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 8DC6926A08F; Fri, 26 Jun 2026 14:30:53 +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=1782484254; cv=none; b=mQMvkHSznikil5iHaFxp1kDwiItJwPP+cf8uL0cKSECZSYU8UEE3wb1QCbUIYSMRv+McYx4gYpIwpmBBoiea/dQFoN7ed8OcL23EKDGnQ+9LHja4HzNJ4ebiwmvq0yvro4RHF7y2VAQYnl8nxphcabcAEcFUVeTjIOi3v0owaQI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782484254; c=relaxed/simple; bh=3XLmTiO6eadTaOv9ADltHdISY35t/Nox0ZgjOhRXpIM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=iBXotUJ4ZL+KUtAO/6gHv1PVSxutP5vZnIKaIp+awoTJkgkZDnP7Zx99Knh+BW4S/ZPo901Y/xe290UHfr+Cv1d0+4+qxy5/s1vfCc6YJTSlKhltuJ4k1h4dqHQfDLZ7BcaXLXdhtPxZd3SjwzgNicpkTUcBo0owES8uGJj8hRQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GDwsXadG; 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="GDwsXadG" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C30871F000E9; Fri, 26 Jun 2026 14:30:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782484253; bh=vu8WzrwqApX7wsPkaWrxMrSifCAb56jkTiTB7+zyurI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=GDwsXadGCDqv1qq8R5wyE2mYy9bMCwO74e6g644EFGVGk8OCUpOgICLOZigXvg31l msYkZOAKKBaMsNkAoPdHAOL0y/JJ2DG0ZWG97k6CBMgNKKxC2LH4sVq1EtSQqSPv6e WomcAvBIFxWyzIL5IaliZZ2ahLx8xemr1KBnsrMOzmXpp8wDTY5WDHVZ1MGpwVcRz5 mgltLTDa30yepVyh6w5NzhetDMxe2wfi3oKjgVwBHOK5I58qJQcTDnhFaF7/z5A+9T Crhks2tl+4q+ILFS2p4CopOCrQP4YclPdwyQl9bUX3AZSTEhNOV1GwgyhlkYTx3fV7 7Hngm+Y4fB7XQ== Message-ID: <104794ff-9f04-48be-91e8-89d5d2269f95@kernel.org> Date: Fri, 26 Jun 2026 16:30:48 +0200 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 1/4] mm/memcontrol: do not drain objcg stock when spinning is not allowed Content-Language: en-US To: "Harry Yoo (Oracle)" , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Alexei Starovoitov , Pedro Falcato Cc: cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org References: <20260624-kmalloc-nolock-fixes-v1-0-fdf4d17351dd@kernel.org> <20260624-kmalloc-nolock-fixes-v1-1-fdf4d17351dd@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/24/26 15:11, Harry Yoo (Oracle) wrote: > When kmalloc_nolock() drains objcg stock, the stock might be holding > the last reference to the objcg. Since obj_cgroup_release() is a > callback for percpu refcount and does not know whether spinning is > allowed, it is not safe to invoke obj_cgroup_put(). > > This was caught by lockdep on PREEMPT_RT because acquiring > a sleeping lock (objcg_lock) violates lock nesting rules: > > kernel: BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48 > kernel: in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1267, name: systemd-resolve > preempt_count: 1, expected: 0 > RCU nest depth: 3, expected: 3 > 6 locks held by systemd-resolve/1267: > #0: ffff888a8165fa20 ((&pcs->lock)){+.+.}-{3:3}, at: kmem_cache_alloc_noprof+0x185/0xa20 > #1: ffffffff9658a4c0 (rcu_read_lock){....}-{1:3}, at: rt_spin_trylock+0x74/0x2a0 > #2: ffff888a81648598 ((lock)#4){+.+.}-{3:3}, at: trylock_stock+0x118/0x380 > #3: ffffffff9658a4c0 (rcu_read_lock){....}-{1:3}, at: rt_spin_trylock+0x74/0x2a0 > #4: ffffffff9658a4c0 (rcu_read_lock){....}-{1:3}, at: percpu_ref_put_many.constprop.0+0x40/0x270 > #5: ffffffff96af11d8 (objcg_lock){+.+.}-{3:3}, at: obj_cgroup_release+0x8a/0x410 > [...] > Call Trace: > > dump_stack_lvl+0x8a/0xe0 > dump_stack+0x14/0x1c > __might_resched.cold+0x233/0x2bb > rt_spin_lock+0xd3/0x410 > obj_cgroup_release+0x8a/0x410 > percpu_ref_put_many.constprop.0+0x226/0x270 > drain_obj_stock_slot+0x27e/0x8d0 > __refill_obj_stock+0x409/0x6d0 > __memcg_slab_post_alloc_hook+0xa45/0x1500 > __kmalloc_nolock_noprof+0x988/0xc40 > [...] > > However, this is illegal in !RT kernels too because the objcg release > callback acquires a spinlock even when spinning is not allowed. > > To fix this issue, fall back to atomics when the cached objcg doesn't > match, but it is unsafe to drain because spinning is not allowed. > > This is expected to affect performance of kmalloc_nolock() since > it can no longer drain