linux-kernel - Re: [PATCH V3] mm: slub: fix flush_cpu_slab()/__free

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Date:   Mon, 19 Sep 2022 19:30:40 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Maurizio Lombardi <mlombard@...hat.com>
Cc:     linux-mm@...ck.org, rientjes@...gle.com, penberg@...nel.org,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, bigeasy@...utronix.de,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH V3] mm: slub: fix flush_cpu_slab()/__free_slab()
 invocations in task context.

On 9/19/22 18:39, Maurizio Lombardi wrote:
> Commit 5a836bf6b09f ("mm: slub: move flush_cpu_slab() invocations
> __free_slab() invocations out of IRQ context") moved all flush_cpu_slab()
> invocations to the global workqueue to avoid a problem related
> with deactivate_slab()/__free_slab() being called from an IRQ context
> on PREEMPT_RT kernels.
> 
> When the flush_all_cpu_locked() function is called from a task context
> it may happen that a workqueue with WQ_MEM_RECLAIM bit set ends up
> flushing the global workqueue, this will cause a dependency issue.
> 
>  workqueue: WQ_MEM_RECLAIM nvme-delete-wq:nvme_delete_ctrl_work [nvme_core]
>    is flushing !WQ_MEM_RECLAIM events:flush_cpu_slab
>  WARNING: CPU: 37 PID: 410 at kernel/workqueue.c:2637
>    check_flush_dependency+0x10a/0x120
>  Workqueue: nvme-delete-wq nvme_delete_ctrl_work [nvme_core]
>  RIP: 0010:check_flush_dependency+0x10a/0x120[  453.262125] Call Trace:
>  __flush_work.isra.0+0xbf/0x220
>  ? __queue_work+0x1dc/0x420
>  flush_all_cpus_locked+0xfb/0x120
>  __kmem_cache_shutdown+0x2b/0x320
>  kmem_cache_destroy+0x49/0x100
>  bioset_exit+0x143/0x190
>  blk_release_queue+0xb9/0x100
>  kobject_cleanup+0x37/0x130
>  nvme_fc_ctrl_free+0xc6/0x150 [nvme_fc]
>  nvme_free_ctrl+0x1ac/0x2b0 [nvme_core]
> 
> Fix this bug by creating a workqueue for the flush operation with
> the WQ_MEM_RECLAIM bit set.
> 
> v2: Create a workqueue with WQ_MEM_RECLAIM
>     instead of trying to revert the changes.
> 
> v3: replace create_workqueue() with alloc_workqueue() and BUG_ON() with
>     WARN_ON()
> 
> Signed-off-by: Maurizio Lombardi <mlombard@...hat.com>

Thanks, I added a Fixes: and Cc stable as AFAICS the warnings are not under
a debugging config option, and it could bite somebody near OOM. Added to
slab.git for-6.0/fixes and will include in pullrq this week.