Re: [PATCH v3 14/21] slab: simplify kmalloc_nolock()

Harry Yoo
2026-01-22T01:53:42Z

On Fri, Jan 16, 2026 at 03:40:34PM +0100, Vlastimil Babka wrote:
> The kmalloc_nolock() implementation has several complications and
> restrictions due to SLUB's cpu slab locking, lockless fastpath and
> PREEMPT_RT differences. With cpu slab usage removed, we can simplify
> things:
>
> - relax the PREEMPT_RT context checks as they were before commit
> a4ae75d1b6a2 ("slab: fix kmalloc_nolock() context check for
> PREEMPT_RT") and also reference the explanation comment in the page
> allocator
>
> - the local_lock_cpu_slab() macros became unused, remove them
>
> - we no longer need to set up lockdep classes on PREEMPT_RT
>
> - we no longer need to annotate ___slab_alloc as NOKPROBE_SYMBOL
> since there's no lockless cpu freelist manipulation anymore
>
> - __slab_alloc_node() can be called from kmalloc_nolock_noprof()
> unconditionally. It can also no longer return EBUSY. But trylock
> failures can still happen so retry with the larger bucket if the
> allocation fails for any reason.
>
> Note that we still need __CMPXCHG_DOUBLE, because while it was removed
> we don't use cmpxchg16b on cpu freelist anymore, we still use it on
> slab freelist, and the alternative is slab_lock() which can be
> interrupted by a nmi. Clarify the comment to mention it specifically.
>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---

What a nice cleanup!

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

with a nit below.

> mm/slab.h | 1 -
> mm/slub.c | 144 +++++++++++++-------------------------------------------------
> 2 files changed, 29 insertions(+), 116 deletions(-)
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 4efec41b6445..e9a0738133ed 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -5268,10 +5196,11 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node)
> if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s))
> /*
> * kmalloc_nolock() is not supported on architectures that
> - * don't implement cmpxchg16b, but debug caches don't use
> - * per-cpu slab and per-cpu partial slabs. They rely on
> - * kmem_cache_node->list_lock, so kmalloc_nolock() can
> - * attempt to allocate from debug caches by
> + * don't implement cmpxchg16b and thus need slab_lock()
> + * which could be preempted by a nmi.

nit: I think now this limitation can be removed because the only slab
lock used in the allocation path is get_partial_node() ->
__slab_update_freelist(), but it is always used under n->list_lock.

Being preempted by a NMI while holding the slab lock is fine because
NMI context should fail to acquire n->list_lock and bail out.

But no hurry on this, it's probably not important enough to delay
this series :)

> + * But debug caches don't use that and only rely on
> + * kmem_cache_node->list_lock, so kmalloc_nolock() can attempt
> + * to allocate from debug caches by
> * spin_trylock_irqsave(&n->list_lock, ...)
> */
> return NULL;
>

--
Cheers,
Harry / Hyeonggon