linux-kernel - Re: [PATCH] mm/memory: Pass head page to do_set

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Date: Tue, 11 Jun 2024 11:24:28 -0400
From: Andrew Bresticker <abrestic@...osinc.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm/memory: Pass head page to do_set_pmd()

On Tue, Jun 11, 2024 at 10:46 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 11.06.24 16:18, Andrew Bresticker wrote:
> > The requirement that the head page be passed to do_set_pmd() was added
> > in commit ef37b2ea08ac ("mm/memory: page_add_file_rmap() ->
> > folio_add_file_rmap_[pte|pmd]()") and prevents pmd-mapping in the
> > finish_fault() path if vmf->page is anything but the head page for an
> > otherwise suitable vma and pmd-sized page. Have finish_fault() pass in
> > the head page instead.
> >
> > Fixes: ef37b2ea08ac ("mm/memory: page_add_file_rmap() -> folio_add_file_rmap_[pte|pmd]()")
> > Signed-off-by: Andrew Bresticker <abrestic@...osinc.com>
> > ---
> >   mm/memory.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 0f47a533014e..f13b953b507c 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4764,7 +4764,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> >
> >       if (pmd_none(*vmf->pmd)) {
> >               if (PageTransCompound(page)) {
> > -                     ret = do_set_pmd(vmf, page);
> > +                     ret = do_set_pmd(vmf, compound_head(page));
> >                       if (ret != VM_FAULT_FALLBACK)
> >                               return ret;
> >               }
>
> That certainly makes the "page != &folio->page" check happy.
>
> It is *likely* incorrect if we would ever have folios > PMD size (which
> we don't have on that path yet).
>
> I assume that the thp_vma_suitable_order() check would detect any kind
> of "different placement of the folio in virtual address space", where we
> could mess up.

thp_vma_suitable_order() checks that the folio fits in the vma, if
that's what you mean.

> Question is: should we instead drop the "page != &folio->page" check
> that I added?

Indeed that's probably better as I'm now noticing that
filemap_map_pmd() has the same issue. Will send a v2.

Thanks.
-Andrew

>
> I think I added that check because I saw the "compound_order(page)"
> check and assumed it would return 0 for tail pages, but missed that we
> get the compound head first.
>
> --
> Cheers,
>
> David / dhildenb
>