linux-kernel - Re: [PATCH v9 4/7] mm,page_owner: Implement the tracking of the stacks count

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Date: Thu, 15 Feb 2024 12:57:39 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Michal Hocko <mhocko@...e.com>,
	Marco Elver <elver@...gle.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v9 4/7] mm,page_owner: Implement the tracking of the
 stacks count

On Thu, Feb 15, 2024 at 12:08:53PM +0100, Vlastimil Babka wrote:
> On 2/14/24 18:01, Oscar Salvador wrote:
> > Implement {inc,dec}_stack_record_count() which increments or
> > decrements on respective allocation and free operations, via
> > __reset_page_owner() (free operation) and __set_page_owner() (alloc
> > operation).
> > Newly allocated stack_record structs will be added to the list stack_list
> > via add_stack_record_to_list().
> > Modifications on the list are protected via a spinlock with irqs
> > disabled, since this code can also be reached from IRQ context.
> > 
> > Signed-off-by: Oscar Salvador <osalvador@...e.de>
> > Reviewed-by: Marco Elver <elver@...gle.com>
> 
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!


> > +		if (atomic_try_cmpxchg_relaxed(&stack_record->count.refs, &old, 1))
> > +			/* Add the new stack_record to our list */
> > +			add_stack_record_to_list(stack_record, gfp_mask);
> 			
> 			Not returning here...
> 
> > +	}
> > +	refcount_inc(&stack_record->count);
> 
> ... means we'll increase the count to 2 on the first store, so there's a
> bias. Which would be consistent with the failure and dummy stacks that also
> start with a refcount of 1. But then the stack count reporting should
> decrement by 1 to prevent confusion? (in the following patch). Imagine
> somebody debugging an allocation stack where there are not so many of them,
> but the allocation is large, and being sidetracked by an off-by-one error.

Good catch Vlastimil!
Yes, we should substract one from the total count in stack_print.

-- 
Oscar Salvador
SUSE Labs