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Date:   Fri, 11 Feb 2022 11:29:50 +0100
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Subject: Re: [PATCH 04/13] mm/munlock: rmap call mlock_vma_page()
 munlock_vma_page()
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To:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@suse.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Alistair Popple <apopple@nvidia.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Rik van Riel <riel@surriel.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Yu Zhao <yuzhao@google.com>, Greg Thelen <gthelen@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <8e4356d-9622-a7f0-b2c-f116b5f2efea@google.com>
 <9f9ca113-ffb9-498e-4bd6-6bfeaaa10b7@google.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <9f9ca113-ffb9-498e-4bd6-6bfeaaa10b7@google.com>
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Precedence: bulk

On 2/6/22 22:36, Hugh Dickins wrote:
> Add vma argument to mlock_vma_page() and munlock_vma_page(), make them
> inline functions which check (vma->vm_flags & VM_LOCKED) before calling
> mlock_page() and munlock_page() in mm/mlock.c.
> 
> Add bool compound to mlock_vma_page() and munlock_vma_page(): this is
> because we have understandable difficulty in accounting pte maps of THPs,
> and if passed a PageHead page, mlock_page() and munlock_page() cannot
> tell whether it's a pmd map to be counted or a pte map to be ignored.
> 
> Add vma arg to page_add_file_rmap() and page_remove_rmap(), like the
> others, and use that to call mlock_vma_page() at the end of the page
> adds, and munlock_vma_page() at the end of page_remove_rmap() (end or
> beginning? unimportant, but end was easier for assertions in testing).
> 
> No page lock is required (although almost all adds happen to hold it):
> delete the "Serialize with page migration" BUG_ON(!PageLocked(page))s.
> Certainly page lock did serialize with page migration, but I'm having
> difficulty explaining why that was ever important.
> 
> Mlock accounting on THPs has been hard to define, differed between anon
> and file, involved PageDoubleMap in some places and not others, required
> clear_page_mlock() at some points.  Keep it simple now: just count the
> pmds and ignore the ptes, there is no reason for ptes to undo pmd mlocks.
> 
> page_add_new_anon_rmap() callers unchanged: they have long been calling
> lru_cache_add_inactive_or_unevictable(), which does its own VM_LOCKED
> handling (it also checks for not VM_SPECIAL: I think that's overcautious,
> and inconsistent with other checks, that mmap_region() already prevents
> VM_LOCKED on VM_SPECIAL; but haven't quite convinced myself to change it).
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Yeah, much better when most of the sites are encapsulated in rmap operations
like this.

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