linux-kernel - Re: [PATCH v2] memcg: simple cleanup of stats update functions

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Message-ID: <86006806-4ffc-4330-ab4b-29215ab2c98c@kernel.org>
Date: Mon, 27 May 2024 18:34:24 +0200
From: "Vlastimil Babka (SUSE)" <vbabka@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] memcg: simple cleanup of stats update functions

On 5/27/24 5:22 PM, Sebastian Andrzej Siewior wrote:
> On 2024-04-20 16:25:05 [-0700], Shakeel Butt wrote:
>> mod_memcg_lruvec_state() is never called from outside of memcontrol.c
>> and with always irq disabled. So, replace it with the irq disabled
>> version and add an assert that irq is disabled in the caller.
> 
> unless PREEMPT_RT is enabled. In that case IRQs are not disabled as part
> of local_lock_irqsave(&memcg_stock.stock_lock, …) leading to:

But then the "interrupts are handled by a kernel thread that can sleep" part
of RT also means it's ok to just have the stock_lock taken with no
interrupts disabled as no actual raw interrupt handler will interrupt the
holder and deadlock, right?

> | ------------[ cut here ]------------
> | WARNING: CPU: 0 PID: 1 at mm/memcontrol.c:3150 __mod_objcg_mlstate+0xc2/0x110
> | CPU: 0 PID: 1 Comm: systemd Not tainted 6.10.0-rc1-rt0+ #17
> | Call Trace:
> |  <TASK>
> |  mod_objcg_state+0x2b3/0x320
> |  __memcg_slab_post_alloc_hook+0x13c/0x340
> |  kmem_cache_alloc_lru_noprof+0x2bd/0x2e0
> |  alloc_inode+0x59/0xc0
> |  iget_locked+0xf0/0x290
> 
> suggestions?

So in that case the appropriate thing would be to replace the assert with
lockdep_assert_held(&memcg_stock.stock_lock);
?

It seems all the code paths leading here have that one.

> Sebastian
>