From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 06CAD35A384; Tue, 16 Jun 2026 07:55:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781596518; cv=none; b=Xkc2xjQPM3KoHrJAgFDGoRA86DLshJXCxzCZMw4MoFQRcVN0oD4qwc1If/p30rE9wUjBUNsBWnFsMkBlxqjyKk5MeZgx6o7HMVKcCuTvbXYWqAm/oeeN+UfXLGx/fY46n8GvIIddZvJvNbtIEsdWfo1elgU7nWsVzsy9i7VzbA0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781596518; c=relaxed/simple; bh=eJdqyv4hJpz+5xYRRlWO8e2A6D+CHZ+zYvuR3m+8C+0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=dS1SI3pbJZTOcigx3RPfQbYebHHOB4TO8NRLCjf5ape8CwHy0UiqJAEMiEGUNI3iE2esOxe/e7DTxCSvf6w8YnCudxMbLMc71RA/uH2zSioPIsHj6w0qxQaS5VGMHa1Avk3hZOWelVRwve7SDx/k/mwzjQMsq4T+VOyR3Vsi/kw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QkdPriGn; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="QkdPriGn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 228D21F000E9; Tue, 16 Jun 2026 07:55:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781596516; bh=n8yo6IeeQUch0PIwpPtioYGk9SmXvxlaWBe70kkkR6g=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=QkdPriGnJTwVj05m+T5Ki7nROr14VsrYgXqzl3WhfN6z5X6o97l6fEr+yv4mwKqlA egTkmsB603voSA1Lzh3+IWSacBh7KwYcZLAr6LGc9ci+uzG79Lv3CGn5XxXD/2J8iQ PzrwDdzl/KcLcDH7Jub2Ntu8EK9HrtMD9R8tyCHLUkT9aTUpmzBcha6lOZtXUg4//s VAKDL1cTrOTlI1ihwV4RnevI0BoWph25XnR8MgDcT0OpWDLWSJBLLBgZ+kuNw6dY2g hgb+EujlBevwthD6KMkeJKCK42JL92MWjmTmzbzx6H13AVXC1q0gHPVUSNgzzBL6aq i8jZ42xICef+g== Message-ID: <4d7ebcd5-f3b7-4c73-94f1-b7d3ac12a521@kernel.org> Date: Tue, 16 Jun 2026 09:55:09 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH for-next v3 3/9] mm/slab: handle the !allow_spin case in kfree_rcu_sheaf() Content-Language: en-US To: "Harry Yoo (Oracle)" , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Alexei Starovoitov , Andrii Nakryiko , Puranjay Mohan , Amery Hung , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Pedro Falcato , Suren Baghdasaryan Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, rcu@vger.kernel.org, bpf@vger.kernel.org References: <20260615-kfree_rcu_nolock-v3-0-70a54f3775bb@kernel.org> <20260615-kfree_rcu_nolock-v3-3-70a54f3775bb@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: <20260615-kfree_rcu_nolock-v3-3-70a54f3775bb@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/15/26 13:05, Harry Yoo (Oracle) wrote: > Teach kfree_rcu_sheaf() how to handle the !allow_spin case. Try to get > an empty sheaf from pcs->spare or the barn even when spinning is not > allowed. Unlike __pcs_replace_full_main(), try harder to allocate > an empty sheaf because the fallback path will be more expensive than > kfree_nolock(). You mean that it will try to allocate an empty sheaf, while __pcs_replace_full_main() doesn't with allow_spin == false. I just noticed the comment there is a bit stale... nevermind. > > When trylock fails or the kernel observes non-NULL pcs->rcu_free after > lock acquisition, free the sheaf instead of putting it to the barn. > This is rare and not worth complicating the code. > > Since call_rcu() cannot be called in an unknown context, > kfree_rcu_sheaf() fails when the rcu sheaf becomes full. > > Signed-off-by: Harry Yoo (Oracle) > --- > mm/slab.h | 2 +- > mm/slab_common.c | 2 +- > mm/slub.c | 39 ++++++++++++++++++++++++++++++--------- > 3 files changed, 32 insertions(+), 11 deletions(-) > > diff --git a/mm/slab.h b/mm/slab.h > index 509f330654b8..b1bd33a16544 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -429,7 +429,7 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s) > return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT)); > } > > -bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj); > +bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, bool allow_spin); > void flush_all_rcu_sheaves(void); > void flush_rcu_sheaves_on_cache(struct kmem_cache *s); > > diff --git a/mm/slab_common.c b/mm/slab_common.c > index b6426d7ceec9..bc1a8ec938d9 100644 > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -1605,7 +1605,7 @@ static bool kfree_rcu_sheaf(void *obj) > > s = slab->slab_cache; > if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())) > - return __kfree_rcu_sheaf(s, obj); > + return __kfree_rcu_sheaf(s, obj, /* allow_spin = */ true); > > return false; > } > diff --git a/mm/slub.c b/mm/slub.c > index 87ca154ccd80..b0d38d515386 