Re: [PATCH v4] mm: slub: avoid wake up kswapd in set_track_prepare

[David Rientjes <rientjes@google.com>]

On Sat, 30 Aug 2025, yangshiguang1011@163.com wrote:

> From: yangshiguang <yangshiguang@xiaomi.com>
> 
> From: yangshiguang <yangshiguang@xiaomi.com>
> 

Duplicate lines.

> set_track_prepare() can incur lock recursion.
> The issue is that it is called from hrtimer_start_range_ns
> holding the per_cpu(hrtimer_bases)[n].lock, but when enabled
> CONFIG_DEBUG_OBJECTS_TIMERS, may wake up kswapd in set_track_prepare,
> and try to hold the per_cpu(hrtimer_bases)[n].lock.
> 
> Avoid deadlock caused by implicitly waking up kswapd by
> passing in allocation flags. And the slab caller context has
> preemption disabled, so __GFP_KSWAPD_RECLAIM must not appear in gfp_flags.
> 

This mentions __GFP_KSWAPD_RECLAIM, but the patch actually masks off 
__GFP_DIRECT_RECLAIM which would be a heavierweight operation.  Disabling 
direct reclaim does not necessarily imply that kswapd will be disabled as 
well.

Are you meaning to clear __GFP_RECLAIM in set_track_prepare()?

