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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHqk3-009uWh-Sc; Tue, 17 Jan 2023 18:31:27 +0000 Date: Tue, 17 Jan 2023 18:31:27 +0000 From: Matthew Wilcox To: Suren Baghdasaryan Cc: Jann Horn , 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, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@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, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.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 Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock Message-ID: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-42-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: gdgg8xypwo3rfg1r3xzczfpwk7d4g3in X-Rspamd-Queue-Id: B7FCDC0010 X-HE-Tag: 1673980305-789049 X-HE-Meta: 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 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 Tue, Jan 17, 2023 at 10:26:32AM -0800, Suren Baghdasaryan wrote: > On Tue, Jan 17, 2023 at 10:12 AM Jann Horn wrote: > > > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan wrote: > > > rw_semaphore is a sizable structure of 40 bytes and consumes > > > considerable space for each vm_area_struct. However vma_lock has > > > two important specifics which can be used to replace rw_semaphore > > > with a simpler structure: > > [...] > > > static inline void vma_read_unlock(struct vm_area_struct *vma) > > > { > > > - up_read(&vma->vm_lock->lock); > > > + if (atomic_dec_and_test(&vma->vm_lock->count)) > > > + wake_up(&vma->vm_mm->vma_writer_wait); > > > } > > > > I haven't properly reviewed this, but this bit looks like a > > use-after-free because you're accessing the vma after dropping your > > reference on it. You'd have to first look up the vma->vm_mm, then do > > the atomic_dec_and_test(), and afterwards do the wake_up() without > > touching the vma. Or alternatively wrap the whole thing in an RCU > > read-side critical section if the VMA is freed with RCU delay. > > vm_lock->count does not control the lifetime of the VMA, it's a > counter of how many readers took the lock or it's negative if the lock > is write-locked. Yes, but ... Task A: atomic_dec_and_test(&vma->vm_lock->count) Task B: munmap() write lock free VMA synchronize_rcu() VMA is really freed wake_up(&vma->vm_mm->vma_writer_wait); ... vma is freed. Now, I think this doesn't occur. I'm pretty sure that every caller of vma_read_unlock() is holding the RCU read lock. But maybe we should have that assertion?