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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 30, 2025 at 08:34:02AM +0000, ranxiaokai627@163.com wrote: > From: Ran Xiaokai > > 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: > > 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 > > > 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 > --- > 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