On 9/30/25 13:19, Alexei Starovoitov wrote:

On Tue, Sep 30, 2025 at 12:54 PM Harry Yoo [off-list ref] wrote:

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On Tue, Sep 30, 2025 at 08:34:02AM +0000, ranxiaokai627@163.com wrote:

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From: Ran Xiaokai <redacted>

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

Please share config and repro details, since the stack trace
looks theoretical, but you somehow got it?
This is not CONFIG_SLUB_TINY, but kfree_nolock()
sees locked per-cpu slab?

Could it be just the "slab != c->slab" condition in do_slab_free()? That's
more likely. However...

Is this PREEMPT_RT ?

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 __alloc_tagging_slab_alloc_hook+0xa0/0x300
 __kmalloc_cache_noprof+0x1c4/0x5c0
 __set_page_owner+0x10d/0x1c0

This is the part that puzzles me, where do we call kmalloc from
__set_page_owner()? And in a way that it loses the GFP_KERNEL passed all the
way? I don't even see a lib/stackdepot function here.

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 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

This has a kcalloc(GFP_KERNEL).

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 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

Here we've set the full gfp_allowed_mask already so it's not masking our
GFP_KERNEL.

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 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.

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Signed-off-by: Ran Xiaokai <redacted>
---
 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?

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      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()?

+1.
Though irq_work_queue() works for any irq_work it should
be used for current cpu, since it IPIs itself.
So pls use guard(preempt)(); instead.

Agreed. But we should fix it like this. But the report is strange.