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Date:   Mon, 18 Feb 2019 14:50:02 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Lars Persson <lists@...h.nu>
Cc:     Jan Stancek <jstancek@...hat.com>, linux-mm@...ck.org,
        lersek@...hat.com, alex williamson <alex.williamson@...hat.com>,
        aarcange@...hat.com, rientjes@...gle.com, kirill@...temov.name,
        mgorman@...hsingularity.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: page_mapped: don't assume compound page is huge
 or THP

On Mon 18-02-19 14:43:58, Lars Persson wrote:
> On Tue, Feb 5, 2019 at 8:14 AM Jan Stancek <jstancek@...hat.com> 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.

Please make sure to cc linux-mm for the patch
-- 
Michal Hocko
SUSE Labs

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