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

[Matthew Wilcox <willy@infradead.org>]

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.