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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHdMg-009MFh-CK; Tue, 17 Jan 2023 04:14:26 +0000 Date: Tue, 17 Jan 2023 04:14:26 +0000 From: Matthew Wilcox To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Suren Baghdasaryan , 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, jannh@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-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: EF15118000A X-Stat-Signature: 8hybd4tgcfkkhqx6mj6ed7fcczoonteh X-Rspam-User: X-HE-Tag: 1673928905-702465 X-HE-Meta: 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 81JucHmi 09zbr 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 Mon, Jan 16, 2023 at 11:14:38AM +0000, Hyeonggon Yoo wrote: > > @@ -643,20 +647,28 @@ static inline void vma_write_lock(struct vm_area_struct *vma) > > static inline bool vma_read_trylock(struct vm_area_struct *vma) > > { > > /* Check before locking. A race might cause false locked result. */ > > - if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) > > + if (vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) > > return false; > > > > - if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0)) > > + if (unlikely(!atomic_inc_unless_negative(&vma->vm_lock->count))) > > return false; > > > > + /* If atomic_t overflows, restore and fail to lock. */ > > + if (unlikely(atomic_read(&vma->vm_lock->count) < 0)) { > > + if (atomic_dec_and_test(&vma->vm_lock->count)) > > + wake_up(&vma->vm_mm->vma_writer_wait); > > + return false; > > + } > > + > > /* > > * Overflow might produce false locked result. > > * False unlocked result is impossible because we modify and check > > * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq > > * modification invalidates all existing locks. > > */ > > - if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { > > - up_read(&vma->vm_lock->lock); > > + if (unlikely(vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { > > + if (atomic_dec_and_test(&vma->vm_lock->count)) > > + wake_up(&vma->vm_mm->vma_writer_wait); > > return false; > > } > > With this change readers can cause writers to starve. > What about checking waitqueue_active() before or after increasing > vma->vm_lock->count? I don't understand how readers can starve a writer. Readers do atomic_inc_unless_negative() so a writer can always force readers to fail.