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t=1788537869; bh=kfO05etABMv82BXz4+vLRRcFamVzSGusRFH1mdkPthE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=YKLd9aDjjzXaQ4SfS3x8Hrp3POL1gK2WdY77i3qp+ramm5iyO91UoC/iFAsMbwNnq 3RAnVVfnLjNjOv+8Sj1IwZT0Kp77cCf/Iim7UBGVSQVFt+9tlobHceHzhgMbS8XypL Ks0y4rW62Zfjrwc/uSzqErFGK8OkezAc4VreJyl3DQ7yzYtHKSG5aw3Nt9ENi4aHF8 Gl6JBit9umqQFGaVMR3re/TRFPoJIRHPU3DZIz6CQIcXE85atIZ/7C/U6RDSin45m+ Y9zzZCv6MGim/GrcPxnbPt2+q8Et9Ew9HW8d3MmVh5cu2ObIMVyXcwwK1BuCfCZBoj vOgdB3e/7s+iw== Message-ID: Date: Fri, 4 Sep 2026 18:04:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/2] mm/slub: make the case handling in __slab_free() easier to follow Content-Language: en-US To: Hao Li , harry@kernel.org, akpm@linux-foundation.org Cc: cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260824122004.3652-1-hao.li@linux.dev> <20260824122513.3829-1-hao.li@linux.dev> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 82EE980012 X-Stat-Signature: wauk1m5u9zrfjnqqc6gs8fhx4yg8mics X-Rspam-User: X-HE-Tag: 1788537870-143174 X-HE-Meta: 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 XgziYsoN hP75y4u17QQLKwHM77tuMV3lnqqc0yr9zUl6abxk/KzmBodHieHLHg4xQX99M4/Rxmz4m3YtWIdy0PzxL22aehUApf0oqHYbdReUKL34+er8ZA2hoOb0fo2GcRso8NJRKTXZrauXz3q6E7/tgVUoUwrY1ItDVQH/3Fp7sqqQkw6jqZZKeaiJ9FshxH4szeIykWFqkkFZAEYw8JOLdGQ5Hp5piIrkOAJI4LcMrSFH+8vBtWuaEK6vPLtCk69tAvXpGSKxSAG9DDP12A6nJXumz8FDE8q3I8Lw2DQWKC2ChNfuhlvxSCGVNNk3ym4O4bMc1MQ4atZ8uX6PG1So= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/24/26 14:25, Hao Li wrote: > There are 7 possible transitions in __slab_free(): > > a. partial->partial maybe add "(offlist/onlist doesn't matter)" > b. partial->empty, offlist > c. partial->empty, onlist, exceeding min_partial > d. partial->empty, onlist, not exceeding min_partial > e. full->empty, exceeding min_partial > f. full->empty, not exceeding min_partial > g. full->partial > > (There is no offlist variant of e, f and g as a full slab is on no > list.) > > Clarify which case each branch handles, and replace the goto with a > return at the end of the skipped block so that every branch explicitly > states its coverage. > > Case 'a' is the only path that needs neither list_lock nor list > handling. Give it an early continue: handling it upfront is much clearer > than forcing every other case into a nested block. > > Also, read SL_partial once after the loop right where it is used, rather > than re-reading it on every iteration. > > No functional change. > > Signed-off-by: Hao Li Reviewed-by: Vlastimil Babka (SUSE) Nit: > --- > mm/slub.c | 95 ++++++++++++++++++++++++++++--------------------------- > 1 file changed, 49 insertions(+), 46 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index b0cd0572e2f2..e20375307770 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -5748,76 +5748,79 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab, > new.inuse -= cnt; > > /* > - * Might need to be taken off (due to becoming empty) or added > - * to (due to not being full anymore) the partial list. > - * Unless it's frozen. > + * partial->partial: the slab was on the node partial list and > + * stays there, so we need no list handling and no list_lock. I think more accurate is "if the slab was on the node partial list, it stays there, and if it was off, it stays off, so we need no..." ? > + * > + * Note that continue in a do-while goes on to evaluate the > + * condition below, so we do perform the freelist update. > */ > - if (!new.inuse || was_full) { > - > - n = get_node(s, slab_nid(slab)); > - /* > - * Speculatively acquire the list_lock. > - * If the cmpxchg does not succeed then we may > - * drop the list_lock without any processing. > - * > - * Otherwise the list_lock will synchronize with > - * other processors updating the list of slabs. > - */ > - spin_lock_irqsave(&n->list_lock, flags); > + if (!was_full && new.inuse) > + continue; > > - on_node_partial = slab_test_node_partial(slab); > - } > + /* > + * The slab might need to be taken off (due to becoming empty) > + * or added to (due to not being full anymore) the partial > + * list. > + * > + * Speculatively acquire list_lock before calling cmpxchg(), as > + * performing cmpxchg() prior to lock acquisition races with > + * concurrent paths, such as the shrinker. > + * > + * If the cmpxchg does not succeed then we will drop the > + * list_lock and retry. > + */ > + n = get_node(s, slab_nid(slab)); > + spin_lock_irqsave(&n->list_lock, flags); > > } while (!slab_update_freelist(s, slab, &old, &new, "__slab_free")); > > if (likely(!n)) { > + /* partial->partial: we didn't take the list_lock */ > + return; > + } > + > + on_node_partial = slab_test_node_partial(slab); > + > + if (!was_full && !on_node_partial) { > /* > - * We didn't take the list_lock because the slab was already on > - * the partial list and will remain there. > + * partial->empty, offlist: a bulk refill has taken the slab > + * off the partial list and will put it back, so its list > + * handling is not ours to do. > */ > + spin_unlock_irqrestore(&n->list_lock, flags); > return; > } > > - /* > - * This slab was partially empty but not on the per-node partial list, > - * in which case we shouldn't manipulate its list, just return. > - */ > - if (!was_full && !on_node_partial) { > + /* full/partial->empty, exceed: we have enough partial slabs already */ > + if (unlikely(!new.inuse && n->nr_partial >= s->min_partial)) { > + /* partial->empty, onlist, exceed */ > + if (likely(!was_full)) { > + remove_partial(n, slab); > + stat(s, FREE_REMOVE_PARTIAL); > + } > + /* full->empty, exceed: it is on no list to remove from */ > + > spin_unlock_irqrestore(&n->list_lock, flags); > + stat(s, FREE_SLAB); > + discard_slab(s, slab); > return; > } > > /* > - * If slab became empty, should we add/keep it on the partial list or we > - * have enough? > + * At this point, only three cases remain: > + * full->partial > + * full->empty, not exceed > + * partial->empty, onlist, not exceed > */ > - if (unlikely(!new.inuse && n->nr_partial >= s->min_partial)) > - goto slab_empty; > > - /* > - * Objects left in the slab. If it was not on the partial list before > - * then add it. > - */ > + /* full->partial; full->empty, not exceed */ > if (unlikely(was_full)) { > add_partial(n, slab, ADD_TO_TAIL); > stat(s, FREE_ADD_PARTIAL); > } > - spin_unlock_irqrestore(&n->list_lock, flags); > - return; > - > -slab_empty: > - /* > - * The slab could have a single object and thus go from full to empty in > - * a single free, but more likely it was on the partial list. Remove it. > - */ > - if (likely(!was_full)) { > - remove_partial(n, slab); > - stat(s, FREE_REMOVE_PARTIAL); > - } > + /* partial->empty, onlist, not exceed: it stays where it is */ > > spin_unlock_irqrestore(&n->list_lock, flags); > - stat(s, FREE_SLAB); > - discard_slab(s, slab); > } > > /*