> The oops looks something like:
> 
> BUG: spinlock recursion on CPU#3, swapper/3/0
>  lock: 0xffffff8a4bf29c80, .magic: dead4ead, .owner: swapper/3/0, .owner_cpu: 3
> Hardware name: Qualcomm Technologies, Inc. Popsicle based on SM8850 (DT)
> Call trace:
> spin_bug+0x0
> _raw_spin_lock_irqsave+0x80
> hrtimer_try_to_cancel+0x94
> task_contending+0x10c
> enqueue_dl_entity+0x2a4
> dl_server_start+0x74
> enqueue_task_fair+0x568
> enqueue_task+0xac
> do_activate_task+0x14c
> ttwu_do_activate+0xcc
> try_to_wake_up+0x6c8
> default_wake_function+0x20
> autoremove_wake_function+0x1c
> __wake_up+0xac
> wakeup_kswapd+0x19c
> wake_all_kswapds+0x78
> __alloc_pages_slowpath+0x1ac
> __alloc_pages_noprof+0x298
> stack_depot_save_flags+0x6b0
> stack_depot_save+0x14
> set_track_prepare+0x5c
> ___slab_alloc+0xccc
> __kmalloc_cache_noprof+0x470
> __set_page_owner+0x2bc
> post_alloc_hook[jt]+0x1b8
> prep_new_page+0x28
> get_page_from_freelist+0x1edc
> __alloc_pages_noprof+0x13c
> alloc_slab_page+0x244
> allocate_slab+0x7c
> ___slab_alloc+0x8e8
> kmem_cache_alloc_noprof+0x450
> debug_objects_fill_pool+0x22c
> debug_object_activate+0x40
> enqueue_hrtimer[jt]+0xdc
> hrtimer_start_range_ns+0x5f8
> ...
> 
> Signed-off-by: yangshiguang <yangshiguang@xiaomi.com>
> Fixes: 5cf909c553e9 ("mm/slub: use stackdepot to save stack trace in objects")
> Cc: stable@vger.kernel.org
> ---
> 
> v1 -> v2:
>     propagate gfp flags to set_track_prepare()
> v2 -> v3:
>     Remove the gfp restriction in set_track_prepare()
> v3 -> v4:
>     Re-describe the comments in set_track_prepare.
> 
> [1]https://lore.kernel.org/all/20250801065121.876793-1-yangshiguang1011@163.com/
> [2]https://lore.kernel.org/all/20250814111641.380629-2-yangshiguang1011@163.com/
> [3]https://lore.kernel.org/all/20250825121737.2535732-1-yangshiguang1011@163.com/
> ---
>  mm/slub.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 30003763d224..b0af51a5321b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -962,19 +962,25 @@ static struct track *get_track(struct kmem_cache *s, void *object,
>  }
>  
>  #ifdef CONFIG_STACKDEPOT
> -static noinline depot_stack_handle_t set_track_prepare(void)
> +static noinline depot_stack_handle_t set_track_prepare(gfp_t gfp_flags)
>  {
>  	depot_stack_handle_t handle;
>  	unsigned long entries[TRACK_ADDRS_COUNT];
>  	unsigned int nr_entries;
> +	/*
> +	 * Preemption is disabled in ___slab_alloc() so we need to disallow
> +	 * blocking. The flags are further adjusted by gfp_nested_mask() in
> +	 * stack_depot itself.
> +	 */
> +	gfp_flags &= ~(__GFP_DIRECT_RECLAIM);
>  
>  	nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 3);
> -	handle = stack_depot_save(entries, nr_entries, GFP_NOWAIT);
> +	handle = stack_depot_save(entries, nr_entries, gfp_flags);
>  
>  	return handle;
>  }
>  #else
> -static inline depot_stack_handle_t set_track_prepare(void)
> +static inline depot_stack_handle_t set_track_prepare(gfp_t gfp_flags)
>  {
>  	return 0;
>  }
> @@ -996,9 +1002,9 @@ static void set_track_update(struct kmem_cache *s, void *object,
>  }
>  
>  static __always_inline void set_track(struct kmem_cache *s, void *object,
> -				      enum track_item alloc, unsigned long addr)
> +				      enum track_item alloc, unsigned long addr, gfp_t gfp_flags)
>  {
> -	depot_stack_handle_t handle = set_track_prepare();
> +	depot_stack_handle_t handle = set_track_prepare(gfp_flags);
>  
>  	set_track_update(s, object, alloc, addr, handle);
>  }
> @@ -1921,9 +1927,9 @@ static inline bool free_debug_processing(struct kmem_cache *s,
>  static inline void slab_pad_check(struct kmem_cache *s, struct slab *slab) {}
>  static inline int check_object(struct kmem_cache *s, struct slab *slab,
>  			void *object, u8 val) { return 1; }
> -static inline depot_stack_handle_t set_track_prepare(void) { return 0; }
> +static inline depot_stack_handle_t set_track_prepare(gfp_t gfp_flags) { return 0; }
>  static inline void set_track(struct kmem_cache *s, void *object,
> -			     enum track_item alloc, unsigned long addr) {}
> +			     enum track_item alloc, unsigned long addr, gfp_t gfp_flags) {}
>  static inline void add_full(struct kmem_cache *s, struct kmem_cache_node *n,
>  					struct slab *slab) {}
>  static inline void remove_full(struct kmem_cache *s, struct kmem_cache_node *n,
> @@ -3878,7 +3884,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>  			 * tracking info and return the object.
>  			 */
>  			if (s->flags & SLAB_STORE_USER)
> -				set_track(s, freelist, TRACK_ALLOC, addr);
> +				set_track(s, freelist, TRACK_ALLOC, addr, gfpflags);
>  
>  			return freelist;
>  		}
> @@ -3910,7 +3916,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>  			goto new_objects;
>  
>  		if (s->flags & SLAB_STORE_USER)
> -			set_track(s, freelist, TRACK_ALLOC, addr);
> +			set_track(s, freelist, TRACK_ALLOC, addr, gfpflags);
>  
>  		return freelist;
>  	}
> @@ -4422,7 +4428,7 @@ static noinline void free_to_partial_list(
>  	depot_stack_handle_t handle = 0;
>  
>  	if (s->flags & SLAB_STORE_USER)
> -		handle = set_track_prepare();
> +		handle = set_track_prepare(__GFP_NOWARN);
>  
>  	spin_lock_irqsave(&n->list_lock, flags);
>  
> -- 
> 2.43.0
> 
>