linux-kernel - Re: [PATCH v2 1/2] mm,page_owner: Fix refcount imbalance

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Message-ID: <71e4b82b-8f32-41f1-afd1-5238e88bf0e7@suse.cz>
Date: Thu, 21 Mar 2024 11:36:57 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Oscar Salvador <osalvador@...e.de>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: 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>,
 Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
 syzbot+41bbfdb8d41003d12c0f@...kaller.appspotmail.com
Subject: Re: [PATCH v2 1/2] mm,page_owner: Fix refcount imbalance

On 3/19/24 19:32, Oscar Salvador 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.
> 
> Reported-by: syzbot+41bbfdb8d41003d12c0f@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-mm/00000000000090e8ff0613eda0e5@google.com
> Fixes: 217b2119b9e2 ("mm,page_owner: implement the tracking of the stacks count")
> Signed-off-by: Oscar Salvador <osalvador@...e.de>

With the fixup,

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

But I think you'll need to resend with the missing hunk already applied, it
had broken whitespace in your email and IIUC this is was dropped from mm tree.

Also I'd suggest a change:

> +++ b/mm/page_owner.c
> @@ -196,9 +196,11 @@ static void add_stack_record_to_list(struct stack_record *stack_record,
>  	spin_unlock_irqrestore(&stack_list_lock, flags);
>  }
>  
> -static void inc_stack_record_count(depot_stack_handle_t handle, gfp_t gfp_mask)
> +static void inc_stack_record_count(depot_stack_handle_t handle, gfp_t gfp_mask,
> +				   int nr_base_pages)
>  {
>  	struct stack_record *stack_record = __stack_depot_get_stack_record(handle);
> +	int old = REFCOUNT_SATURATED;
>  
>  	if (!stack_record)
>  		return;
> @@ -210,22 +212,18 @@ static void inc_stack_record_count(depot_stack_handle_t handle, gfp_t gfp_mask)
>  	 * Since we do not use STACK_DEPOT_FLAG_GET API, let us
>  	 * set a refcount of 1 ourselves.
>  	 */
> -	if (refcount_read(&stack_record->count) == REFCOUNT_SATURATED) {
> -		int old = REFCOUNT_SATURATED;

I think this was useful optimization in that most cases the count is not
REFCOUNT_SATURATED so we don't have to go for the expensive cmpxchg all the
time. Or do I miss a reason why this was changed?

> -
> -		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);
> -	}
> -	refcount_inc(&stack_record->count);
> +	if (atomic_try_cmpxchg_relaxed(&stack_record->count.refs, &old, 1))
> +		add_stack_record_to_list(stack_record, gfp_mask);
> +	refcount_add(nr_base_pages, &stack_record->count);
>  }
>