linux-kernel - Re: [PATCH 7/7] mm: compaction: Introduce sync-light migration for use by compaction

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Date:	Wed, 23 Nov 2011 22:35:37 +0800
From:	Nai Xia <nai.xia@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Shaohua Li <shaohua.li@...el.com>, Linux-MM <linux-mm@...ck.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>, Jan Kara <jack@...e.cz>,
	Andy Isaacson <adi@...apodia.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] mm: compaction: Introduce sync-light migration for
 use by compaction

On Wed, Nov 23, 2011 at 9:45 PM, Mel Gorman <mgorman@...e.de> wrote:
> On Wed, Nov 23, 2011 at 09:05:08PM +0800, Nai Xia wrote:
>> > <SNIP>
>> >
>> > Where are you adding this check?
>> >
>> > If you mean in __unmap_and_move(), the check is unnecessary unless
>> > another subsystem starts using sync-light compaction. With this series,
>> > only direct compaction cares about MIGRATE_SYNC_LIGHT. If the page is
>>
>> But I am still a little bit confused that if MIGRATE_SYNC_LIGHT is only
>> used by direct compaction and  another mode can be used by it:
>> MIGRATE_ASYNC also does not write dirty pages, then why not also
>> do an (current->flags & PF_MEMALLOC) test before writing out pages,
>
> Why would it be necessary?
> Why would it be better than what is there now?

I mean, if MIGRATE_SYNC_LIGHT -->  (current->flags & PF_MEMALLOC),
and MIGRATE_SYNC_LIGHT --> no dirty writeback, and
 (current->flags & PF_MEMALLOC)
---> (MIGRATE_SYNC_LIGHT || MIGRATE_ASYNC)
and   MIGRATE_ASYNC --> no dirty writeback, then
why not simply  (current->flags & PF_MEMALLOC) ---> no dirty writeback
and keep the sync meaning as it was?

Hoping I get myself clear this time......

>
>> like we already did for the page lock condition, but adding a new mode
>> instead?
>>
>
> I'm afraid I am missing the significance of your question or how it
> might apply to the problem at hand.

Sorry, It always takes some effort for me to get myself understood
when expressing a complicated thing. That's always my fault ;)

Thanks,
Nai

>
> --
> Mel Gorman
> SUSE Labs
>
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