linux-kernel - Re: [PATCH 02/13] mm/munlock: delete FOLL_MLOCK and FOLL

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Date:   Thu, 10 Feb 2022 12:35:19 +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 02/13] mm/munlock: delete FOLL_MLOCK and FOLL_POPULATE

On 2/6/22 22:32, Hugh Dickins wrote:
> If counting page mlocks, we must not double-count: follow_page_pte() can
> tell if a page has already been Mlocked or not, but cannot tell if a pte
> has already been counted or not: that will have to be done when the pte
> is mapped in (which lru_cache_add_inactive_or_unevictable() already tracks
> for new anon pages, but there's no such tracking yet for others).
> 
> Delete all the FOLL_MLOCK code - faulting in the missing pages will do
> all that is necessary, without special mlock_vma_page() calls from here.
> 
> But then FOLL_POPULATE turns out to serve no purpose - it was there so
> that its absence would tell faultin_page() not to faultin page when
> setting up VM_LOCKONFAULT areas; but if there's no special work needed
> here for mlock, then there's no work at all here for VM_LOCKONFAULT.
> 
> Have I got that right?  I've not looked into the history, but see that
> FOLL_POPULATE goes back before VM_LOCKONFAULT: did it serve a different
> purpose before?  Ah, yes, it was used to skip the old stack guard page.
> 
> And is it intentional that COW is not broken on existing pages when
> setting up a VM_LOCKONFAULT area?  I can see that being argued either
> way, and have no reason to disagree with current behaviour.

Yeah I think it's consistent with the two usecases stated for VM_LOCKONFAULT:
https://lore.kernel.org/all/1441720742-7803-1-git-send-email-emunson@akamai.com/

> Signed-off-by: Hugh Dickins <hughd@...gle.com>

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