and refill the stock and falls back to a > per-objcg atomic counter (objcg->nr_charged_bytes). Maybe it's not bad that the constrained operation will not do a drain? Could we say it's likely that both _nolock() and normal operations will likely be happening on the same cpu for the same memcg, so a normal one will soon handle it? > Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().") > Cc: stable@vger.kernel.org > Signed-off-by: Harry Yoo (Oracle) > --- > mm/memcontrol.c | 34 +++++++++++++++++++++++----------- > mm/slab.h | 3 ++- > mm/slub.c | 29 +++++++++++++++++++---------- > 3 files changed, 44 insertions(+), 22 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 29390ba13baa..5bb5e75ef5b0 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -3316,18 +3316,19 @@ static bool obj_stock_flush_required(struct obj_stock_pcp *stock, > static void __refill_obj_stock(struct obj_cgroup *objcg, > struct obj_stock_pcp *stock, > unsigned int nr_bytes, > - bool allow_uncharge) > + bool allow_uncharge, > + bool allow_spin) > { > unsigned int nr_pages = 0; > > - if (!stock) { > - nr_pages = nr_bytes >> PAGE_SHIFT; > - nr_bytes = nr_bytes & (PAGE_SIZE - 1); > - atomic_add(nr_bytes, &objcg->nr_charged_bytes); > - goto out; > - } > + if (!stock) > + goto fallback; > > if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */ > + /* Not safe to drain since objcg release acquires spinlock */ > + if (unlikely(!allow_spin)) > + goto fallback; > + > drain_obj_stock(stock); > obj_cgroup_get(objcg); > stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes) > @@ -3346,6 +3347,13 @@ static void __refill_obj_stock(struct obj_cgroup *objcg, > out: > if (nr_pages) > obj_cgroup_uncharge_pages(objcg, nr_pages); > + return; > + > +fallback: > + nr_pages = nr_bytes >> PAGE_SHIFT; > + nr_bytes = nr_bytes & (PAGE_SIZE - 1); > + atomic_add(nr_bytes, &objcg->nr_charged_bytes); > + goto out; > } > > static void refill_obj_stock(struct obj_cgroup *objcg, > @@ -3353,7 +3361,8 @@ static void refill_obj_stock(struct obj_cgroup *objcg, > bool allow_uncharge) > { > struct obj_stock_pcp *stock = trylock_stock(); > - __refill_obj_stock(objcg, stock, nr_bytes, allow_uncharge); > + __refill_obj_stock(objcg, stock, nr_bytes, allow_uncharge, > + /* allow_spin = */ true); > unlock_stock(stock); > } > > @@ -3428,6 +3437,7 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru, > size_t size, void **p) > { > size_t obj_size = obj_full_size(s); > + bool allow_spin = alloc_flags_allow_spinning(slab_alloc_flags); > struct obj_cgroup *objcg; > struct slab *slab; > unsigned long off; > @@ -3497,7 +3507,8 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru, > return false; > stock = trylock_stock(); > if (remainder) > - __refill_obj_stock(objcg, stock, remainder, false); > + __refill_obj_stock(objcg, stock, remainder, false, > + allow_spin); > } > __account_obj_stock(objcg, stock, obj_size, > slab_pgdat(slab), cache_vmstat_idx(s)); > @@ -3516,7 +3527,8 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru, > } > > void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, > - void **p, int objects, unsigned long obj_exts) > + void **p, int objects, unsigned long obj_exts, > + bool allow_spin) > { > size_t obj_size = obj_full_size(s); > > @@ -3535,7 +3547,7 @@ void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, > obj_ext->objcg = NULL; > > stock = trylock_stock(); > - __refill_obj_stock(objcg, stock, obj_size, true); > + __refill_obj_stock(objcg, stock, obj_size, true, allow_spin); > __account_obj_stock(objcg, stock, -obj_size, > slab_pgdat(slab), cache_vmstat_idx(s)); > unlock_stock(stock); > diff --git a/mm/slab.h b/mm/slab.h > index 281a65233795..a6b4ac298d08 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -660,7 +660,8 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru, > gfp_t flags, unsigned int slab_alloc_flags, > size_t size, void **p); > void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, > - void **p, int objects, unsigned