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From:   Shakeel Butt <shakeelb@google.com>
Date:   Mon, 31 Aug 2020 07:44:33 -0700
Message-ID: <CALvZod793qTH_i82nrcFCuyXVW2YoDDqcRoXrgkEd2E2_7rTrA@mail.gmail.com>
Subject: Re: [PATCH 4/5] mm: fix check_move_unevictable_pages() on 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 2:08 PM Hugh Dickins <hughd@google.com> wrote:
>
> check_move_unevictable_pages() is used in making unevictable shmem pages
> evictable: by shmem_unlock_mapping(), drm_gem_check_release_pagevec() and
> i915/gem check_release_pagevec().  Those may pass down subpages of a huge
> page, when /sys/kernel/mm/transparent_hugepage/shmem_enabled is "force".
>
> That does not crash or warn at present, but the accounting of vmstats
> unevictable_pgs_scanned and unevictable_pgs_rescued is inconsistent:
> scanned being incremented on each subpage, rescued only on the head
> (since tails already appear evictable once the head has been updated).
>
> 5.8 commit 5d91f31faf8e ("mm: swap: fix vmstats for huge page") has
> established that vm_events in general (and unevictable_pgs_rescued in
> particular) should count every subpage: so follow that precedent here.
>
> Do this in such a way that if mem_cgroup_page_lruvec() is made stricter
> (to check page->mem_cgroup is always set), no problem: skip the tails
> before calling it, and add thp_nr_pages() to vmstats on the head.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Thanks for catching this.

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