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

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Message-ID: <aLAUhXeRVWzLCNNd@devbig569.cln6.facebook.com>
Date: Thu, 28 Aug 2025 01:34:13 -0700
From: Yueyang Pan <pyyjason@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, 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 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.
> > 
> 
> 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.

> 
> >  static inline void show_node(struct zone *zone)
> >  {
> >  	if (IS_ENABLED(CONFIG_NUMA))
> > @@ -419,7 +421,7 @@ void __show_mem(unsigned int filter, nodemask_t *nodemask, int max_zone_idx)
> >  	printk("%lu pages hwpoisoned\n", atomic_long_read(&num_poisoned_pages));
> >  #endif
> >  #ifdef CONFIG_MEM_ALLOC_PROFILING
> > -	if (mem_alloc_profiling_enabled()) {
> > +	if (mem_alloc_profiling_enabled() && mutex_trylock(&mem_alloc_profiling_mutex)) {
> >  		struct codetag_bytes tags[10];
> >  		size_t i, nr;
> >  
> > @@ -445,6 +447,7 @@ void __show_mem(unsigned int filter, nodemask_t *nodemask, int max_zone_idx)
> >  						  ct->lineno, ct->function);
> >  			}
> >  		}
> > +		mutex_unlock(&mem_alloc_profiling_mutex);
> >  	}
> 
> If we're going to suppress the usual output then how about we let
> people know this happened, rather than silently dropping it?
> 
> pr_notice("memory allocation output suppressed due to show_mem() contention\n")
> 
> or something like that?

For this point, I am sort of on Shakeel's side. Probably I won't call 
it suppressed as two concurrent printers is actually sharing this 
global information.

Thanks,
Pan