Re: [PATCH v2] mm: page_mapped: don't assume compound page is huge or THP

[Jan Stancek <jstancek@redhat.com>]

----- Original Message -----
> On Fri, Nov 30, 2018 at 1:07 PM Jan Stancek <jstancek@redhat.com> wrote:
> >
> > LTP proc01 testcase has been observed to rarely trigger crashes
> > on arm64:
> >     page_mapped+0x78/0xb4
> >     stable_page_flags+0x27c/0x338
> >     kpageflags_read+0xfc/0x164
> >     proc_reg_read+0x7c/0xb8
> >     __vfs_read+0x58/0x178
> >     vfs_read+0x90/0x14c
> >     SyS_read+0x60/0xc0
> >
> > Issue is that page_mapped() assumes that if compound page is not
> > huge, then it must be THP. But if this is 'normal' compound page
> > (COMPOUND_PAGE_DTOR), then following loop can keep running
> > (for HPAGE_PMD_NR iterations) until it tries to read from memory
> > that isn't mapped and triggers a panic:
> >         for (i = 0; i < hpage_nr_pages(page); i++) {
> >                 if (atomic_read(&page[i]._mapcount) >= 0)
> >                         return true;
> >         }
> >
> > I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> > with a custom kernel module [1] which:
> > - allocates compound page (PAGEC) of order 1
> > - allocates 2 normal pages (COPY), which are initialized to 0xff
> >   (to satisfy _mapcount >= 0)
> > - 2 PAGEC page structs are copied to address of first COPY page
> > - second page of COPY is marked as not present
> > - call to page_mapped(COPY) now triggers fault on access to 2nd
> >   COPY page at offset 0x30 (_mapcount)
> >
> > [1]
> > https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
> >
> > Fix the loop to iterate for "1 << compound_order" pages.
> >
> > Debugged-by: Laszlo Ersek <lersek@redhat.com>
> > Suggested-by: "Kirill A. Shutemov" <kirill@shutemov.name>
> > Signed-off-by: Jan Stancek <jstancek@redhat.com>
> > ---
> >  mm/util.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > Changes in v2:
> > - change the loop instead so we check also mapcount of subpages
> >
> > diff --git a/mm/util.c b/mm/util.c
> > index 8bf08b5b5760..5c9c7359ee8a 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
> >                 return true;
> >         if (PageHuge(page))
> >                 return false;
> > -       for (i = 0; i < hpage_nr_pages(page); i++) {
> > +       for (i = 0; i < (1 << compound_order(page)); i++) {
> >                 if (atomic_read(&page[i]._mapcount) >= 0)
> >                         return true;
> >         }
> > --
> > 1.8.3.1
> 
> Hi all
> 
> This patch landed in the 4.9-stable tree starting from 4.9.151 and it
> broke our MIPS1004kc system with CONFIG_HIGHMEM=y.

Hi,

are you using THP (CONFIG_TRANSPARENT_HUGEPAGE)?

The changed line should affect only THP and normal compound pages,
so a test with THP disabled might be interesting. 

> 
> The breakage consists of random processes dying with SIGILL or SIGSEGV
> when we stress test the system with high memory pressure and explicit
> memory compaction requested through /proc/sys/vm/compact_memory.
> Reverting this patch fixes the crashes.
> 
> We can put some effort on debugging if there are no obvious
> explanations for this. Keep in mind that this is 32-bit system with
> HIGHMEM.

Nothing obvious that I can see. I've been trying to reproduce on
32-bit x86 Fedora with no luck so far.

Regards,
Jan