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Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Oct 02, 2025 at 10:14:55AM +0200, Vlastimil Babka wrote: > On 9/30/25 13:19, Alexei Starovoitov wrote: > > On Tue, Sep 30, 2025 at 12:54 PM Harry Yoo wrote: > >> > >> 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 > > > > 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. Agreed that we're seeing the case. > However... > > Is this PREEMPT_RT ? > > > >> > __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()? It's __set_page_owner() -> inc_stack_record_count() -> add_stack_record_to_list() -> kmalloc(). > And in a way that it loses the GFP_KERNEL passed all the > way? I don't even see a lib/stackdepot function here. Oh wait, we clear __GFP_RECLAIM on the first attempt to allocate high-order slabs. so gfpflags_allow_spinning() returns false. > >> > 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). Right. > >> > 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. Right. -- Cheers, Harry / Hyeonggon > >> > 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()? > > > > +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.