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[99.254.144.39]) by smtp.gmail.com with ESMTPSA id c21-20020ac853d5000000b004181d77e08fsm5406196qtq.85.2023.10.02.08.23.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Oct 2023 08:23:08 -0700 (PDT) Date: Mon, 2 Oct 2023 11:23:05 -0400 From: Peter Xu To: David Hildenbrand Cc: Suren Baghdasaryan , akpm@linux-foundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, hughd@google.com, mhocko@suse.com, axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com, jannh@google.com, zhangpeng362@huawei.com, bgeffon@google.com, kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation Message-ID: References: <20230923013148.1390521-1-surenb@google.com> <20230923013148.1390521-2-surenb@google.com> <27f177c9-1035-3277-cd62-dc81c12acec4@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <27f177c9-1035-3277-cd62-dc81c12acec4@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Oct 02, 2023 at 04:42:50PM +0200, David Hildenbrand wrote: > On 23.09.23 03:31, Suren Baghdasaryan wrote: > > From: Andrea Arcangeli > > > > As far as the rmap code is concerned, UFFDIO_REMAP only alters the > > page->mapping and page->index. It does it while holding the page > > lock. However folio_referenced() is doing rmap walks without taking the > > folio lock first, so folio_lock_anon_vma_read() must be updated to > > re-check that the folio->mapping didn't change after we obtained the > > anon_vma read lock. > > I'm curious: why don't we need this for existing users of > page_move_anon_rmap()? What's special about UFFDIO_REMAP? Totally no expert on anon vma so I'm prone to errors, but IIUC the difference here is root anon vma cannot change in page_move_anon_rmap(), while UFFDIO_REMAP can. Thanks, -- Peter Xu