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

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Message-ID: <c8db3486-4f13-4922-a270-1fbf61e2d782@suse.cz>
Date: Thu, 28 Aug 2025 10:41:23 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Yueyang Pan <pyyjason@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: 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 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?

>> > 
>> 
>> Fair enough, I guess.
>> 
>> > --- a/mm/show_mem.c
>> > +++ b/mm/show_mem.c
>> > @@ -23,6 +23,8 @@ EXPORT_SYMBOL(_totalram_pages);
>> >  unsigned long totalreserve_pages __read_mostly;
>> >  unsigned long totalcma_pages __read_mostly;
>> >  
>> > +static DEFINE_MUTEX(mem_alloc_profiling_mutex);
>> 
>> It would be a bit neater to make this local to __show_mem() - it didn't
>> need file scope.
> 
> Thanks for your feedback, Andrew. I will move it the next version.
> 
>> 
>> Also, mutex_unlock() isn't to be used from interrupt context, so
>> problem.
>> 
>> Something like atomic cmpxchg or test_and_set_bit could be used and
>> wouldn't involve mutex_unlock()'s wakeup logic, which isn't needed
>> here.
> 
> I was not aware of interrupt context before. I will change to test-and-set 
> lock in the next version.

Perhaps simply spinlock_t with spin_trylock()?