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b=WmNaAty0/yAlFHU43CCV2MnKPupK91y4CnUMGEPeh8kUxiMJUVFllGCnEn+XOmwGQ jtlvJlSlTo3pXUrwqWFokdpYCApi0lr1RmxxvWBXUTbVZf1W3uMFwAY1NHYkIBYb7h nuRxiRXpupiJkqa41j5HZAfQSgHa4I1bvNweGE1VLB12K3Xb9yp8565qAjjqByqEdr YErL9gNtCzYJriHq48dJYL0TnKGxgmyjrXSWvOdY9cZzlMTcKiLi3OUVthdWHc684f 5zQqvMwgkRmNx6lztZ2a4kpyFR8UaBglnNyLfEZInlMVmnff0Vwz5sDSgVeUEwYY8H qm1dmQ8eGJU6Q== Message-ID: Date: Wed, 23 Sep 2026 11:46:55 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: madvise: drop MADV_PAGEOUT folios at swap writeback completion To: Kairui Song Cc: Barry Song , Alexandre Ghiti , akpm@linux-foundation.org, willy@infradead.org, jack@suse.cz, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, chrisl@kernel.org, shikemeng@huaweicloud.com, nphamcs@gmail.com, baoquan.he@linux.dev, youngjun.park@lge.com, qi.zheng@linux.dev, shakeel.butt@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, hannes@cmpxchg.org, mhocko@kernel.org, yosry@kernel.org, chengming.zhou@linux.dev, kunwu.chan@gmail.com, tz2294@columbia.edu, hch@lst.de, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260921152449.629486-1-alex@ghiti.fr> <70cae945-3a4a-40db-96ac-5ce66a3fa186@kernel.org> <108487b7-0529-4282-b5f4-355804b4c0cf@kernel.org> <2e695c8a-4212-42b5-bb51-fbe1aa162e1d@kernel.org> <4c7c6ea0-84a7-45f6-989c-d1502b90694c@kernel.org> 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-Stat-Signature: eddau67cj7md71qexuhmfu4899fu1e4g X-Rspamd-Queue-Id: 879B118000A X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1790156826-604070 X-HE-Meta: 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 NfZypoUl ue50rX7SbO59n+QnSMVmT5TGv9su+N5ti3uzBCMKHdzdnJutRvwzRH57jBN1OjpySwj5w3rZuTJcRhfaJClyVhK3FRWZgtx9N8dRJnJsi8iPsi2PFoZjyewZNBbIpHUymFHFWngSJvgTkzrtsU8zKSjzWbK6zWj6pf7z/23JsAMPU/BMkNTC2Kioq7z0KfnZhVrhxPWsErvvHS+oD/DnJAGv4x8y/0+aUKexwJWxTsDkLXBzkqhMbreC/gCXrnqgnJMw+h3KZUAbeA1JIZ4qN7Rxb6R8cZKM5msz0hkbZMhl6JbS3n1znwINPRgf5eHj9tmGcwSrT/qVs058Mxy96vTiQJKd8d2wirouz8VOLE3BRCgJHPPdfl4YcLch9Y1/DAwWhluzCX5zpW04IAGjvCVOabdPXVL60eLKOScMrTDeJjsI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/23/26 11:35, Kairui Song wrote: > On Wed, Sep 23, 2026 at 10:46 AM David Hildenbrand (Arm) > wrote: >> >> On 9/22/26 22:55, Barry Song wrote: >>> On Tue, Sep 22, 2026 at 7:26 PM David Hildenbrand (Arm) >>> wrote: >>> >>> I guess that's because the LRU is not always accurate. There are cases >>> where our prediction of future access patterns may be wrong, resulting >>> in refaults? > > Hi All, > >> >> Right. But these would happen in both the SYNC and the ASYNC case. Which seems >> to indicate that making the SYNC case behave like the ASYNC case (keep in >> swapcache before evicting) could actually improve performance? >> >> Or is the SYNC case in general so fast that it is not a problem and the >> swapcache is just not a good use? > > I think the problem is not LRU being inaccurate, but ASYNC case could > overshoot the anon reclaim very easily. File folios can be simply > dropped, and increase nr_reclaimed, but anon writeback (swap) can't be > done durig the reclaim iteration so nr_reclaimed never increase during > the scan iteration. So the scan tries hard to scan and reclaim more > anon folios, more than it really needed. > > I think the real fix is some kind of different watermark, throttling, > or a different counter to limit the scan of async swap. I mentioned > this at LSFMM this year but I guess I presented too much at the same > time so few people remembered it :D. :D > >> >>> >>> Even on Android, which uses zram with very fast synchronous swap-out, >>> I can still hit the swapcache from time to time. So perhaps with a >>> slower device, such as an HDD, keeping the swapcache around for longer >>> could allow applications to hit it more often? >> >> Why is that specific for HDD? It's the exact same app behavior independent of >> the underlying swap technology. > > If you have a shared folio, you must use swap cache, and even for > single used folios swap cache is the way to do synchronization, > regardless of the device. > >>> >>> I don't know. Maybe Alexandre can share more details about this >>> benchmark. I'm also a little surprised by the 15% regression in >>> Sysbench OLTP. My initial feeling was the same as yours: we should >>> release the memory immediately after writeback completes, for both >>> synchronous and asynchronous I/O. >> >> Right. And see if we can identify why we are swapping out the wrong things :) > > I think we are not? It's about over-reclaim. In the worst case we > might put every anon folio under writeback, while no folio is > reclaimed, if the device is super slow. Ah, that hints at the real problem then? How confident are we that that's what's happening? -- Cheers, David