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Mon, 15 Dec 2025 12:22:16 -0800 (PST) Date: Mon, 15 Dec 2025 15:22:12 -0500 From: Johannes Weiner To: Yuanchu Xie Cc: Deepanshu Kartikey , akpm@linux-foundation.org, axelrasmussen@google.com, weixugc@google.com, david@kernel.org, mhocko@kernel.org, zhengqi.arch@bytedance.com, shakeel.butt@linux.dev, lorenzo.stoakes@oracle.com, yuzhao@google.com, heftig@archlinux.org, oleksandr@natalenko.name, bgeffon@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+90fcab4d88cffed6d0d8@syzkaller.appspotmail.com Subject: Re: [PATCH] mm: vmscan: always allow writeback during memcg reclaim Message-ID: <20251215202212.GD905277@cmpxchg.org> References: <20251213083639.364539-1-kartikey406@gmail.com> <20251215041200.GB905277@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Stat-Signature: 7qxaurw7huzutzn39ijjnihqifc7kz6t X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 239CA1C0023 X-Rspam-User: X-HE-Tag: 1765830137-364913 X-HE-Meta: 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 nTN/ev87 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Dec 15, 2025 at 01:42:27PM -0600, Yuanchu Xie wrote: > On Sun, Dec 14, 2025 at 10:52 PM Deepanshu Kartikey > wrote: > > > > On Mon, Dec 15, 2025 at 9:42 AM Johannes Weiner wrote: > > > > > > Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks") > > > > > > That seems unrelated? > > > > Sorry for the wrong fixes. Correct Fixes: ee814fe23daf ("mm: vmscan: > > clean up struct scan_control") > > > > I'll wait for input from someone with MGLRU context on the broader discussion. > > > This warning came from commit e9d4e1ee7880 ("mm: multi-gen LRU: > clarify scan_control flags") [1]. > > The original rationale: > > 4. sc->may_writepage and sc->may_unmap, which indicates opportunistic > > reclaim, are rejected, since unmapped clean folios are already > > prioritized. Scanning for more of them is likely futile and can > > cause high reclaim latency when there is a large number of memcgs. > > As far as I can tell this was a sanity check to ensure > `lru_gen_shrink_lruvec` avoids extra work for minimal gain. Perhaps > this shouldn't be a warning? Always setting may_writepage in this case > would free more folios. I'm not against removing the warning either. The premise doesn't seem correct. Aside from laptop_mode, they're used in those scenarios: - zone_reclaim_mode: local node is full and user would prefer clean and/or unmapped pages over spilling to remote nodes - watermark_boost: the page allocator finds itself in a situation where it needs to fragment pageblocks, and it calls for additional reclaim to get out of that situation Neither of them are opportunistic. It's user-requested behavior.