linux-kernel - Re: [PATCH 10/13] mm/munlock: mlock_page() munlock

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Date:   Fri, 11 Feb 2022 19:26:40 +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 10/13] mm/munlock: mlock_page() munlock_page() batch by
 pagevec

On 2/6/22 22:47, Hugh Dickins wrote:
> A weakness of the page->mlock_count approach is the need for lruvec lock
> while holding page table lock.  That is not an overhead we would allow on
> normal pages, but I think acceptable just for pages in an mlocked area.
> But let's try to amortize the extra cost by gathering on per-cpu pagevec
> before acquiring the lruvec lock.
> 
> I have an unverified conjecture that the mlock pagevec might work out
> well for delaying the mlock processing of new file pages until they have
> got off lru_cache_add()'s pagevec and on to LRU.
> 
> The initialization of page->mlock_count is subject to races and awkward:
> 0 or !!PageMlocked or 1?  Was it wrong even in the implementation before
> this commit, which just widens the window?  I haven't gone back to think
> it through.  Maybe someone can point out a better way to initialize it.

Not me, it seems.

> Bringing lru_cache_add_inactive_or_unevictable()'s mlock initialization
> into mm/mlock.c has helped: mlock_new_page(), using the mlock pagevec,
> rather than lru_cache_add()'s pagevec.
> 
> Experimented with various orderings: the right thing seems to be for
> mlock_page() and mlock_new_page() to TestSetPageMlocked before adding to
> pagevec, but munlock_page() to leave TestClearPageMlocked to the later
> pagevec processing.
> 
> Dropped the VM_BUG_ON_PAGE(PageTail)s this time around: they have made
> their point, and the thp_nr_page()s already contain a VM_BUG_ON_PGFLAGS()
> for that.
> 
> This still leaves acquiring lruvec locks under page table lock each time
> the pagevec fills (or a THP is added): which I suppose is rather silly,
> since they sit on pagevec waiting to be processed long after page table
> lock has been dropped; but I'm disinclined to uglify the calling sequence
> until some load shows an actual problem with it (nothing wrong with
> taking lruvec lock under page table lock, just "nicer" to do it less).
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

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