linux-kernel - Re: [PATCH 4/5] mm: fix check_move_unevictable

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Message-ID: <CALvZod793qTH_i82nrcFCuyXVW2YoDDqcRoXrgkEd2E2_7rTrA@mail.gmail.com>
Date:   Mon, 31 Aug 2020 07:44:33 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>, Qian Cai <cai@....pw>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Kuo-Hsin Yang <vovoy@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH 4/5] mm: fix check_move_unevictable_pages() on THP

On Sun, Aug 30, 2020 at 2:08 PM Hugh Dickins <hughd@...gle.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@...gle.com>

Thanks for catching this.

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