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

[Lars Persson]

On Tue, Feb 5, 2019 at 8:14 AM Jan Stancek <jstancek@xxxxxxxxxx> wrote:
> 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.
>

Hi

Thanks for looking in to it. After some deep dive in MM code, I think
it is safe to say this patch was innocent.

All traces studied so far points to a missing cache coherency call in
mm/migrate.c:migrate_page that is needed only for those evil MIPSes
that lack I/D cache coherency. I will send a write-up to linux-mips
about this. Basically for a non-mapped page it does only a copy of
page data and metadata but no flush_dcache_page() call will be done.
This races with subsequent use of the page.

BR,
 Lars