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

[Yueyang Pan <pyyjason@gmail.com>]

On Wed, Aug 27, 2025 at 03:28:41PM -0700, Shakeel Butt 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@gmail.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.
> 
> +1, something static to __show_mem().

Thanks for your feedback, Shakeel. See my reply to Andrew for this.

> 
> > 
> > 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.
> 
> +1

Again, see my reply to Andrew.

> 
> > 
> > >  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?
> 
> Personally I think this is not needed as this patch is suppressing only
> the memory allocation profiling output which is global, will be same
> for all the consumers and context does not matter. All consumers will
> get the memory allocation profiling data eventually.

For this point, I sort of agree with you. Wait for others' opinions?

Thanks
Pan