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From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 6EF26A0012 X-Rspam-User: X-Stat-Signature: 4e7rcjpz9jyytot6778ea4n3zcxtecux X-Rspamd-Server: rspam09 X-HE-Tag: 1780487615-10048 X-HE-Meta: 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 BGyN2r7I doBA0k88kWpW4hfGFqLLfDBDOtDypuQC1w7srO7fHGmyE0Gsl/MtHya7+pp7XjrJyG4ZtoG8+lQK6qQxWZqiAgBK66fFQP4duj/YNUhC16UqcFp2v511Fue4gixuEZd9KVlJIGtVsmfTu97qRu4n6T4p8T6Ekes7jPTM8oUVC1tLYtgpNDFT8bBYbj2UWFAXHUYts1sOB6oCN1hACGjsJmyhebnAa41HaICID+A4C7+3f2/MLfY+gNFOrXytK/lzUOE8g Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026/6/3 19:41, Johannes Weiner wrote: > 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. Thanks for clarifying! No strong objection from me. Just wanted to call out the nokmem behavior change and hear what folks think :D >> Not sure what the right fix is, as I am not a memcg expert ...