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From:   Shakeel Butt <shakeelb@google.com>
Date:   Mon, 31 Aug 2020 07:41:05 -0700
Message-ID: <CALvZod5zQZR8_vSMCRC7bXv2Ou2A9YF1V9ZTRCmwuVW2TaWODQ@mail.gmail.com>
Subject: Re: [PATCH 3/5] shmem: shmem_writepage() split unlikely i915 THP
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alex Shi <alex.shi@linux.alibaba.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Matthew Wilcox <willy@infradead.org>, Qian Cai <cai@lca.pw>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Kuo-Hsin Yang <vovoy@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>
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On Sun, Aug 30, 2020 at 2:04 PM Hugh Dickins <hughd@google.com> wrote:
>
> drivers/gpu/drm/i915/gem/i915_gem_shmem.c contains a shmem_writeback()
> which calls shmem_writepage() from a shrinker: that usually works well
> enough; but if /sys/kernel/mm/transparent_hugepage/shmem_enabled has
> been set to "force" (documented as "Force the huge option on for all -
> very useful for testing"), shmem_writepage() is surprised to be called
> with a huge page, and crashes on the VM_BUG_ON_PAGE(PageCompound) (I
> did not find out where the crash happens when CONFIG_DEBUG_VM is off).
>
> LRU page reclaim always splits the shmem huge page first: I'd prefer not
> to demand that of i915, so check and split compound in shmem_writepage().
>
> Fixes: 2d6692e642e7 ("drm/i915: Start writeback from the shrinker")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: stable@vger.kernel.org # v5.3+

Reviewed-by: Shakeel Butt <shakeelb@google.com>