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

[Miaohe Lin <linmiaohe@huawei.com>]

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@linux.intel.com>
>>> 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;
	}

Thanks.

> 
> thanks,
> 
> -jane
> 
> .