linux-kernel - Re: [PATCH -mm -v4 01/21] mm, THP, swap: Enable PMD swap operations for CONFIG_THP

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Message-ID: <871scbkicl.fsf@yhuang-dev.intel.com>
Date:   Tue, 10 Jul 2018 09:08:42 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "Shaohua Li" <shli@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [PATCH -mm -v4 01/21] mm, THP, swap: Enable PMD swap operations for CONFIG_THP_SWAP

Dave Hansen <dave.hansen@...ux.intel.com> writes:

> On 06/21/2018 08:51 PM, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@...el.com>
>> 
>> Previously, the PMD swap operations are only enabled for
>> CONFIG_ARCH_ENABLE_THP_MIGRATION.  Because they are only used by the
>> THP migration support.  We will support PMD swap mapping to the huge
>> swap cluster and swapin the THP as a whole.  That will be enabled via
>> CONFIG_THP_SWAP and needs these PMD swap operations.  So enable the
>> PMD swap operations for CONFIG_THP_SWAP too.
>
> This commit message kinda skirts around the real reasons for this patch.
>  Shouldn't we just say something like:
>
> 	Currently, "swap entries" in the page tables are used for a
> 	number of things outside of actual swap, like page migration.
> 	We support THP/PMD "swap entries" for page migration currently
> 	and the functions behind this are tied to page migration's
> 	config option (CONFIG_ARCH_ENABLE_THP_MIGRATION).
>
> 	But, we also need them for THP swap.
> 	...
>
> It would also be nice to explain a bit why you are moving code around.

This looks much better than my original words.  Thanks for help!

> Would this look any better if we made a Kconfig option:
>
> 	config HAVE_THP_SWAP_ENTRIES
> 		def_bool n
> 		# "Swap entries" in the page tables are used
> 		# both for migration and actual swap.
> 		depends on THP_SWAP || ARCH_ENABLE_THP_MIGRATION
>
> You logically talked about this need for PMD swap operations in your
> commit message, so I think it makes sense to codify that in a single
> place where it can be coherently explained.

Because both you and Dan thinks it's better to add new Kconfig option, I
will do that.

Best Regards,
Huang, Ying