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

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Message-Id: <20250827150619.4e468e68988f224f9f9bea6f@linux-foundation.org>
Date: Wed, 27 Aug 2025 15:06:19 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Yueyang Pan <pyyjason@...il.com>
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, 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.

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.

>  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?