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    SubjectRe: [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@redhat.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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