 3/12/2024 7:14 AM, Matthew Wilcox wrote:

On Tue, Mar 12, 2024 at 03:07:39PM +0800, Miaohe Lin wrote:
On 2024/3/11 20:31, Matthew Wilcox wrote:
Assuming we have a refcount on this page so it can't be simultaneously
split/freed/whatever, these three sequences are equivalent:
If page is stable after page refcnt is held, I agree below three sequences are equivalent.

1	if (PageCompound(p))

2	struct page *head = compound_head(p);
2	if (PageHead(head))

3	struct folio *folio = page_folio(p);
3	if (folio_test_large(folio))

.

But please see below commit:

"""
commit f37d4298aa7f8b74395aa13c728677e2ed86fdaf
Author: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Date:   Wed Aug 6 16:06:49 2014 -0700

     hwpoison: fix race with changing page during offlining

     When a hwpoison page is locked it could change state due to parallel
     modifications.  The original compound page can be torn down and then
     this 4k page becomes part of a differently-size compound page is is a
     standalone regular page.

     Check after the lock if the page is still the same compound page.
I can't speak to what the rules were ten years ago, but this is not
true now.  Compound pages cannot be split if you hold a refcount.
Since we don't track a per-page refcount, we wouldn't know which of
the split pages to give the excess refcount to.

I noticed this recently

 * GUP pin and PG_locked transferred to @page. Rest subpages can be freed if
 * they are not mapped.
 *
 * Returns 0 if the hugepage is split successfully.
 * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
 * us.
 */
int split_huge_page_to_list(struct page *page, struct list_head *list)
{

I have a test case with poisoned shmem THP page that was mlocked and

GUP pinned (FOLL_LONGTERM|FOLL_WRITE), but the split succeeded.

thanks,

-jane





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