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Tue, 03 Oct 2023 10:56:16 -0700 (PDT) MIME-Version: 1.0 References: <20230923013148.1390521-1-surenb@google.com> <20230923013148.1390521-2-surenb@google.com> <27f177c9-1035-3277-cd62-dc81c12acec4@redhat.com> In-Reply-To: From: Suren Baghdasaryan Date: Tue, 3 Oct 2023 10:56:01 -0700 Message-ID: Subject: Re: [PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation To: David Hildenbrand Cc: Peter Xu , 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Stat-Signature: jwwcy8w4didmftkecof3n547stwfdqcf X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 7F5294000F X-HE-Tag: 1696355777-699783 X-HE-Meta: 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 wUqf5X/x AxIg+g5hn5kqezFUcWUsj5P+RAyz+42dlYkB4WoPl2w9coVE5GelbiP8urk3mV1aBx+wpiAAnVB+vAmHXNp6aPZnj2x9hzLVW9iwKWpODS7zSJqGZ8oSEX0ekwTgmidRG+BdGwbJlDsED5JYo+6ktbbY13nNpB73eWA0ILux9ZUlAj43YtWLCYRHY9rk/VlDGbzwV1ahY9vuVkavzPfeCcLl2C0c9DE+opx2YqDFNYC+iKhq7d9diouapLTnEJJ62GV7SeNQsYw4vBeYcyvT7UgE6yvO4dKFojw1pfk3+zP50JUuiAOnVD8ehbpBeQ2URStn6DzdvgT4TGib0yEiHHmSqMgcQGmivTg8LNvCA7EZbQHM= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000066, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Oct 2, 2023 at 10:30=E2=80=AFAM David Hildenbrand wrote: > > On 02.10.23 17:23, Peter Xu wrote: > > 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 t= he > >>> 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. > > That does sound reasonable, thanks. > > Probably we can do better with the patch description (once [1] is used > to move the folio to the other anon_vma). I'll develop the next version with your patches in the baseline. Hopefully by the time of my posting your patches will be in the mm-unstable. > > "mm/rmap: support move to different root anon_vma in folio_move_anon_rmap= () > > For now, folio_move_anon_rmap() was only used to move a folio to a > different anon_vma after fork(), whereby the root anon_vma stayed > unchanged. For that, it was sufficient to hold the page lock when > calling folio_move_anon_rmap(). > > However, we want to make use of folio_move_anon_rmap() to move folios > between VMAs that have a different root anon_vma. As folio_referenced() > performs an RMAP walk without holding the page lock but only holding the > anon_vma in read mode, holding the page lock is insufficient. > > When moving to an anon_vma with a different root anon_vma, we'll have to > hold both, the page lock and the anon_vma lock in write mode. > Consequently, whenever we succeeded in folio_lock_anon_vma_read() to > read-lock the anon_vma, we have to re-check if the mapping was changed > in the meantime. If that was the case, we have to retry. > > Note that folio_move_anon_rmap() must only be called if the anon page is > exclusive to a process, and must not be called on KSM folios. > > This is a preparation for UFFDIO_REMAP, which will hold the page lock, > the anon_vma lock in write mode, and the mmap_lock in read mode. > " Thanks for taking time to write this up! Looks really clear to me. I'll reuse it. > > In addition, we should document these locking details for > folio_move_anon_rmap() and probably not mention UFFDIO_REMAP in the > comment in folio_lock_anon_vma_read(), but instead say > "folio_move_anon_rmap() might have changed the anon_vma as we might not > hold the page lock here." Sounds good. Will add. > > > [1] https://lkml.kernel.org/r/20231002142949.235104-3-david@redhat.com > > -- > Cheers, > > David / dhildenb >