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Message-ID: <11735e2e-781f-492f-7a1a-71b91e0876dc@linux.intel.com>
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.
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