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

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Message-ID: <87y3voyjj1.fsf@yhuang-dev.intel.com>
Date:   Wed, 29 Mar 2017 09:10:58 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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

"Kirill A. Shutemov" <kirill@...temov.name> writes:

> On Tue, Mar 28, 2017 at 01:32:01PM +0800, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@...el.com>
>> 
>> In this patch, the size of the swap cluster is changed to that of the
>> THP (Transparent Huge Page) on x86_64 architecture (512).  This is for
>> the THP swap support on x86_64.  Where one swap cluster will be used to
>> hold the contents of each THP swapped out.  And some information of the
>> swapped out THP (such as compound map count) will be recorded in the
>> swap_cluster_info data structure.
>> 
>> For other architectures which want THP swap support,
>> ARCH_USES_THP_SWAP_CLUSTER need to be selected in the Kconfig file for
>> the architecture.
>
> Intreseting case could be architecture with HPAGE_PMD_NR < 256.
> Can current code pack more than one THP per claster.

No.  Only one THP for each swap cluster is supported.  But in current
implementation, if HPAGE_PMD_NR < 256, the swap cluster will be < 256
too.  The size of swap cluster will be exact same as HPAGE_PMD_NR.

Best Regards,
Huang, Ying

> If not we need to have BUILG_BUG_ON() to catch attempt of such enabling.