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

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Message-ID: <aLAXsptN4cw05IoZ@devbig569.cln6.facebook.com>
Date: Thu, 28 Aug 2025 01:47:46 -0700
From: Yueyang Pan <pyyjason@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
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 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.
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?

> 
> >> > 
> >> 
> >> 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()?
> 

Agreed.

Thanks
Pan