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

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Date: Thu, 4 Apr 2024 13:23:08 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>, 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, 
	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 1:16 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 04.04.24 22:07, Suren Baghdasaryan wrote:
> > On Thu, Apr 4, 2024 at 10:21 AM Matthew Wilcox <willy@...radeadorg> 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].
>
> Agreed, I don't think it is required for ->index. (I also don't spot any
> corresponding READ_ONCE)

Since this patch just got Ack'ed, I'll wait for Andrew to take it into
mm-unstable and then will send a fix removing those WRITE_ONCE(). That
way we won't have merge conflicts,

>
> --
> Cheers,
>
> David / dhildenb
>