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.

Can you elaborate your test case a little bit more detail? There is a check in split_huge_page_to_list():

/* Racy check whether the huge page can be split */
bool can_split_folio(struct folio *folio, int *pextra_pins)
{
	int extra_pins;

	/* Additional pins from page cache */
	if (folio_test_anon(folio))
		extra_pins = folio_test_swapcache(folio) ?
				folio_nr_pages(folio) : 0;
	else
		extra_pins = folio_nr_pages(folio);
	if (pextra_pins)
		*pextra_pins = extra_pins;
	return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - 1;
}

So a large folio can only be split if only one extra page refcnt is held. It means large folio won't be split from
under us if we hold an page refcnt. Or am I miss something?

Thanks.

> 
> thanks,
> 
> -jane
> 
> .