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2815,7 +2815,8 @@ static inline struct slab_sheaf *alloc_empty_sheaf(struct kmem_cache *s, > return __alloc_empty_sheaf(s, gfp, alloc_flags, s->sheaf_capacity); > } > > -static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf) > +static void __free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf, > + bool allow_spin) > { > /* > * If the sheaf was created with SLAB_ALLOC_NO_RECURSE flag then its > @@ -2827,11 +2828,20 @@ static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf) > mark_obj_codetag_empty(sheaf); > > VM_WARN_ON_ONCE(sheaf->size > 0); > - kfree(sheaf); > + > + if (likely(allow_spin)) > + kfree(sheaf); > + else > + kfree_nolock(sheaf); Hmm what if the sheaf wasn't allocated with kmalloc_nolock()? > > stat(s, SHEAF_FREE); > } > > +static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf) > +{ > + __free_empty_sheaf(s, sheaf, /* allow_spin = */ true); > +} > + > static unsigned int > refill_objects(struct kmem_cache *s, void **p, gfp_t gfp, unsigned int min, > unsigned int max); > @@ -3132,7 +3142,6 @@ static struct slab_sheaf *barn_get_empty_sheaf(struct node_barn *barn, > * intended action due to a race or cpu migration. Thus they do not check the > * empty or full sheaf limits for simplicity. > */ > - > static void barn_put_empty_sheaf(struct node_barn *barn, struct slab_sheaf *sheaf) > { > unsigned long flags; > @@ -6065,7 +6074,7 @@ static void rcu_free_sheaf(struct rcu_head *head) > */ > static DEFINE_WAIT_OVERRIDE_MAP(kfree_rcu_sheaf_map, LD_WAIT_CONFIG); > > -bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj) > +bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, bool allow_spin) > { > struct slub_percpu_sheaves *pcs; > struct slab_sheaf *rcu_sheaf; > @@ -6081,9 +6090,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj) > pcs = this_cpu_ptr(s->cpu_sheaves); > > if (unlikely(!pcs->rcu_free)) { > - > struct slab_sheaf *empty; > struct node_barn *barn; > + unsigned int alloc_flags = SLAB_ALLOC_DEFAULT; > + gfp_t gfp = GFP_NOWAIT; > > /* Bootstrap or debug cache, fall back */ > if (unlikely(!cache_has_sheaves(s))) { > @@ -6103,7 +6113,7 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj) > goto fail; > } > > - empty = barn_get_empty_sheaf(barn, true); > + empty = barn_get_empty_sheaf(barn, allow_spin); Here we might be getting a sheaf allocated previously with kmalloc(). > > if (empty) { > pcs->rcu_free = empty; > @@ -6112,20 +6122,25 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj) > > local_unlock(&s->cpu_sheaves->lock); > > - empty = alloc_empty_sheaf(s, GFP_NOWAIT, SLAB_ALLOC_DEFAULT); > + if (unlikely(!allow_spin)) { > + alloc_flags = SLAB_ALLOC_TRYLOCK; > + gfp = 0; Maybe __GFP_NOWARN? Here we would get a sheaf with kmalloc_nolock() so that's ok even if it's later freed by someone else by kfree(), right. > + } > + > + empty = alloc_empty_sheaf(s, gfp, alloc_flags); > > if (!empty) > goto fail; > > if (!local_trylock(&s->cpu_sheaves->lock)) { > - barn_put_empty_sheaf(barn, empty); > + __free_empty_sheaf(s, empty, allow_spin); Well we could still use the barn with allow_spin == true. But more crucially, here we might be freeing with kfree_nolock() a sheaf from the barn previously allocated with kmalloc()? Maybe we need to track if it's the case and defer-free it or something. Also maybe there could be a wrapper kfree_maybe_nolock() (~better name?) That means "I want to kfree safely in kfree_nolock() context something that MIGHT have been kmalloc()" And maybe depending on the debugging options that make kmalloc() -> kfree_nolock() incompatible, if those are not enabled, it wouldn't have to defer, but proceed normally? > goto fail; > } > > pcs = this_cpu_ptr(s->cpu_sheaves); > > if (unlikely(pcs->rcu_free)) > - barn_put_empty_sheaf(barn, empty); > + __free_empty_sheaf(s, empty, allow_spin); > else > pcs->rcu_free = empty; > } > @@ -6143,6 +6158,12 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj) > if (likely(rcu_sheaf->size < s->sheaf_capacity)) { > rcu_sheaf = NULL; > } else { > + if (unlikely(!allow_spin)) { > + /* call_rcu() cannot be called in an unknown context */ > + rcu_sheaf->size--; > + local_unlock(&s->cpu_sheaves->lock); > + goto fail; > + } > pcs->rcu_free = NULL; > rcu_sheaf->node = numa_node_id(); > } >