linux-kernel - Re: [PATCH] mm: do not consider SWAP to calculate available when not necessary

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Date:   Sun, 25 Nov 2018 18:01:15 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Yang Yang <yang.yang29@....com.cn>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        pavel.tatashin@...rosoft.com, vbabka@...e.cz, osalvador@...e.de,
        rppt@...ux.vnet.ibm.com, iamjoonsoo.kim@....com,
        alexander.h.duyck@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, zhong.weidong@....com.cn,
        wang.yi59@....com.cn
Subject: Re: [PATCH] mm: do not consider SWAP to calculate available when not
 necessary

On Mon, Nov 26, 2018 at 07:58:23AM +0800, Yang Yang wrote:
> When si_mem_available() calculates 'available', it takes SWAP
> into account. But if CONFIG_SWAP is N or SWAP is off(some embedded system
> would like to do that), there is no need to consider it.

I don't understand this patch.  The pagecache can be written back to
storage if it is dirty, regardless of whether there is swap space.

> @@ -4724,9 +4726,13 @@ long si_mem_available(void)
>  	 * Not all the page cache can be freed, otherwise the system will
>  	 * start swapping. Assume at least half of the page cache, or the
>  	 * low watermark worth of cache, needs to stay.
> +	 * But if CONFIG_SWAP is N or SWAP is off, do not consider it.
>  	 */
>  	pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
> -	pagecache -= min(pagecache / 2, wmark_low);
> +#ifdef CONFIG_SWAP
> +	if (i.totalswap > 0)
> +		pagecache -= min(pagecache / 2, wmark_low);
> +#endif
>  	available += pagecache;
>  
>  	/*
> -- 
> 2.15.2
>