linux-kernel - Re: [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check in rmqueue

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Date:   Thu, 19 May 2022 12:57:01 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check
 in rmqueue

On 5/12/22 10:50, Mel Gorman wrote:
> The VM_BUG_ON check for a valid page can be avoided with a simple
> change in the flow. The ZONE_BOOSTED_WATERMARK is unlikely in general
> and even more unlikely if the page allocation failed so mark the
> branch unlikely.

Hm, so that makes a DEBUG_VM config avoid the check. On the other hand,
it puts it on the path returning from rmqueue_pcplist() for all configs,
and that should be the fast path. So unless things further change in the
following patches, it doesn't seem that useful?

> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> Tested-by: Minchan Kim <minchan@...nel.org>
> Acked-by: Minchan Kim <minchan@...nel.org>
> ---
>  mm/page_alloc.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1c4c54503a5d..b543333dce8f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3765,17 +3765,18 @@ struct page *rmqueue(struct zone *preferred_zone,
>  
>  	page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags,
>  							migratetype);
> -	if (unlikely(!page))
> -		return NULL;
>  
>  out:
>  	/* Separate test+clear to avoid unnecessary atomics */
> -	if (test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags)) {
> +	if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) {
>  		clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
>  		wakeup_kswapd(zone, 0, 0, zone_idx(zone));
>  	}
>  
> -	VM_BUG_ON_PAGE(page && bad_range(zone, page), page);
> +	if (unlikely(!page))
> +		return NULL;
> +
> +	VM_BUG_ON_PAGE(bad_range(zone, page), page);
>  	return page;
>  }
>