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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 3/3] mm/khugepaged: release contended mmap read lock after scanning enough VMAs Message-ID: References: <20261110-khugepaged-latency-fix-v3-0-6eea743cf9a3@kernel.org> <20261110-khugepaged-latency-fix-v3-3-6eea743cf9a3@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261110-khugepaged-latency-fix-v3-3-6eea743cf9a3@kernel.org> X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 0989DC0004 X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-Stat-Signature: ow7ms6867wewp5uz61qiedjzmx6wfur3 X-Rspam-User: X-HE-Tag: 1794337799-271919 X-HE-Meta: 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 UMc+T/ia W/HTVV6UBa8OFpLGCjOKPgsniyGophxHHp1vtOJhwImHGREA9nV6Hw/pxtQhfV1MyoICNj4sMgEqU1rT6w/7qZcZaj86zVh9MYTv/Ra/XtFChwQkBuRafSasjRRhuIXfGWLQLMadfLPNHozIpgme0Oq7pCl/CiMQdZyQu1VjwYSMa2PzIsv0TZe0yQraARETj27bGtLQzWwejproJ1nAuhCDoR64kH4ml8UWvBe+7AIoyRZsccCdhrztcI6XOF6UHI3To Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Nov 10, 2026 at 05:19:13PM +0000, Lorenzo Stoakes (ARM) wrote: > khugepaged walks an mm's VMAs under the mmap read lock in > collapse_scan_mm_slot(), checking for ineligible VMAs first, and skipping > them if encountered. > > It charges cc->progress 1 page against the /sys/.../pages_to_scan budget > when these are encountered, however only checking this budget once an > eligible PMD-spanning VMA is encountered. > > It can therefore hold the mmap read lock for a long time if there are a > large number of ineligible VMAs present in an mm, over which time no mmap > write locks (and thus no VMA write locks) can be acquired. > > Address this by checking whether the mmap lock is contended after 1-10 > ms (depending on CONFIG_HZ), i.e. the granularity of jiffies. This is a > long time for a lock to be held, so is a reasonable compromise for > determining when best to do this. Oops I left this in from the previous approach :) Replace with: Address this by checking whether the mmap lock is contended after 1024 VMAs scanned. This was measured to take around ~100 us on a fast machine, and given VMA eligibility checking is a light operation, it is likely this will remain at the same order of magnitude for slower machines. This therefore establishes a reasonable upper bound on how long a contended mmap lock is held. Thanks! :) -- Cheers, Lorenzo