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Wed, 03 Jun 2026 04:41:36 -0700 (PDT) Date: Wed, 3 Jun 2026 07:41:32 -0400 From: Johannes Weiner To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, shakeel.butt@linux.dev, mhocko@kernel.org, david@fromorbit.com, roman.gushchin@linux.dev, muchun.song@linux.dev, qi.zheng@linux.dev, yosry.ahmed@linux.dev, ziy@nvidia.com, liam@infradead.org, usama.arif@linux.dev, kas@kernel.org, vbabka@kernel.org, ryncsn@gmail.com, zaslonko@linux.ibm.com, gor@linux.ibm.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, dev.jain@arm.com, npache@redhat.com, ryan.roberts@arm.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 0/9] mm: switch THP shrinker to list_lru Message-ID: References: <20260603044426.54863-1-lance.yang@linux.dev> 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: <20260603044426.54863-1-lance.yang@linux.dev> On Wed, Jun 03, 2026 at 12:44:26PM +0800, Lance Yang wrote: > > On Tue, Jun 02, 2026 at 05:46:02PM -0400, Johannes Weiner wrote: > >On Mon, Jun 01, 2026 at 04:36:52PM +0800, Lance Yang wrote: > >> As the changelog above says, the old queue is per-memcg only, rather > >> than per-memcg-per-node. So reclaim on one node can still walk the whole > >> memcg queue and split underused THPs from other nodes in the same memcg. > >> > >> But I think the new one can lose reclaim in the cgroup.memory=nokmem > >> case ... > >> > >> With nokmem, the deferred shrinker can still run from memcg reclaim, > >> because it is SHRINKER_NONSLAB. But the list_lru is no longer per-memcg: > >> > >> __list_lru_init() clears memcg_aware, > >> > >> if (mem_cgroup_kmem_disabled()) > >> memcg_aware = false; > >> > >> so list_lru_from_memcg_idx() falls back to the shared node list: > >> > >> static inline struct list_lru_one * > >> list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx) > >> { > >> if (list_lru_memcg_aware(lru) && idx >= 0) { > >> [...] > >> } > >> return &lru->node[nid].lru; > >> } > >> > >> That makes the shrinker bit unreliable. __list_lru_add() still sets the > >> bit on the memcg passed in, but only when the list goes from empty to > >> non-empty: > >> > >> bool __list_lru_add(struct list_lru *lru, struct list_lru_one *l, > >> struct list_head *item, int nid, > >> struct mem_cgroup *memcg) > >> { > >> if (list_empty(item)) { > >> [...] > >> if (!l->nr_items++) > >> set_shrinker_bit(memcg, nid, lru_shrinker_id(lru)); > >> [...] > >> return true; > >> } > >> return false; > >> } > >> > >> If memcg A adds the first folio, A gets the bit. If memcg B later adds a > >> folio to the same shared list, B does not get a bit, because the list > >> was already non-empty. > >> > >> So in the A-first/B-later case, reclaim from B may not call the deferred > >> shrinker at all. The shared list is scanned from memcg reclaim only if > >> reclaim runs from the memcg that has the bit, such as A here, or from > >> global reclaim :) > >> > >> Anyway, only after the shared list is emptied does the next memcg to add > >> a folio get to be the one with the bit, IIUC :) > > > >Sorry for the delay, this took me a bit to think about. The shrinker > >code is a mess. > > > >I read it the same way you do. And this is true for all list_lru users > >when nokmem is set: we just set random nonsense shrinker bits. > > > >HOWEVER, the generic shrinker code fixes that up by IGNORING random > >shrinker bits like this when !memcg_kmem_online(). And shrinking > >correctly happens only against the shared root queue when the reclaim > >iterator walks root_mem_cgroup. > > > >HOWEVER, the THP shrinker explicitly sets SHRINKER_NONSLAB, which in > >turn overrides the previous override. So yes there is a weirdness: we > >get the root cgroup invocation against the shared queue, and then one > >more time triggered by that random memcg bit. > > > >The most direct fix is to just drop SHRINKER_NONSLAB. It declares > >independence from kmem, which is no longer true. > > > >Cleaning up the shrinker code is left for another day. > > Thanks for working on this! > > Wondering if this fix trades one problem for another, though ... > > Before this series, the deferred split shrinker had a real per-memcg > queue. Even with cgroup.memory=nokmem, memcg reclaim could still scan > that memcg's own deferred_split_queue: > > memcg reclaim -> deferred split shrinker -> sc->memcg->deferred_split_queue > > With the fix, nokmem + w/o SHRINKER_NONSLAB falls back to a > non-memcg-aware shrinker: > > memcg reclaim -> skip deferred split shrinker > > root/global reclaim -> deferred split shrinker -> shared list_lru > > Is that expected? There woud be no memcg-driven deferred split reclaim > under nokmem, IIUC ... Yes, this is all correct. list_lru is still inherently tied to the kmem component of memcg (memcg_kmem_id()). So without kmem, no isolation. But without kmem, no isolation *for a lot of stuff*. It's a legacy knob when slab accounting was new and expensive. But so many things depend on it now, disabling it just punches a nassive hole into memcg functionality and isolation coverage. It's not a sanctioned production use flag. This change is negligible from a memcg semantics POV. > Not sure what the right fix is, as I am not a memcg expert ...