Re: [PATCH v4 29/35] mm: slub: Move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context

[Qian Cai]

On 8/10/2021 10:33 AM, Vlastimil Babka wrote:
> On 8/9/21 3:41 PM, Qian Cai wrote:
> 
>>>  static void flush_all(struct kmem_cache *s)
>>>  {
>>> -	on_each_cpu_cond(has_cpu_slab, flush_cpu_slab, s, 1);
>>> +	struct slub_flush_work *sfw;
>>> +	unsigned int cpu;
>>> +
>>> +	mutex_lock(&flush_lock);
>>
>> Vlastimil, taking the lock here could trigger a warning during memory offline/online due to the locking order:
>>
>> slab_mutex -> flush_lock
> 
> Here's the full fixup, also incorporating Mike's fix. Thanks.
> 
> ----8<----
> From c2df67d5116d4615c322e262556e34117e268104 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@xxxxxxx>
> Date: Tue, 10 Aug 2021 10:58:07 +0200
> Subject: [PATCH] mm, slub: fix memory and cpu hotplug related lock ordering
>  issues
> 
> Qian Cai reported [1] a lockdep splat on memory offline.
> 
> [   91.374541] WARNING: possible circular locking dependency detected
> [   91.381411] 5.14.0-rc5-next-20210809+ #84 Not tainted
> [   91.387149] ------------------------------------------------------
> [   91.394016] lsbug/1523 is trying to acquire lock:
> [   91.399406] ffff800018e76530 (flush_lock){+.+.}-{3:3}, at: flush_all+0x50/0x1c8
> [   91.407425] but task is already holding lock:
> [   91.414638] ffff800018e48468 (slab_mutex){+.+.}-{3:3}, at: slab_memory_callback+0x44/0x280
> [   91.423603] which lock already depends on the new lock.
> 
> To fix it, we need to change the order in flush_all() so that cpus_read_lock()
> is first and mutex_lock(&flush_lock) second.
> 
> Also when called from slab_mem_going_offline_callback() we are already under
> cpus_read_lock() and cannot take it again, so create a flush_all_cpus_locked()
> variant and decouple flushing from actual shrinking for this call path.
> 
> Additionally, Mike Galbraith reported [2] wrong order of cpus_read_lock() and
> slab_mutex in kmem_cache_destroy() path and proposed a fix to reverse it.
> 
> This patch is a fixup for the mmotm patch
> mm-slub-move-flush_cpu_slab-invocations-__free_slab-invocations-out-of-irq-context.patch
> 
> [1] https://lore.kernel.org/lkml/0b36128c-3e12-77df-85fe-a153a714569b@xxxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/2eb3cf340716c40f03a0a342ab40219b3d1de195.camel@xxxxxx/
> 
> Reported-by: Qian Cai <quic_qiancai@xxxxxxxxxxx>
> Reported-by: Mike Galbraith <efault@xxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

This is running fine for me. There is a separate hugetlb crash while fuzzing and will
report to where it belongs.