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Date: Tue, 19 Mar 2024 16:24:30 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Oscar Salvador <osalvador@suse.de>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Michal Hocko
 <mhocko@suse.com>, Vlastimil Babka <vbabka@suse.cz>, Marco Elver
 <elver@google.com>, Andrey Konovalov <andreyknvl@gmail.com>, Alexander
 Potapenko <glider@google.com>, Tetsuo Handa
 <penguin-kernel@i-love.sakura.ne.jp>,
 syzbot+41bbfdb8d41003d12c0f@syzkaller.appspotmail.com
Subject: Re: [PATCH v2 1/2] mm,page_owner: Fix refcount imbalance
Message-Id: <20240319162430.3e76504d06f12edc107029a4@linux-foundation.org>
In-Reply-To: <20240319183212.17156-2-osalvador@suse.de>
References: <20240319183212.17156-1-osalvador@suse.de>
	<20240319183212.17156-2-osalvador@suse.de>
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On Tue, 19 Mar 2024 19:32:11 +0100 Oscar Salvador <osalvador@suse.de> wrote:

> Current code does not contemplate scenarios were an allocation and
> free operation on the same pages do not handle it in the same amount
> at once.
> To give an example, page_alloc_exact(), where we will allocate a page
> of enough order to stafisfy the size request, but we will free the
> remainings right away.
> 
> In the above example, we will increment the stack_record refcount
> only once, but we will decrease it the same number of times as number
> of unused pages we have to free.
> This will lead to a warning because of refcount imbalance.
> 
> Fix this by recording the number of base pages in the refcount field.
> 
> ...
>
> -static void dec_stack_record_count(depot_stack_handle_t handle)
> +static void dec_stack_record_count(depot_stack_handle_t handle,
> +				   int nr_base_pages)
>  {
>  	struct stack_record *stack_record = __stack_depot_get_stack_record(handle);
>  
>  	if (stack_record)
> -		refcount_dec(&stack_record->count);
> +		refcount_sub_and_test(nr_base_pages, &stack_record->count);
>  }

mm/page_owner.c: In function 'dec_stack_record_count':
mm/page_owner.c:226:17: error: ignoring return value of 'refcount_sub_and_test' declared with attribute 'warn_unused_result' [-Werror=unused-result]
  226 |                 refcount_sub_and_test(nr_base_pages, &stack_record->count);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors