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b=N8K+TmDdK88xGDMzYGmuOcdg0t5dGhwKYyZ7n2PsCOVVHvv7wX9H5CeAn2j3CEHBA HSKrdxMGbZ4oixqqjn3EN85PrlHxhnfvqz97xDGD5nQ76YgW5P2Elwd/rEQxqjCC22 hM1DZkfv+0GSv/tdmbUGIBgWfDs9d6Rsw3/Rx847M/We4L0deN9/eHy1AXCPT++k4i yM1q6GVm4HsUIk1JamCd9QR4yxMk9CqAE8y/lPdRExkcozL3f950c9gNjL+EzYJAxf d2vRLOHdeNlsnCT+cKatkIU1cDiOHTUtCX2+FnndsoTqXPRIOrbHC6GrY1NVmx8scJ sERJGYNvzSeeA== Message-ID: <2e695c8a-4212-42b5-bb51-fbe1aa162e1d@kernel.org> Date: Tue, 22 Sep 2026 13:26:00 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: madvise: drop MADV_PAGEOUT folios at swap writeback completion To: Barry Song Cc: 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, kasong@tencent.com, 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> 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: rspam05 X-Rspamd-Queue-Id: 35CEB10000A X-Stat-Signature: cf9ham7u3z4iefwy68wqbku6ceiamm5d X-Rspam-User: X-HE-Tag: 1790076374-822665 X-HE-Meta: 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 A1rua/AY SC8u0OCob7KXjmYH86zuUeV/dJ3NY6zNWpddOKaNDM+gmfCW+KfrnQdgsImnZGpgGoF/5FKTHh4D+zC/sU1OAzRp9yaVCZadTwBj7Lx+RA0wcaNnznOeygw8px2QQgvWvqyKNtzbN106hJVpBSyxswmxPjU/D0RUPYaj1gNF4IDK0yKf0WZVqkSbJT1vqvJhf9RSA47wTVdcI8trzMhHWZ2XwjzcVbJOIqHPU4uBkcluhMM6SXHosVThKnuctIdkAV4crtwPqHF12MwyWKi5atWZxCgr3/2V8jSyVmCKWtoUNcGbi9AFzSFeSOJwEahnXCstF80TCBjU4m2AfdiD0Ozdi+N7VTG6qjhhlMfVH5TLO0QjhXW2uP25wZzyToxwKQlN0p9w1kJJAGabNjbbCNNYyJNzA6dvrV43smNestx2gzX6/5bjiQ2SoLA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/22/26 12:37, Barry Song wrote: > On Tue, Sep 22, 2026 at 6:20 PM David Hildenbrand (Arm) > wrote: >> >> On 9/21/26 23:56, Barry Song wrote: >>> On Mon, Sep 21, 2026 at 11:37 PM David Hildenbrand (Arm) >>> wrote: >>> >>> For synchronous I/O, such as zswap and zram, the memory is released >>> immediately after sync I/O is done. >>> >>> For asynchronous I/O, such as NVMe, the swapcache is currently >>> expected to be rotated back to the tail of the LRU and wait for >>> another scan. Alexandre once mentioned that when he tried handling >>> async I/O the same way as sync I/O—releasing the memory once the I/O >>> completed—he saw some regression. So, delaying the release until a >>> later scan may allow swapcache hits before the folios are eventually >>> reclaimed. >> >> "may", do we have any evidence that this actually is relevant in practice? >> >> We asked to reclaim memory. We wrote the memory out to disk. We unmapped it from >> the page tables. We made the workload the could, access the page immediately >> again suffer already. >> >> We should just evict them as soon as possible to free up memory. > > I suggested this to Alexandre, and he found that it could regress some > workloads [1]. That is why Alexandre is only making the folios > immediately reclaimable for `MADV_PAGEOUT`. I really hate to add special sauce for MADV_PAGEOUT. Also, I don't quite like that we do something fundamentally different for sync vs. async. I assume, though, that async writeback that is very fast might just find the !folio_test_writeback() scenario just like the sync case. ... or if we reschedule after pageout()? I am wondering: if we can show that letting async swapped out pages stay in memory a bit longer improves performance, aren't we swapping out the wrong pages in the first place, and not swapping them out would help the sync case as well? > > See Alexandre's description: > > "Future work > ----------- > Barry suggested extending this to MADV_PAGEOUT and general reclaim. I > prototyped dropbehind for all reclaimed swap folios and it regressed > sysbench OLTP throughput by ~15% on NVMe swap: dropping the swap cache > immediately turns cheap in-cache refaults into disk reads and collapses > swap readahead clustering. Thanks, does this represent a workload pattern we care about? -- Cheers, David