linux-kernel - Re: [PATCH] userfaultfd: change src_folio after ensuring it's unpinned in UFFDIO

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Date: Thu, 4 Apr 2024 13:07:25 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Lokesh Gidra <lokeshgidra@...gle.com>, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, kernel-team@...roid.com, aarcange@...hat.com, 
	peterx@...hat.com, david@...hat.com, zhengqi.arch@...edance.com, 
	kaleshsingh@...gle.com, ngeoffray@...gle.com
Subject: Re: [PATCH] userfaultfd: change src_folio after ensuring it's
 unpinned in UFFDIO_MOVE

On Thu, Apr 4, 2024 at 10:21 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Apr 04, 2024 at 10:17:26AM -0700, Lokesh Gidra wrote:
> > -             folio_move_anon_rmap(src_folio, dst_vma);
> > -             WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> > -
> >               src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
> >               /* Folio got pinned from under us. Put it back and fail the move. */
> >               if (folio_maybe_dma_pinned(src_folio)) {
> > @@ -2270,6 +2267,9 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
> >                       goto unlock_ptls;
> >               }
> >
> > +             folio_move_anon_rmap(src_folio, dst_vma);
> > +             WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
> > +
>
> This use of WRITE_ONCE scares me.  We hold the folio locked.  Why do
> we need to use WRITE_ONCE?  Who's looking at folio->index without
> holding the folio lock?

Indeed that seems to be unnecessary here. Both here and in
move_present_pte() we are holding folio lock while moving the page. I
must have just blindly copied that from Andrea's original patch [1].

https://gitlab.com/aarcange/aa/-/commit/2aec7aea56b10438a3881a20a411aa4b1fc19e92