long obj_exts); > + void **p, int objects, unsigned long obj_exts, > + bool allow_spin); > #endif > > void kvfree_rcu_cb(struct rcu_head *head); > diff --git a/mm/slub.c b/mm/slub.c > index 917635203f73..32672a92581b 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2488,7 +2488,7 @@ bool memcg_slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags, > > static __fastpath_inline > void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p, > - int objects) > + int objects, bool allow_spin) > { > unsigned long obj_exts; > > @@ -2500,7 +2500,7 @@ void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p, > return; > > get_slab_obj_exts(obj_exts); > - __memcg_slab_free_hook(s, slab, p, objects, obj_exts); > + __memcg_slab_free_hook(s, slab, p, objects, obj_exts, allow_spin); > put_slab_obj_exts(obj_exts); > } > > @@ -2575,7 +2575,7 @@ static inline bool memcg_slab_post_alloc_hook(struct kmem_cache *s, > } > > static inline void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, > - void **p, int objects) > + void **p, int objects, bool allow_spin) > { > } > > @@ -2946,11 +2946,12 @@ static bool __rcu_free_sheaf_prepare(struct kmem_cache *s, > void **p = &sheaf->objects[0]; > unsigned int i = 0; > bool pfmemalloc = false; > + bool allow_spin = true; > > while (i < sheaf->size) { > struct slab *slab = virt_to_slab(p[i]); > > - memcg_slab_free_hook(s, slab, p + i, 1); > + memcg_slab_free_hook(s, slab, p + i, 1, allow_spin); > alloc_tagging_slab_free_hook(s, slab, p + i, 1); > > if (unlikely(!slab_free_hook(s, p[i], init, true))) { > @@ -6215,12 +6216,13 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p) > struct node_barn *barn; > void *remote_objects[PCS_BATCH_MAX]; > unsigned int remote_nr = 0; > + bool allow_spin = true; > > next_remote_batch: > while (i < size) { > struct slab *slab = virt_to_slab(p[i]); > > - memcg_slab_free_hook(s, slab, p + i, 1); > + memcg_slab_free_hook(s, slab, p + i, 1, allow_spin); > alloc_tagging_slab_free_hook(s, slab, p + i, 1); > > if (unlikely(!slab_free_hook(s, p[i], init, false))) { > @@ -6398,13 +6400,16 @@ static __fastpath_inline > void slab_free(struct kmem_cache *s, struct slab *slab, void *object, > unsigned long addr) > { > - memcg_slab_free_hook(s, slab, &object, 1); > + bool allow_spin = true; > + > + memcg_slab_free_hook(s, slab, &object, 1, allow_spin); > alloc_tagging_slab_free_hook(s, slab, &object, 1); > > if (unlikely(!slab_free_hook(s, object, slab_want_init_on_free(s), false))) > return; > > - if (likely(can_free_to_pcs(slab)) && likely(free_to_pcs(s, object, true))) > + if (likely(can_free_to_pcs(slab)) && > + likely(free_to_pcs(s, object, allow_spin))) > return; > > __slab_free(s, slab, object, object, 1, addr); > @@ -6429,7 +6434,9 @@ static __fastpath_inline > void slab_free_bulk(struct kmem_cache *s, struct slab *slab, void *head, > void *tail, void **p, int cnt, unsigned long addr) > { > - memcg_slab_free_hook(s, slab, p, cnt); > + bool allow_spin = true; > + > + memcg_slab_free_hook(s, slab, p, cnt, allow_spin); > alloc_tagging_slab_free_hook(s, slab, p, cnt); > /* > * With KASAN enabled slab_free_freelist_hook modifies the freelist > @@ -6734,6 +6741,7 @@ void kfree_nolock(const void *object) > struct slab *slab; > struct kmem_cache *s; > void *x = (void *)object; > + bool allow_spin = false; > > if (unlikely(ZERO_OR_NULL_PTR(object))) > return; > @@ -6746,7 +6754,7 @@ void kfree_nolock(const void *object) > > s = slab->slab_cache; > > - memcg_slab_free_hook(s, slab, &x, 1); > + memcg_slab_free_hook(s, slab, &x, 1, allow_spin); > alloc_tagging_slab_free_hook(s, slab, &x, 1); > /* > * Unlike slab_free() do NOT call the following: > @@ -6776,7 +6784,8 @@ void kfree_nolock(const void *object) > */ > kasan_slab_free(s, x, false, false, /* skip quarantine */true); > > - if (likely(can_free_to_pcs(slab)) && likely(free_to_pcs(s, x, false))) > + if (likely(can_free_to_pcs(slab)) && > + likely(free_to_pcs(s, x, allow_spin))) > return; > > /* >