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[109.81.92.149]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-47bd5f8ae74sm124075e9.0.2025.12.15.09.49.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 15 Dec 2025 09:49:57 -0800 (PST) Date: Mon, 15 Dec 2025 18:49:56 +0100 From: Michal Hocko To: Johannes Weiner Cc: Deepanshu Kartikey , akpm@linux-foundation.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, david@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: References: <20251213083639.364539-1-kartikey406@gmail.com> <20251215041200.GB905277@cmpxchg.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251215041200.GB905277@cmpxchg.org> On Sun 14-12-25 23:12:00, Johannes Weiner wrote: > On Sat, Dec 13, 2025 at 02:06:39PM +0530, Deepanshu Kartikey wrote: > > When laptop_mode is enabled, may_writepage is set to 0 in > > try_to_free_mem_cgroup_pages(). This triggers a warning in MGLRU's > > lru_gen_shrink_lruvec(): > > > > VM_WARN_ON_ONCE(!sc->may_writepage || !sc->may_unmap); > > > > The warning occurs because MGLRU expects full reclaim capabilities to > > function correctly. The call path is: > > > > mem_cgroup_resize_max() > > try_to_free_mem_cgroup_pages() > > do_try_to_free_pages() > > shrink_node() > > shrink_lruvec() > > lru_gen_shrink_lruvec() <-- WARNING > > > > Unlike kswapd or direct reclaim where laptop_mode's disk-saving behavior > > is a reasonable optimization, memcg limit enforcement is a hard > > requirement - memory MUST be freed when a cgroup exceeds its limit. > > That reasoning doesn't make sense to me. Reclaim is always in response > to an allocation need. The laptop_mode idea applies to cgroup reclaim > as much as any other reclaim. > > Now obviously all of this is pretty dated. Reclaim doesn't do > filesystem writes anymore, and I'm not sure there are a whole lot of > laptops with rotational drives left, either. Also I doubt anybody is > still using zone_reclaim_mode (which is where the may_unmap is from). > > But let's not introduce more inconsistencies, please. The only thing > weird here is the MGLRU warning. What is it trying to assert? Clearly > whatever assumption was made here has never been true. Completely agreed. This patch seems to just paper over a warning that seems dubious while doing something that doesn't make much sense in itself. Dropping laptop_mode from the memory reclaim seems like the right direction anyway. I seriously doubt that it makes any practical or measurable difference even on slow rotating storage laptops these days. -- Michal Hocko SUSE Labs