Re: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split

[Yang Shi <shy828301@gmail.com>]

On Tue, May 25, 2021 at 4:58 PM Hugh Dickins <hughd@google.com> wrote:
>
> 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, it is possible. If the THP is being split, some VM_BUG_* might be
triggered if total_mapcount() is called. But it is still feasible to
print total mapcount as long as we implement a more robust version for
dump_page().

>
> 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