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:   Sat, 01 Apr 2017 11:29:41 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        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

Johannes Weiner <hannes@...xchg.org> writes:

> On Thu, Mar 30, 2017 at 08:45:56AM +0800, Huang, Ying wrote:
>> Johannes Weiner <hannes@...xchg.org> writes:
>> 
>> > 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
>> 
>> The user will not see this, because there is no string after "bool" to
>> let user to select it.  The help here is for document only, so that
>> architecture developers could know what this is for.
>
> Oh, I missed that. My bad!
>
>> > config THP_SWAP_CLUSTER
>> >      bool
>> >      default n
>> >
>> > and let the archs with reasonable THP sizes select it.
>> 
>> This will have same effect as the original solution except the document
>> is removed.
>
> Then I still don't understand why we need two config symbols. Can't
> archs select the documented THP_SWAP_CLUSTER directly?
>
> The #ifdef in swapfile.c could check THP && THP_SWAP_CLUSTER.
>
> Am I missing something?

I use two config symbols just to save some typing, instead of

#ifdef CONFIG_THP_SWAP_CLUSTER

it will be,

#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_THP_SWAP_CLUSTER)

or

#if defined(CONFIG_SWAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_THP_SWAP_CLUSTER)

Best Regards,
Huang, Ying