linux-kernel - Re: [PATCH -mm -v7 1/9] mm, swap: Make swap cluster size same of THP size on x86

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Date:   Wed, 29 Mar 2017 12:55:22 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Shaohua Li <shli@...nel.org>, Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH -mm -v7 1/9] mm, swap: Make swap cluster size same of THP
 size on x86_64

On Tue, Mar 28, 2017 at 01:32:01PM +0800, Huang, Ying wrote:
> @@ -499,6 +499,19 @@ config FRONTSWAP
>  
>  	  If unsure, say Y to enable frontswap.
>  
> +config ARCH_USES_THP_SWAP_CLUSTER
> +	bool
> +	default n

This is fine.

> +config THP_SWAP_CLUSTER
> +	bool
> +	depends on SWAP && TRANSPARENT_HUGEPAGE && ARCH_USES_THP_SWAP_CLUSTER
> +	default y
> +	help
> +	  Use one swap cluster to hold the contents of the THP
> +	  (Transparent Huge Page) swapped out.  The size of the swap
> +	  cluster will be same as that of THP.

But this is a super weird thing to ask the user. How would they know
what to say, if we don't know? I don't think this should be a config
knob at all. Merge the two config items into a simple

config THP_SWAP_CLUSTER
     bool
     default n

and let the archs with reasonable THP sizes select it.