linux-kernel - Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration

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Date:	Fri, 18 Jul 2014 17:23:12 +0900
From:	Gioh Kim <gioh.kim@....com>
To:	Marek Szyprowski <m.szyprowski@...sung.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	'김준수' <iamjoonsoo.kim@....com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Minchan Kim <minchan@...nel.org>
CC:	Michal Nazarewicz <mina86@...a86.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, 이건호 <gunho.lee@....com>,
	'Chanho Min' <chanho.min@....com>
Subject: Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration



2014-07-18 오후 4:50, Marek Szyprowski 쓴 글:
> Hello,
>
> On 2014-07-18 08:45, Gioh Kim wrote:
>> For page migration of CMA, buffer-heads of lru should be dropped.
>> Please refer to https://lkml.org/lkml/2014/7/4/101 for the history.
>>
>> I have two solution to drop bhs.
>> One is invalidating entire lru.
>> Another is searching the lru and dropping only one bh that Laura proposed
>> at https://lkml.org/lkml/2012/8/31/313.
>>
>> I'm not sure which has better performance.
>> So I did performance test on my cortex-a7 platform with Lmbench
>> that has "File & VM system latencies" test.
>> I am attaching the results.
>> The first line is of invalidating entire lru and the second is dropping selected bh.
>>
>> File & VM system latencies in microseconds - smaller is better
>> -------------------------------------------------------------------------------
>> Host                 OS   0K File      10K File     Mmap    Prot   Page   100fd
>>                          Create Delete Create Delete Latency Fault  Fault  selct
>> --------- ------------- ------ ------ ------ ------ ------- ----- ------- -----
>> 10.178.33 Linux 3.10.19   25.1   19.6   32.6   19.7  5098.0 0.666 3.45880 6.506
>> 10.178.33 Linux 3.10.19   24.9   19.5   32.3   19.4  5059.0 0.563 3.46380 6.521
>>
>>
>> I tried several times but the result tells that they are the same under 1% gap
>> except Protection Fault.
>> But the latency of Protection Fault is very small and I think it has little effect.
>>
>> Therefore we can choose anything but I choose invalidating entire lru.
>> The try_to_free_buffers() which is calling drop_buffers() is called by many filesystem code.
>> So I think inserting codes in drop_buffers() can affect the system.
>> And also we cannot distinguish migration type in drop_buffers().
>>
>> In alloc_contig_range() we can distinguish migration type and invalidate lru if it needs.
>> I think alloc_contig_range() is proper to deal with bh like following patch.
>>
>> Laura, can I have you name on Acked-by line?
>> Please let me represent my thanks.
>>
>> Thanks for any feedback.
>>
>> ------------------------------- 8< ----------------------------------
>>
>> >From 33c894b1bab9bc26486716f0c62c452d3a04d35d Mon Sep 17 00:00:00 2001
>> From: Gioh Kim <gioh.kim@....com>
>> Date: Fri, 18 Jul 2014 13:40:01 +0900
>> Subject: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration
>>
>> The bh must be free to migrate a page at which bh is mapped.
>> The reference count of bh is increased when it is installed
>> into lru so that the bh of lru must be freed before migrating the page.
>>
>> This frees every bh of lru. We could free only bh of migrating page.
>> But searching lru costs more than invalidating entire lru.
>>
>> Signed-off-by: Gioh Kim <gioh.kim@....com>
>> Acked-by: Laura Abbott <lauraa@...eaurora.org>
>> ---
>>   mm/page_alloc.c |    3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index b99643d4..3b474e0 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -6369,6 +6369,9 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>>          if (ret)
>>                  return ret;
>>
>> +       if (migratetype == MIGRATE_CMA || migratetype == MIGRATE_MOVABLE)
>
> I'm not sure if it really makes sense to check the migratetype here. This check
> doesn't add any new information to the code and make false impression that this
> function can be called for other migratetypes than CMA or MOVABLE. Even if so,
> then invalidating bh_lrus unconditionally will make more sense, IMHO.

I agree. I cannot understand why alloc_contig_range has an argument of migratetype.
Can the alloc_contig_range is called for other migrate type than CMA/MOVABLE?

What do you think about removing the argument of migratetype and
checking migratetype (if (migratetype == MIGRATE_CMA || migratetype == MIGRATE_MOVABLE))?


>
>> +               invalidate_bh_lrus();
>> +
>>          ret = __alloc_contig_migrate_range(&cc, start, end);
>>          if (ret)
>>                  goto done;
>> --
>> 1.7.9.5
>>
>
> Best regards
--
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