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Message-ID: <623a8fd2.8b7b.1990481f065.Coremail.yangshiguang1011@163.com>
Date: Mon, 1 Sep 2025 17:00:48 +0800 (CST)
From: yangshiguang <yangshiguang1011@....com>
To: "Vlastimil Babka" <vbabka@...e.cz>
Cc: "David Rientjes" <rientjes@...gle.com>, harry.yoo@...cle.com,
akpm@...ux-foundation.org, cl@...two.org, roman.gushchin@...ux.dev,
glittao@...il.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
yangshiguang <yangshiguang@...omi.com>, stable@...r.kernel.org
Subject: Re:Re: [PATCH v4] mm: slub: avoid wake up kswapd in
set_track_prepare
At 2025-09-01 16:46:13, "Vlastimil Babka" <vbabka@...e.cz> wrote:
>On 9/1/25 10:29, yangshiguang wrote:
>>
>>
>> At 2025-09-01 16:15:04, "Vlastimil Babka" <vbabka@...e.cz> wrote:
>>>On 9/1/25 09:50, David Rientjes wrote:
>>>> On Sat, 30 Aug 2025, yangshiguang1011@....com wrote:
>>>>
>>>>> From: yangshiguang <yangshiguang@...omi.com>
>>>>>
>>>>> From: yangshiguang <yangshiguang@...omi.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.
>>>
>>>Yeah I think the changelog should say __GFP_DIRECT_RECLAIM.
>>>
>>>> Are you meaning to clear __GFP_RECLAIM in set_track_prepare()?
>>>
>>>No because if the context context (e.g. the hrtimers) can't support
>>>__GFP_KSWAPD_RECLAIM it won't have it in gfp_flags and we now pass them to
>>
>>>set_track_prepare() so it already won't be there.
>>
>>
>> Sry. Should be __GFP_DIRECT_RECLAIM. I will resend the patch.
>
>I have adjusted it locally already. Also moved the masking of
>__GFP_DIRECT_RECLAIM to ___slab_alloc itself as that's where
>the preemption is disabled so it's more obvious.
>
>Does the result look good to you?
This looks good.
Currently only ___slab_alloc disables preemption context calls to set_track.
In the future, not all callers will disable preemption.
>
>commit 1b7052bc536650f8ca29b4f6f8682dc9f5692d16
>Author: yangshiguang <yangshiguang@...omi.com>
>Date: Sat Aug 30 10:09:46 2025 +0800
>
> mm: slub: avoid wake up kswapd in set_track_prepare
>
> 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, which do not contain __GFP_KSWAPD_RECLAIM in the
> debug_objects_fill_pool() case.
> Since ___slab_alloc() has preemption disabled and thus was using
> GFP_NOWAIT, we instead mask out __GFP_DIRECT_RECLAIM from the flags.
>
> 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@...omi.com>
> Fixes: 5cf909c553e9 ("mm/slub: use stackdepot to save stack trace in objects")
> Cc: stable@...r.kernel.org
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>
>diff --git a/mm/slub.c b/mm/slub.c
>index 1787e4d51e48..d257141896c9 100644
>--- a/mm/slub.c
>+++ b/mm/slub.c
>@@ -962,19 +962,19 @@ 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;
>
> 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 +996,9 @@ 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);
> }
>@@ -1926,9 +1926,9 @@ 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,
>@@ -3881,9 +3881,14 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> * For debug caches here we had to go through
> * alloc_single_from_partial() so just store the
> * tracking info and return the object.
>+ *
>+ * Due to disabled preemption we need to disallow
>+ * blocking. The flags are further adjusted by
>+ * gfp_nested_mask() in stack_depot itself.
> */
> if (s->flags & SLAB_STORE_USER)
>- set_track(s, freelist, TRACK_ALLOC, addr);
>+ set_track(s, freelist, TRACK_ALLOC, addr,
>+ gfpflags & ~(__GFP_DIRECT_RECLAIM));
>
> return freelist;
> }
>@@ -3915,7 +3920,8 @@ 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 & ~(__GFP_DIRECT_RECLAIM));
>
> return freelist;
> }
>@@ -4426,8 +4432,12 @@ static noinline void free_to_partial_list(
> unsigned long flags;
> depot_stack_handle_t handle = 0;
>
>+ /*
>+ * We cannot use GFP_NOWAIT as there are callsites where waking up
>+ * kswapd could deadlock
>+ */
> if (s->flags & SLAB_STORE_USER)
>- handle = set_track_prepare();
>+ handle = set_track_prepare(__GFP_NOWARN);
>
> spin_lock_irqsave(&n->list_lock, flags);
>
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