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[99.254.144.39]) by smtp.gmail.com with ESMTPSA id p4-20020ac84604000000b004181234dd1dsm106401qtn.96.2023.10.12.15.01.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Oct 2023 15:01:40 -0700 (PDT) Date: Thu, 12 Oct 2023 18:01:37 -0400 From: Peter Xu To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, david@redhat.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 v3 1/3] mm/rmap: support move to different root anon_vma in folio_move_anon_rmap() Message-ID: References: <20231009064230.2952396-1-surenb@google.com> <20231009064230.2952396-2-surenb@google.com> MIME-Version: 1.0 In-Reply-To: <20231009064230.2952396-2-surenb@google.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Queue-Id: DD75E4002F X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: 5zrs8hn6wk8ts7gncsme7ohre56zjgfg X-HE-Tag: 1697148103-116406 X-HE-Meta: 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 32ZGxDIz JaFCzzS+1/K+GN99kyo8kDtVhb1j6Uv8VvILJji7jNkU475VVejwzpMmGO+a3M1owxe2990QiM7izZeLkL5lr3/sJ06VhriBKym68VkNrXbOQQP1aAIopmTuWeY8f0qbgRAEDafpfRqQCCCQDnVNf28nTQeDTHOhBaGSyter1YrnJPymPJ3rEXrUqfBmlAwyIVfAedCpmVhr73wVetKmXYT88Cyf7gWAaR+K+K4OTIoxaEH4WdBoPj7kM3QX9OQJ8ZgO6wJSmNR5l59YgxJhbIbT9i0rvAiZBm0XUYGanSnI5aFLb7TJHtamKMZrNQBejbBAqYrN5ZOMJwn+/XHLn4hgKS5DKUEOzdj66d6NdrRdoSz0NNgMfSSYVXl/vCAjV6YILU9KPXU48CeRsPBPDI9cNriiG3IHwxcNdrOdub6YGDJLpY5Beo6OipQvboUqRq5/VbaCfUejTUfY= 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 Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote: > From: Andrea Arcangeli > > 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 folio 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 folio lock but only holding the > anon_vma in read mode, holding the folio lock is insufficient. > > When moving to an anon_vma with a different root anon_vma, we'll have to > hold both, the folio 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_MOVE, which will hold the folio lock, > the anon_vma lock in write mode, and the mmap_lock in read mode. > > Signed-off-by: Andrea Arcangeli > Signed-off-by: Suren Baghdasaryan > --- > mm/rmap.c | 24 ++++++++++++++++++++++++ > 1 file changed, 24 insertions(+) > > diff --git a/mm/rmap.c b/mm/rmap.c > index c1f11c9dbe61..f9ddc50269d2 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, > struct anon_vma *root_anon_vma; > unsigned long anon_mapping; > > +retry: > rcu_read_lock(); > +retry_under_rcu: > anon_mapping = (unsigned long)READ_ONCE(folio->mapping); > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > goto out; > @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, > anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); > root_anon_vma = READ_ONCE(anon_vma->root); > if (down_read_trylock(&root_anon_vma->rwsem)) { > + /* > + * folio_move_anon_rmap() might have changed the anon_vma as we > + * might not hold the folio lock here. > + */ > + if (unlikely((unsigned long)READ_ONCE(folio->mapping) != > + anon_mapping)) { > + up_read(&root_anon_vma->rwsem); > + goto retry_under_rcu; Is adding this specific label worthwhile? How about rcu unlock and goto retry (then it'll also be clear that we won't hold rcu read lock for unpredictable time)? > + } > + > /* > * If the folio is still mapped, then this anon_vma is still > * its anon_vma, and holding the mutex ensures that it will -- Peter Xu