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

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Date:   Mon, 9 Jul 2018 08:59:20 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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

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.

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.