linux-kernel - Re: [PATCH v1 2/2] mm/show_mem: Add trylock while printing alloc info

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Message-ID: <92a3320e-2433-4ee8-8ed5-d5db6098b43e@suse.cz>
Date: Thu, 28 Aug 2025 11:54:58 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Yueyang Pan <pyyjason@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
 Zi Yan <ziy@...dia.com>, Usama Arif <usamaarif642@...il.com>,
 linux-mm@...ck.org, kernel-team@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] mm/show_mem: Add trylock while printing alloc info

On 8/28/25 11:51, Yueyang Pan wrote:
> On Thu, Aug 28, 2025 at 10:53:01AM +0200, Vlastimil Babka wrote:
>> On 8/28/25 10:47, Yueyang Pan wrote:
>> > On Thu, Aug 28, 2025 at 10:41:23AM +0200, Vlastimil Babka wrote:
>> >> On 8/28/25 10:34, Yueyang Pan wrote:
>> >> > On Wed, Aug 27, 2025 at 03:06:19PM -0700, Andrew Morton wrote:
>> >> >> On Wed, 27 Aug 2025 11:34:23 -0700 Yueyang Pan <pyyjason@...il.com> wrote:
>> >> >> 
>> >> >> > In production, show_mem() can be called concurrently from two
>> >> >> > different entities, for example one from oom_kill_process()
>> >> >> > another from __alloc_pages_slowpath from another kthread. This
>> >> >> > patch adds a mutex and invokes trylock before printing out the
>> >> >> > kernel alloc info in show_mem(). This way two alloc info won't
>> >> >> > interleave with each other, which then makes parsing easier.
>> >> 
>> >> What about the rest of the information printed by show_mem() being interleaved?
>> > 
>> > Thanks for your feedback, Vlastimil. We cannot use trylock for the rest 
>> > part as node filter can be different.
>> 
>> Right.
>> 
>> > Do you think we need a lock to prevent the whole show_mem() from being 
>> > interleaved and to acquire it at the very beginning? Will it be too 
>> > heavy?
>> 
>> It might be risky so perhaps let's not. Guess we can disentangle by dmesg
>> showing the thread id prefix.
> 
> I have thought about this. Since each line can interleave with another, we 
> would end up adding tid to each line. Not sure if this is acceptable.

I meant that printk/dmesg already does that so it's fine.

> Thanks
> Pan