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

[Jane Chu <jane.chu@oracle.com>]

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,

-jane