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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHenU-009Pje-4O; Tue, 17 Jan 2023 05:46:12 +0000 Date: Tue, 17 Jan 2023 05:46:12 +0000 From: Matthew Wilcox To: Suren Baghdasaryan Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>, 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-Rspam-User: X-Rspamd-Queue-Id: 5B635180006 X-Rspamd-Server: rspam01 X-Stat-Signature: qm4w3yuui98pikj7dfdtfg6oefqknxxb X-HE-Tag: 1673934394-862122 X-HE-Meta: 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 x7fgFrUC qvo6L 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 08:34:36PM -0800, Suren Baghdasaryan wrote: > On Mon, Jan 16, 2023 at 8:14 PM Matthew Wilcox wrote: > > > > 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. > > I think the point here was that if page faults keep occuring and they > prevent vm_lock->count from reaching 0 then a writer will be blocked > and there is no reader throttling mechanism (no max time that writer > will be waiting). Perhaps I misunderstood your description; I thought that a _waiting_ writer would make the count negative, not a successfully acquiring writer.