linux-kernel - Re: [PATCH] mm, page_alloc: reset the zone->watermark

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Message-Id: <20200511131155.0b40ee443c3367e8f748b16f@linux-foundation.org>
Date:   Mon, 11 May 2020 13:11:55 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Charan Teja Reddy <charante@...eaurora.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        vinmenon@...eaurora.org
Subject: Re: [PATCH] mm, page_alloc: reset the zone->watermark_boost early

On Mon, 11 May 2020 19:10:08 +0530 Charan Teja Reddy <charante@...eaurora.org> wrote:

> Updating the zone watermarks by any means, like extra_free_kbytes,
> min_free_kbytes, water_mark_scale_factor e.t.c, when watermark_boost is
> set will result into the higher low and high watermarks than the user
> asks. This can be avoided by resetting the zone->watermark_boost to zero
> early.

Does this solve some problem which has been observed in testing?

> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7746,9 +7746,9 @@ static void __setup_per_zone_wmarks(void)
>  			    mult_frac(zone_managed_pages(zone),
>  				      watermark_scale_factor, 10000));
>  
> +		zone->watermark_boost = 0;
>  		zone->_watermark[WMARK_LOW]  = min_wmark_pages(zone) + tmp;
>  		zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
> -		zone->watermark_boost = 0;
>  
>  		spin_unlock_irqrestore(&zone->lock, flags);
>  	}

This could only be a problem if code is accessing these things without
holding zone->lock.  Is that ever the case?