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    SubjectRe: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split
    On Tue, 25 May 2021, Yang Shi wrote:
    > On Tue, May 25, 2021 at 3:06 PM Hugh Dickins <hughd@google.com> wrote:
    > > On Tue, 25 May 2021, Yang Shi wrote:
    > >
    > > > When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
    > > > return false positive for PTE-mapped THP since page_mapcount() is used
    > > > to check if the THP is unmapped, but it just checks compound mapount and
    > > > head page's mapcount. If the THP is PTE-mapped and head page is not
    > > > mapped, it may return false positive.
    > >
    > > But those false positives did not matter because there was a separate
    > > DEBUG_VM check later.
    > >
    > > It's good to have the link to Wang Yugui's report, but that paragraph
    > > is not really about this patch, as it has evolved now: this patch
    > > consolidates the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE.
    > >
    > > >
    > > > The try_to_unmap() has been changed to void function, so check
    > > > page_mapped() after it. And changed BUG_ON to WARN_ON since it is not a
    > > > fatal issue.
    > >
    > > The change from DEBUG_VM BUG to VM_WARN_ON_ONCE is the most important
    > > part of this, and the reason it's good for stable: and the patch title
    > > ought to highlight that, not the page_mapcount business.
    >
    > Will update the subject and the commit log accordingly.

    Thanks!

    >
    > >
    > > >
    > > > [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/
    > > >
    > > > Reviewed-by: Zi Yan <ziy@nvidia.com>
    > > > Signed-off-by: Yang Shi <shy828301@gmail.com>
    > >
    > > This will be required Cc: stable@vger.kernel.org
    > > (but we don't want to Cc them on this mail).
    > >
    > > As I said on the other, I think this should be 1/2 not 2/2.
    >
    > Sure.

    Great.

    >
    > >
    > > > ---
    > > > v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag
    > > > since there is no fundamental change against v2.
    > > > v2: Removed dead code and updated the comment of try_to_unmap() per Zi
    > > > Yan.
    > > > mm/huge_memory.c | 17 +++++------------
    > > > 1 file changed, 5 insertions(+), 12 deletions(-)
    > > >
    > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    > > > index 80fe642d742d..72d81d8e01b1 100644
    > > > --- a/mm/huge_memory.c
    > > > +++ b/mm/huge_memory.c
    > > > @@ -2343,6 +2343,8 @@ static void unmap_page(struct page *page)
    > > > ttu_flags |= TTU_SPLIT_FREEZE;
    > > >
    > > > try_to_unmap(page, ttu_flags);
    > > > +
    > > > + VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
    > >
    > > There is one useful piece of information that dump_page() will not show:
    > > total_mapcount(page). Is there a way of crafting that into the output?
    > >
    > > Not with the macros available, I think. Maybe we should be optimistic
    > > and assume I already have the fixes, so not worth trying to refine the
    > > message (but I'm not entirely convinced of that!).
    > >
    > > The trouble with
    > > if (VM_WARN_ON_ONCE_PAGE(page_mapped(page), page))
    > > pr_warn("total_mapcount:%d\n", total_mapcount(page));
    > > is that it's printed regardless of the ONCEness. Another "trouble"
    > > is that it's printed so long after the page_mapped(page) check that
    > > it may be 0 by now - but one can see that as itself informative.
    >
    > We should be able to make dump_page() print total mapcount, right? The
    > dump_page() should be just called in some error paths so taking some
    > extra overhead to dump more information seems harmless, or am I
    > missing something? Of course, this can be done in a separate patch.

    I didn't want to ask that of you, but yes, if you're willing to add
    total_mapcount() into dump_page(), I think that would be ideal; and
    could be helpful for other cases too.

    Looking through total_mapcount(), I think it's safe to call from
    dump_page() - I always worry about extending crash info with
    something that depends on a maybe-corrupted pointer which would
    generate a further crash and either recurse or truncate the output -
    but please check that carefully.

    Yes, a separate patch please: which can come later on, and no
    need for stable for that one, but good to know it's coming.

    Thanks,
    Hugh

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