Re: [PATCH 08/11] mm/page_alloc: Explicitly acquire the zone lock in __free_pages_ok

[Vlastimil Babka]

On 4/14/21 3:39 PM, Mel Gorman wrote:
> __free_pages_ok() disables IRQs before calling a common helper
> free_one_page() that acquires the zone lock. This is not safe according
> to Documentation/locking/locktypes.rst and in this context, IRQ disabling
> is not protecting a per_cpu_pages structure either or a local_lock would
> be used.
> 
> This patch explicitly acquires the lock with spin_lock_irqsave instead of
> relying on a helper. This removes the last instance of local_irq_save()
> in page_alloc.c.

\o/

> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/page_alloc.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1ed370668e7f..6791e9361076 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1559,21 +1559,18 @@ static void __free_pages_ok(struct page *page, unsigned int order,
>  	unsigned long flags;
>  	int migratetype;
>  	unsigned long pfn = page_to_pfn(page);
> +	struct zone *zone = page_zone(page);
>  
>  	if (!free_pages_prepare(page, order, true))
>  		return;
>  
>  	migratetype = get_pfnblock_migratetype(page, pfn);
>  
> -	/*
> -	 * TODO FIX: Disable IRQs before acquiring IRQ-safe zone->lock
> -	 * and protect vmstat updates.
> -	 */
> -	local_irq_save(flags);
> +	spin_lock_irqsave(&zone->lock, flags);
>  	__count_vm_events(PGFREE, 1 << order);
> -	free_one_page(page_zone(page), page, pfn, order, migratetype,
> -		      fpi_flags);
> -	local_irq_restore(flags);
> +	migratetype = check_migratetype_isolated(zone, page, pfn, migratetype);
> +	__free_one_page(page, pfn, zone, order, migratetype, fpi_flags);
> +	spin_unlock_irqrestore(&zone->lock, flags);
>  }
>  
>  void __free_pages_core(struct page *page, unsigned int order)
>