On Sun, Aug 24, 2025 at 9:46 PM Harry Yoo <harry.yoo@xxxxxxxxxx> wrote:
>
> On Tue, Jul 15, 2025 at 07:29:49PM -0700, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@xxxxxxxxxx>
> >
> > 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 <ast@xxxxxxxxxx>
> > ---
> > 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() ?
PS
I just got back from 2+ week PTO. Going through backlog.
