linux-kernel - Re: [PATCH] mm/vmscan.c: no need to double-check if free pages are under high-watermark

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Date:   Wed, 5 Jan 2022 16:17:06 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     skseofh@...il.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] mm/vmscan.c: no need to double-check if free pages are
 under high-watermark

On Sun,  2 Jan 2022 12:31:29 +0900 skseofh@...il.com wrote:

> From: Daero Lee <skseofh@...il.com>
> 
> In kswapd_try_to_sleep function, to check whether kswapd can sleep,
> the prepare_kswapd_sleep function is called twice.
> 
> If free pages are below high-watermark in the first call,
> the @remaining variable is not updated at 0 and the
> prepare_kswapd_sleep function is called for the second time.
> 
> I think it is necessary to set the initial value of the
> @remaining to a non-zero value to prevent consecutive calls
> to the same function.

Seems right.  Mel & Joonsoo, could you please check?

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4331,7 +4331,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
>  	/*
>  	 * Return the order kswapd stopped reclaiming at as
>  	 * prepare_kswapd_sleep() takes it into account. If another caller
> -	 * entered the allocator slow path while kswapd was awake, order will
> +	 * entered the allqocator slow path while kswapd was awake, order will

whoops.

>  	 * remain at the higher level.
>  	 */
>  	return sc.order;
> @@ -4355,7 +4355,7 @@ static enum zone_type kswapd_highest_zoneidx(pg_data_t *pgdat,
>  static void kswapd_try_to_sleep(pg_data_t *pgdat, int alloc_order, int reclaim_order,
>  				unsigned int highest_zoneidx)
>  {
> -	long remaining = 0;
> +	long remaining = ~0;
>  	DEFINE_WAIT(wait);
>  
>  	if (freezing(current) || kthread_should_stop())