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

[Hugh Dickins]

On Tue, 25 May 2021, Yang Shi wrote:
> On Tue, May 25, 2021 at 3:06 PM Hugh Dickins <hughd@xxxxxxxxxx> 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@xxxxxxxxxxxx/
> > >
> > > Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> > > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> >
> > This will be required Cc: stable@xxxxxxxxxxxxxxx
> > (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