Re: [PATCH] slab: Fix using this_cpu_ptr() in preemptible context

Harry Yoo
2025-09-30T10:54:25Z

On Tue, Sep 30, 2025 at 08:34:02AM +0000, ranxiaokai627@163.com wrote:
> From: Ran Xiaokai <ran.xiaokai@zte.com.cn>
>
> defer_free() maybe called in preemptible context, this will
> trigger the below warning message:
>
> BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
> caller is defer_free+0x1b/0x60
> Call Trace:
> <TASK>
> dump_stack_lvl+0xac/0xc0
> check_preemption_disabled+0xbe/0xe0
> defer_free+0x1b/0x60
> kfree_nolock+0x1eb/0x2b0
> alloc_slab_obj_exts+0x356/0x390
> __alloc_tagging_slab_alloc_hook+0xa0/0x300
> __kmalloc_cache_noprof+0x1c4/0x5c0
> __set_page_owner+0x10d/0x1c0
> post_alloc_hook+0x84/0xf0
> get_page_from_freelist+0x73b/0x1380
> __alloc_frozen_pages_noprof+0x110/0x2c0
> alloc_pages_mpol+0x44/0x140
> alloc_slab_page+0xac/0x150
> allocate_slab+0x78/0x3a0
> ___slab_alloc+0x76b/0xed0
> __slab_alloc.constprop.0+0x5a/0xb0
> __kmalloc_noprof+0x3dc/0x6d0
> __list_lru_init+0x6c/0x210
> alloc_super+0x3b6/0x470
> sget_fc+0x5f/0x3a0
> get_tree_nodev+0x27/0x90
> vfs_get_tree+0x26/0xc0
> vfs_kern_mount.part.0+0xb6/0x140
> kern_mount+0x24/0x40
> init_pipe_fs+0x4f/0x70
> do_one_initcall+0x62/0x2e0
> kernel_init_freeable+0x25b/0x4b0
> kernel_init+0x1a/0x1c0
> ret_from_fork+0x290/0x2e0
> ret_from_fork_asm+0x11/0x20
> </TASK>
>
> Replace this_cpu_ptr with raw_cpu_ptr to eliminate
> the above warning message.
>
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")

There's no mainline commit hash yet, should be adjusted later.

> Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
> ---
> mm/slub.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 1433f5b988f7..67c57f1b5a86 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6432,7 +6432,7 @@ static void free_deferred_objects(struct irq_work *work)
>
> static void defer_free(struct kmem_cache *s, void *head)
> {
> - struct defer_free *df = this_cpu_ptr(&defer_free_objects);
> + struct defer_free *df = raw_cpu_ptr(&defer_free_objects);

This suppresses warning, but let's answer the question;
Is it actually safe to not disable preemption here?

> if (llist_add(head + s->offset, &df->objects))

Let's say a task was running on CPU X and migrated to a different CPU
(say, Y) after returning from llist_add() or before calling llist_add(),
then we're queueing the irq_work of CPU X on CPU Y.

I think technically this should be safe because, although we're using
per-cpu irq_work here, the irq_work framework itself is designed to handle
concurrent access from multiple CPUs (otherwise it won't be safe to use
a global irq_work like in other places) by using lockless list, which
uses try_cmpxchg() and xchg() for atomic update.

So if I'm not missing something it should be safe, but it was very
confusing to confirm that it's safe as we're using per-cpu irq_work...

I don't think these paths are very performance critical, so why not disable
preemption instead of replacing it with raw_cpu_ptr()?

> irq_work_queue(&df->work);
> @@ -6440,7 +6440,7 @@ static void defer_free(struct kmem_cache *s, void *head)
>
> static void defer_deactivate_slab(struct slab *slab, void *flush_freelist)
> {
> - struct defer_free *df = this_cpu_ptr(&defer_free_objects);
> + struct defer_free *df = raw_cpu_ptr(&defer_free_objects);
>
> slab->flush_freelist = flush_freelist;
> if (llist_add(&slab->llnode, &df->slabs))
> --
> 2.25.1

--
Cheers,
Harry / Hyeonggon