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

[Vlastimil Babka <vbabka@suse.cz>]

On 9/30/25 10:34, 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().")
> Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>

Since the slab PR was merged, we should fix this ideally before rc1 so I
updated the code myself per the discussion and added to slab/for-next

Thanks!

----8<----
From d6306a3d5577769b179ae4e448fd144e2b0f7717 Mon Sep 17 00:00:00 2001
From: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Date: Tue, 30 Sep 2025 08:34:02 +0000
Subject: [PATCH] slab: Fix using this_cpu_ptr() in preemptible context

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>

Disable preemption in defer_free() and also defer_deactivate_slab() to
make it safe.

[vbabka@suse.cz: disable preemption instead of using raw_cpu_ptr() per
 the discussion ]
Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Link: https://lore.kernel.org/r/20250930083402.782927-1-ranxiaokai627@163.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 1433f5b988f7..44aa0e3f48ee 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6432,17 +6432,24 @@ 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;
 
+	guard(preempt)();
+
+	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, void *flush_freelist)
 {
-	struct defer_free *df = this_cpu_ptr(&defer_free_objects);
+	struct defer_free *df;
 
 	slab->flush_freelist = flush_freelist;
+
+	guard(preempt)();
+
+	df = this_cpu_ptr(&defer_free_objects);
 	if (llist_add(&slab->llnode, &df->slabs))
 		irq_work_queue(&df->work);
 }
-- 
2.51.0