Re: [PATCH mm] mm/page_alloc: Avoid second trylock of zone->lock

[Michal Hocko]

On Sun 30-03-25 17:28:09, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@xxxxxxxxxx>
> 
> spin_trylock followed by spin_lock will cause extra write cache
> access. If the lock is contended it may cause unnecessary cache
> line bouncing and will execute redundant irq restore/save pair.
> Therefore, check alloc/fpi_flags first and use spin_trylock or
> spin_lock.
> 
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Fixes: 97769a53f117 ("mm, bpf: Introduce try_alloc_pages() for opportunistic page allocation")
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>

Makes sense. Fixes tag is probably over reaching but whatever.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
>  mm/page_alloc.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e3ea5bf5c459..ffbb5678bc2f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1268,11 +1268,12 @@ static void free_one_page(struct zone *zone, struct page *page,
>  	struct llist_head *llhead;
>  	unsigned long flags;
>  
> -	if (!spin_trylock_irqsave(&zone->lock, flags)) {
> -		if (unlikely(fpi_flags & FPI_TRYLOCK)) {
> +	if (unlikely(fpi_flags & FPI_TRYLOCK)) {
> +		if (!spin_trylock_irqsave(&zone->lock, flags)) {
>  			add_page_to_zone_llist(zone, page, order);
>  			return;
>  		}
> +	} else {
>  		spin_lock_irqsave(&zone->lock, flags);
>  	}
>  
> @@ -2341,9 +2342,10 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>  	unsigned long flags;
>  	int i;
>  
> -	if (!spin_trylock_irqsave(&zone->lock, flags)) {
> -		if (unlikely(alloc_flags & ALLOC_TRYLOCK))
> +	if (unlikely(alloc_flags & ALLOC_TRYLOCK)) {
> +		if (!spin_trylock_irqsave(&zone->lock, flags))
>  			return 0;
> +	} else {
>  		spin_lock_irqsave(&zone->lock, flags);
>  	}
>  	for (i = 0; i < count; ++i) {
> @@ -2964,9 +2966,10 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
>  
>  	do {
>  		page = NULL;
> -		if (!spin_trylock_irqsave(&zone->lock, flags)) {
> -			if (unlikely(alloc_flags & ALLOC_TRYLOCK))
> +		if (unlikely(alloc_flags & ALLOC_TRYLOCK)) {
> +			if (!spin_trylock_irqsave(&zone->lock, flags))
>  				return NULL;
> +		} else {
>  			spin_lock_irqsave(&zone->lock, flags);
>  		}
>  		if (alloc_flags & ALLOC_HIGHATOMIC)
> -- 
> 2.47.1

-- 
Michal Hocko
SUSE Labs