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b=Vr2iM/6nX7JbyPyoq5XKeype3OGHyJtV+KQ187Fo6SL7mgco1ZwXRRb2z1aOSr31O 8mKDyrPwKyk5Na3J9wAwaNUV8vVnTkrrp8wpUFOPQKRCRAFWWmQdIHlQomz2Yzpaw6 HLx7nt84vK+4Y4nLlnLoqumTgUcecMcuT5hyVtU0ay3CfSLhp0zcz7IduEdmZmrDfH Z3nKZl7cp9xCmP0ibzGMWyd6rY3AarEKD6ublZ//ebpe9YMgQexTi99XjiEZHprNtD u/bp+kYTSOOzAvJnP7uQbIWW3E9GnBpm8lAtporwQlwpcBSJchoDMtTlR06cy4Uq1R rEtpTVpXx306Q== Message-ID: <0ce6071b-0c99-4119-9ab4-a2d9c00b99a8@kernel.org> Date: Wed, 13 May 2026 09:39:52 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/3] use vma locks for proc/pid/{smaps|numa_maps} reads To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, willy@infradead.org, jannh@google.com, paulmck@kernel.org, pfalcato@suse.de, shuah@kernel.org, hsukrut3@gmail.com, richard.weiyang@gmail.com, reddybalavignesh9979@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260426062718.1238437-1-surenb@google.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: E26C012000B X-Rspam-User: X-Stat-Signature: qgcke7u39d6a5ac8arcehzsxfhnoqc15 X-HE-Tag: 1778657998-41206 X-HE-Meta: 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 ilwRowgX eglUPHLoDhemIbnz4urKqzZqcGX2ao+Z7Gg1PiNUPwfTAwmIrs4SgP47MYDkCulHH0S5V/VPyd02ymmS/vVdHn0Nh/X2rNR6ojq7KvPDWjN/ATiFRLHQUC/wjy/iIg2lKVSkfI0C2WnlbGBnnJIDDH6cDg6BzMXdFXBz81aDDTEKQiBqdoiszANjD7SwNrPi1Ug14Pc8gP9IQ+mGdJ9JnWEZT8wNl/CYbSmovK9MrCs4LdJiQ+uJL1yyRhAE8vlcXAK0wNaoLe/MQhQs0ChGS2N51z/lf4wpwFrOsn7mS+4Cs2lIwa0WWLaSVK20o9T4vC4szHIBVbF0A9eqi7He9ggJpMoO9eNrlNSP9pSrLpYQeeVf6cPvsE0vDoiKOZNIUHnZDn1SRbmtY4ukagRbvPXTMD4/tHG/U+3/oeIr3m4izmY749bl3fOH0JPl8zOokPtE/ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/13/26 05:58, Suren Baghdasaryan wrote: > On Tue, May 12, 2026 at 2:28 AM David Hildenbrand (Arm) > wrote: >> >> On 4/26/26 08:27, Suren Baghdasaryan wrote: >>> Use per-vma locks when reading /proc/pid/smaps and /proc/pid/numa_maps >>> similar to /proc/pid/maps to reduce contention on central mmap_lock. One >>> major difference between maps and smaps/numa_maps reading is that the >>> latter executes page table walk which can't be done under RCU due to a >>> possibility of sleeping. Therefore we drop RCU read lock before this walk >>> while keeping the VMA locked. After the walk we retake RCU read lock, >>> reset VMA iterator and proceed with the next VMA. >> >> With many small VMAs, is that overhead noticable? > > It might be but the point of this patchset (and the previous one that > made a similar change for /proc/pid/maps) is to reduce mmap_lock > contention, not to speed up the read operation, which is not a > performance critical part. Well, this interface has been around .. forever, so if there is a noticeable change in performance it should be called out. > The original problem that Paul McKenney > described and which kicked these series of changes is that a > low-priority monitoring process reading /proc/pid/{maps|smaps|...} can > block a high-priority updates by holding the mmap_lock. You can see > details about this problem and the numbers Paul obtained with the > previous change in here: > https://lore.kernel.org/all/20250719182854.3166724-1-surenb@google.com/ Yes, I know. A problem that has been around ... forever as well :) -- Cheers, David