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

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Message-ID: <Z-tdWXcv1Zqim2ng@harry>
Date: Tue, 1 Apr 2025 12:28:25 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, bpf@...r.kernel.org,
        daniel@...earbox.net, andrii@...nel.org, martin.lau@...nel.org,
        akpm@...ux-foundation.org, peterz@...radead.org, vbabka@...e.cz,
        bigeasy@...utronix.de, rostedt@...dmis.org, shakeel.butt@...ux.dev,
        mhocko@...e.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm] mm/page_alloc: Avoid second trylock of zone->lock

On Sun, Mar 30, 2025 at 05:28:09PM -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> 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@...ux-foundation.org>
> Fixes: 97769a53f117 ("mm, bpf: Introduce try_alloc_pages() for opportunistic page allocation")
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

>  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
> 
> 

-- 
Cheers,
Harry (formerly known as Hyeonggon)