Re: [v2 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration entry

[Yang Shi <shy828301@gmail.com>]

On Wed, Jan 26, 2022 at 3:57 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 26.01.22 12:48, Jann Horn wrote:
> > On Wed, Jan 26, 2022 at 12:38 PM David Hildenbrand <david@redhat.com> wrote:
> >> On 26.01.22 12:29, Jann Horn wrote:
> >>> On Wed, Jan 26, 2022 at 11:51 AM David Hildenbrand <david@redhat.com> wrote:
> >>>> On 20.01.22 21:28, Yang Shi wrote:
> >>>>> The syzbot reported the below BUG:
> >>>>>
> >>>>> kernel BUG at include/linux/page-flags.h:785!
> > [...]
> >>>>> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline]
> >>>>> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744
> > [...]
> >>>> Does this point at the bigger issue that reading the mapcount without
> >>>> having the page locked is completely unstable?
> >>>
> >>> (See also https://lore.kernel.org/all/CAG48ez0M=iwJu=Q8yUQHD-+eZDg6ZF8QCF86Sb=CN1petP=Y0Q@mail.gmail.com/
> >>> for context.)
> >>
> >> Thanks for the pointer.
> >>
> >>>
> >>> I'm not sure what you mean by "unstable". Do you mean "the result is
> >>> not guaranteed to still be valid when the call returns", "the result
> >>> might not have ever been valid", or "the call might crash because the
> >>> page's state as a compound page is unstable"?
> >>
> >> A little bit of everything :)
> > [...]
> >>> In case you mean "the result might not have ever been valid":
> >>> Yes, even with this patch applied, in theory concurrent THP splits
> >>> could cause us to count some page mappings twice. Arguably that's not
> >>> entirely correct.
> >>
> >> Yes, the snapshot is not atomic and, thereby, unreliable. That what I
> >> mostly meant as "unstable".
> >>
> >>>
> >>> In case you mean "the call might crash because the page's state as a
> >>> compound page could concurrently change":
> >>
> >> I think that's just a side-product of the snapshot not being "correct",
> >> right?
> >
> > I guess you could see it that way? The way I look at it is that
> > page_mapcount() is designed to return a number that's at least as high
> > as the number of mappings (rarely higher due to races), and using
> > page_mapcount() on an unlocked page is legitimate if you're fine with
> > the rare double-counting of references. In my view, the problem here
> > is:
> >
> > There are different types of references to "struct page" - some of
> > them allow you to call page_mapcount(), some don't. And in particular,
> > get_page() doesn't give you a reference that can be used with
> > page_mapcount(), but locking a (real, non-migration) PTE pointing to
> > the page does give you such a reference.
>
> I assume the point is that as long as the page cannot be unmapped
> because you block it from getting unmapped (PT lock), the compound page
> cannot get split. As long as the page cannot get unmapped from that page
> table you should have at least a mapcount of 1.

If you mean holding ptl could prevent THP from splitting, then it is
not true since you may be in the middle of THP split just exactly like
the race condition solved by this patch.

Just page lock or elevated page refcount could serialize against THP
split AFAIK.

>
> But yeah, using the mapcount of a page that is not even mapped
> (migration entry) is clearly wrong.
>
> To summarize: reading the mapcount on an unlocked page will easily
> return a wrong result and the result should not be relied upon. reading
> the mapcount of a migration entry is dangerous and certainly wrong.

Depends on your usecase. Some just want to get a snapshot, just like
smaps, they don't care.

>
> --
> Thanks,
>
> David / dhildenb
>