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Howlett" To: Andrii Nakryiko Cc: Suren Baghdasaryan , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, david@redhat.com, vbabka@suse.cz, peterx@redhat.com, jannh@google.com, hannes@cmpxchg.org, mhocko@kernel.org, paulmck@kernel.org, shuah@kernel.org, adobriyan@gmail.com, brauner@kernel.org, josef@toxicpanda.com, yebin10@huawei.com, linux@weissschuh.net, willy@infradead.org, osalvador@suse.de, andrii@kernel.org, ryan.roberts@arm.com, christophe.leroy@csgroup.eu, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 7/8] mm/maps: read proc/pid/maps under RCU Message-ID: <6ay37xorr35nw4ljtptnfqchuaozu73ffvjpmwopat42n4t6vr@qnr6xvralx2o> Mail-Followup-To: "Liam R. Howlett" , Andrii Nakryiko , Suren Baghdasaryan , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, david@redhat.com, vbabka@suse.cz, peterx@redhat.com, jannh@google.com, hannes@cmpxchg.org, mhocko@kernel.org, paulmck@kernel.org, shuah@kernel.org, adobriyan@gmail.com, brauner@kernel.org, josef@toxicpanda.com, yebin10@huawei.com, linux@weissschuh.net, willy@infradead.org, osalvador@suse.de, andrii@kernel.org, ryan.roberts@arm.com, christophe.leroy@csgroup.eu, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org References: <20250418174959.1431962-1-surenb@google.com> <20250418174959.1431962-8-surenb@google.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: User-Agent: NeoMutt/20240425 X-ClientProxiedBy: YT4PR01CA0205.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:ad::27) To MW5PR10MB5764.namprd10.prod.outlook.com (2603:10b6:303:190::15) MIME-Version: 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2:49=E2=80=AFPM Suren Baghdasaryan wrote: > > > > On Tue, Apr 22, 2025 at 3:49=E2=80=AFPM Andrii Nakryiko > > wrote: > > > > > > On Fri, Apr 18, 2025 at 10:50=E2=80=AFAM Suren Baghdasaryan wrote: > > > > > > > > With maple_tree supporting vma tree traversal under RCU and vma and > > > > its important members being RCU-safe, /proc/pid/maps can be read un= der > > > > RCU and without the need to read-lock mmap_lock. However vma conten= t > > > > can change from under us, therefore we make a copy of the vma and w= e > > > > pin pointer fields used when generating the output (currently only > > > > vm_file and anon_name). Afterwards we check for concurrent address > > > > space modifications, wait for them to end and retry. While we take > > > > the mmap_lock for reading during such contention, we do that moment= arily > > > > only to record new mm_wr_seq counter. This change is designed to re= duce > > > > > > This is probably a stupid question, but why do we need to take a lock > > > just to record this counter? uprobes get away without taking mmap_loc= k > > > even for reads, and still record this seq counter. And then detect > > > whether there were any modifications in between. Why does this change > > > need more heavy-weight mmap_read_lock to do speculative reads? > > > > Not a stupid question. mmap_read_lock() is used to wait for the writer > > to finish what it's doing and then we continue by recording a new > > sequence counter value and call mmap_read_unlock. This is what > > get_vma_snapshot() does. But your question made me realize that we can > > optimize m_start() further by not taking mmap_read_lock at all. > > Instead of taking mmap_read_lock then doing drop_mmap_lock() we can > > try mmap_lock_speculate_try_begin() and only if it fails do the same > > dance we do in the get_vma_snapshot(). I think that should work. >=20 > Ok, yeah, it would be great to avoid taking a lock in a common case! We can check this counter once per 4k block and maintain the same 'tearing' that exists today instead of per-vma. Not that anyone said they had an issue with changing it, but since we're on this road anyways I'd thought I'd point out where we could end up. I am concerned about live locking in either scenario, but I haven't looked too deep into this pattern. I also don't love (as usual) the lack of ensured forward progress. It seems like we have four cases for the vm area state now: 1. we want to read a stable vma or set of vmas (per-vma locking) 2. we want to read a stable mm state for reading (the very short named mmap_lock_speculate_try_begin) 3. we ensure a stable vma/mm state for reading (mmap read lock) 4. we are writing - get out of my way (mmap write lock). Cheers, Liam