linux-kernel - Re: [PATCH -mm] remove __GFP_NO

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Message-ID: <501039F6.7010702@redhat.com>
Date:	Wed, 25 Jul 2012 14:24:54 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH -mm] remove __GFP_NO_KSWAPD

On 07/24/2012 07:34 PM, Minchan Kim wrote:
> Hi Rik,
>
> On Tue, Jul 24, 2012 at 11:12:22AM -0400, Rik van Riel wrote:
>> When transparent huge pages were introduced, memory compaction and
>> swap storms were an issue, and the kernel had to be careful to not
>> make THP allocations cause pageout or compaction.
>>
>> Now that we have working compaction deferral, kswapd is smart enough
>> to invoke compaction and the quadratic behaviour around isolate_free_pages
>> has been fixed, it should be safe to remove __GFP_NO_KSWAPD.
>
> Could you point out specific patches you mentiond which makes kswapd/compaction
> smart? It will make description very clear.

That could be a list of 50+ patches, merged over the
last two or so years.

In other words, such a large amount of data that it
is unlikely to clarify the discussion...

>> Signed-off-by: Rik van Riel <riel@...hat.com>
>
> I support it because I had a concern about that flags which is likely to be
> used by other subsystems without careful thinking when the flag was introduced.
> It's proved by mtd_kmalloc_up_to which was merged with sneaking without catching
> from mm guys's eyes. When I read comment of that function, it seems to be proper
> usage but I don't like it because it requries users of mm to know mm internal
> like kswapd. So it should be avoided if possible.
>
> Plus, it means you need to fix it with show_gfp_flags. :)

Ohh, a place I forgot to grep!

I'll send in an incremental patch right now.

>> ---
>> This has been running fine on my system for a while, but my system
>> only has 12GB and moderate memory pressure. I propose we keep this
>> in -mm and -next for a while, and merge it for 3.7 if nobody complains.
>
> Yes. it should be very careful.
> I guess Mel and Andrea would have opinions and benchmark.

It's not as much benchmarks that I am worried about,
but somebody running something unexpected on their
system.


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