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

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On 3/13/2024 7:34 PM, Miaohe Lin wrote:

On 2024/3/13 9:23, Jane Chu wrote:
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 for points this out. Compound pages can be split even if extra refcnt is held. So folio_test_large
check is not stable if we hold a refcnt now? Will it introduce some obscure races?

Except from that, I think a page cannot become a subpage of a THP when extra refcnt is held now. So below code can be removed.
Any thought?

	/*
	 * We're only intended to deal with the non-Compound page here.
	 * However, the page could have changed compound pages due to
	 * race window. If this happens, we could try again to hopefully
	 * handle the page next round.
	 */
	if (PageCompound(p)) {
		if (retry) {
			ClearPageHWPoison(p);
			unlock_page(p);
			put_page(p);
			flags &= ~MF_COUNT_INCREASED;
			retry = false;
			goto try_again;
		}
		res = action_result(pfn, MF_MSG_DIFFERENT_COMPOUND, MF_IGNORED);
		goto unlock_page;
	}
Not sure of what scenario it was meant to deal with.  How about adding a warning instead of removal? It'll be interesting to see how the warning got triggered. But if after a while nothing happens, then remove it.

thanks!

-jane


Thanks.

thanks,

-jane

.




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