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

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Date:   Mon, 09 Jul 2018 13:40:45 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <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>, <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        <n-horiguchi@...jp.nec.com>, <zi.yan@...rutgers.edu>,
        <daniel.m.jordan@...cle.com>
Subject: Re: [PATCH -mm -v4 01/21] mm, THP, swap: Enable PMD swap operations for CONFIG_THP_SWAP

Dan Williams <dan.j.williams@...el.com> writes:

> On Thu, Jun 21, 2018 at 8:55 PM Huang, Ying <ying.huang@...el.com> 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.
>>
>> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
>> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>> Cc: Andrea Arcangeli <aarcange@...hat.com>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: Shaohua Li <shli@...nel.org>
>> Cc: Hugh Dickins <hughd@...gle.com>
>> Cc: Minchan Kim <minchan@...nel.org>
>> Cc: Rik van Riel <riel@...hat.com>
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>> Cc: Zi Yan <zi.yan@...rutgers.edu>
>> Cc: Daniel Jordan <daniel.m.jordan@...cle.com>
>> ---
>>  arch/x86/include/asm/pgtable.h |  2 +-
>>  include/asm-generic/pgtable.h  |  2 +-
>>  include/linux/swapops.h        | 44 ++++++++++++++++++++++--------------------
>>  3 files changed, 25 insertions(+), 23 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>> index 99ecde23c3ec..13bf58838daf 100644
>> --- a/arch/x86/include/asm/pgtable.h
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -1224,7 +1224,7 @@ static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
>>         return pte_clear_flags(pte, _PAGE_SWP_SOFT_DIRTY);
>>  }
>>
>> -#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
>> +#if defined(CONFIG_ARCH_ENABLE_THP_MIGRATION) || defined(CONFIG_THP_SWAP)
>
> How about introducing a new config symbol representing the common
> infrastructure between the two and have them select that symbol.

The common infrastructure shared by two mechanisms is PMD swap entry.
But I didn't find there are many places where the common infrastructure
is used.  So I think it may be over-engineering to introduce a new
config symbol but use it for so few times.

> Would that also allow us to clean up the usage of
> CONFIG_ARCH_ENABLE_THP_MIGRATION in fs/proc/task_mmu.c? In other
> words, what's the point of having nice ifdef'd alternatives in header
> files when ifdefs are still showing up in C files, all of it should be
> optionally determined by header files.

Unfortunately, I think it is not a easy task to wrap all C code via
#ifdef in header files.  And it may be over-engineering to wrap them
all.  I guess this is why there are still some #ifdef in C files.

Best Regards,
Huang, Ying