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Thu, 23 Feb 2023 09:46:17 -0800 (PST) MIME-Version: 1.0 References: <20230216051750.3125598-1-surenb@google.com> <20230216051750.3125598-17-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Thu, 23 Feb 2023 09:46:06 -0800 Message-ID: Subject: Re: [PATCH v3 16/35] mm/mmap: write-lock VMAs before merging, splitting or expanding them To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com, will@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, chriscli@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, rppt@kernel.org, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, leewalsh@google.com, posk@google.com, michalechner92@googlemail.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Queue-Id: DD83A40022 X-Rspamd-Server: rspam01 X-Stat-Signature: 5kktqob3ty1jezinsy88yak7kfduyoxu X-HE-Tag: 1677174378-362971 X-HE-Meta: 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 beLWiSwD d8OHIe0YAV/0sj48NfVT7CHTjPGJg3fC2sIygiX8z7ksaXNH0ETpsGvq4nxTyky/sWjXwoELPaxHe21dt3DDuZstVHY1qEzjAwpZO+HoPYess4AzVT92P6iyHYdVt8irKcYIksAVvPEpFcLrLW75j6XfFXi3mS35tzU/pZJYnnJrZcfqDWxZ8Niy1e45g76E9crWK7vhXoZqD2yvE+NC+kdG0gkRRJwH52MvjslzusLZpOfn/JemPRtyE8ME0BTT+K+5J4n5jZUjHZCGKJG+WqYAW4MKiZgo1oMGqke5DirgS8QLXzcSfFOZxf5teqRElUCuWVgPwxpRjgNe1VsC/UbP6F4Oda0yKmi+R X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Feb 23, 2023 at 6:51 AM Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote: > > On Wed, Feb 15, 2023 at 09:17:31PM -0800, Suren Baghdasaryan wrote: > > Decisions about whether VMAs can be merged, split or expanded must be > > made while VMAs are protected from the changes which can affect that > > decision. For example, merge_vma uses vma->anon_vma in its decision > > Did you mean vma_merge()? Correct. > > > whether the VMA can be merged. Meanwhile, page fault handler changes > > vma->anon_vma during COW operation. > > Write-lock all VMAs which might be affected by a merge or split operation > > before making decision how such operations should be performed. > > > > It doesn't make sense (to me) to update vma->anon_vma during COW fault. > > AFAIK children's vma->anon_vma is allocated during fork() and > page->anon_vma is updated on COW to reduce rmap walking because it's now > unshared, no? > > As patch 26 just falls back to mmap_lock if no anon_vma is set, > I think we can assume nothing updates vma->anon_vma (and its interval > tree) if we are holding mmap_lock in write mode. > > Or am I missing something? No, I think you are right. Patch 26 was added in the later versions of this patchset and at the time this patch was written vma->anon_vma could change during page fault handling under only per-VMA lock protection. So, this patch can be simplified. We still want to prevent concurrent page faults while VMA is being merged or split (simply because par-VMA lock that page fault handler holds might become the wrong one if the VMA is split or merged from under it) but the timing of taking per-VMA lock does not have to be *before* can_vma_merge_{before|after}. Does that make sense? > > -- > Regards, > Hyeonggon > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com. >