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spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 26, 2025 at 07:31:34PM -0700, Alexei Starovoitov wrote: > On Sun, Aug 24, 2025 at 9:46 PM Harry Yoo wrote: > > > > On Tue, Jul 15, 2025 at 07:29:49PM -0700, Alexei Starovoitov wrote: > > > From: Alexei Starovoitov > > > > > > kmalloc_nolock() relies on ability of local_lock to detect the situation > > > when it's locked. > > > In !PREEMPT_RT local_lock_is_locked() is true only when NMI happened in > > > irq saved region that protects _that specific_ per-cpu kmem_cache_cpu. > > > In that case retry the operation in a different kmalloc bucket. > > > The second attempt will likely succeed, since this cpu locked > > > different kmem_cache_cpu. > > > > > > Similarly, in PREEMPT_RT local_lock_is_locked() returns true when > > > per-cpu rt_spin_lock is locked by current task. In this case re-entrance > > > into the same kmalloc bucket is unsafe, and kmalloc_nolock() tries > > > a different bucket that is most likely is not locked by the current > > > task. Though it may be locked by a different task it's safe to > > > rt_spin_lock() on it. > > > > > > Similar to alloc_pages_nolock() the kmalloc_nolock() returns NULL > > > immediately if called from hard irq or NMI in PREEMPT_RT. > > > > > > kfree_nolock() defers freeing to irq_work when local_lock_is_locked() > > > and in_nmi() or in PREEMPT_RT. > > > > > > SLUB_TINY config doesn't use local_lock_is_locked() and relies on > > > spin_trylock_irqsave(&n->list_lock) to allocate while kfree_nolock() > > > always defers to irq_work. > > > > > > Signed-off-by: Alexei Starovoitov > > > --- > > > include/linux/kasan.h | 13 +- > > > include/linux/slab.h | 4 + > > > mm/Kconfig | 1 + > > > mm/kasan/common.c | 5 +- > > > mm/slab.h | 6 + > > > mm/slab_common.c | 3 + > > > mm/slub.c | 454 +++++++++++++++++++++++++++++++++++++----- > > > 7 files changed, 434 insertions(+), 52 deletions(-) > > > > > +static void defer_free(struct kmem_cache *s, void *head) > > > +{ > > > + struct defer_free *df = this_cpu_ptr(&defer_free_objects); > > > + > > > + if (llist_add(head + s->offset, &df->objects)) > > > + irq_work_queue(&df->work); > > > +} > > > + > > > +static void defer_deactivate_slab(struct slab *slab) > > > +{ > > > + struct defer_free *df = this_cpu_ptr(&defer_free_objects); > > > + > > > + if (llist_add(&slab->llnode, &df->slabs)) > > > + irq_work_queue(&df->work); > > > +} > > > + > > > +void defer_free_barrier(void) > > > +{ > > > + int cpu; > > > + > > > + for_each_possible_cpu(cpu) > > > + irq_work_sync(&per_cpu_ptr(&defer_free_objects, cpu)->work); > > > +} > > > > I think it should also initiate deferred frees, if kfree_nolock() freed > > the last object in some CPUs? > > I don't understand the question. "the last object in some CPU" ? > Are you asking about the need of defer_free_barrier() ? My bad. It slipped my mind. I thought objects freed via kfree_nolock() are not freed before a following kfree(), but since we've switched to IRQ work, that's not the case anymore. > PS > I just got back from 2+ week PTO. Going through backlog. Hope you enjoyed your PTO! -- Cheers, Harry / Hyeonggon