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[99.254.144.39]) by smtp.gmail.com with ESMTPSA id l11-20020ac84ccb000000b004181441cb2dsm3585433qtv.34.2023.09.28.09.23.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 Sep 2023 09:23:32 -0700 (PDT) Date: Thu, 28 Sep 2023 12:23:28 -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 v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation Message-ID: References: <20230923013148.1390521-1-surenb@google.com> <20230923013148.1390521-2-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20230923013148.1390521-2-surenb@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Suren, Sorry to review so late. On Fri, Sep 22, 2023 at 06:31:44PM -0700, Suren Baghdasaryan wrote: > diff --git a/mm/rmap.c b/mm/rmap.c > index ec7f8e6c9e48..c1ebbd23fa61 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -542,6 +542,7 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, > struct anon_vma *root_anon_vma; > unsigned long anon_mapping; > > +repeat: > rcu_read_lock(); > anon_mapping = (unsigned long)READ_ONCE(folio->mapping); > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > @@ -586,6 +587,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, > rcu_read_unlock(); > anon_vma_lock_read(anon_vma); > > + /* > + * Check if UFFDIO_REMAP changed the anon_vma. This is needed > + * because we don't assume the folio was locked. > + */ > + if (unlikely((unsigned long) READ_ONCE(folio->mapping) != > + anon_mapping)) { > + anon_vma_unlock_read(anon_vma); > + put_anon_vma(anon_vma); > + anon_vma = NULL; > + goto repeat; > + } We have an open-coded fast path above this: if (down_read_trylock(&root_anon_vma->rwsem)) { /* * If the folio is still mapped, then this anon_vma is still * its anon_vma, and holding the mutex ensures that it will * not go away, see anon_vma_free(). */ if (!folio_mapped(folio)) { up_read(&root_anon_vma->rwsem); anon_vma = NULL; } goto out; } Would that also need such check? > + > if (atomic_dec_and_test(&anon_vma->refcount)) { > /* > * Oops, we held the last refcount, release the lock > -- > 2.42.0.515.g380fc7ccd1-goog > -- Peter Xu