linux-kernel - Re: [PATCH 05/13] mm/munlock: replace clear_page

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Message-ID: <e7fbbdca-6590-7e45-3efd-279fba7f8376@suse.cz>
Date:   Fri, 11 Feb 2022 12:42:10 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...riel.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Yu Zhao <yuzhao@...gle.com>, Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 05/13] mm/munlock: replace clear_page_mlock() by final
 clearance

On 2/6/22 22:38, Hugh Dickins wrote:
> Placing munlock_vma_page() at the end of page_remove_rmap() shifts most
> of the munlocking to clear_page_mlock(), since PageMlocked is typically
> still set when mapcount has fallen to 0.  That is not what we want: we
> want /proc/vmstat's unevictable_pgs_cleared to remain as a useful check
> on the integrity of of the mlock/munlock protocol - small numbers are
> not surprising, but big numbers mean the protocol is not working.
> 
> That could be easily fixed by placing munlock_vma_page() at the start of
> page_remove_rmap(); but later in the series we shall want to batch the
> munlocking, and that too would tend to leave PageMlocked still set at
> the point when it is checked.
> 
> So delete clear_page_mlock() now: leave it instead to release_pages()
> (and __page_cache_release()) to do this backstop clearing of Mlocked,
> when page refcount has fallen to 0.  If a pinned page occasionally gets
> counted as Mlocked and Unevictable until it is unpinned, that's okay.
> 
> A slightly regrettable side-effect of this change is that, since
> release_pages() and __page_cache_release() may be called at interrupt
> time, those places which update NR_MLOCK with interrupts enabled
> had better use mod_zone_page_state() than __mod_zone_page_state()
> (but holding the lruvec lock always has interrupts disabled).
> 
> This change, forcing Mlocked off when refcount 0 instead of earlier
> when mapcount 0, is not fundamental: it can be reversed if performance
> or something else is found to suffer; but this is the easiest way to
> separate the stats - let's not complicate that without good reason.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>