linux-kernel - Re: [PATCH V5] Allow compaction of unevictable pages

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Message-ID: <55037631.3010402@redhat.com>
Date:	Fri, 13 Mar 2015 19:43:45 -0400
From:	Rik van Riel <riel@...hat.com>
To:	David Rientjes <rientjes@...gle.com>,
	Eric B Munson <emunson@...mai.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Lameter <cl@...ux.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.cz>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5] Allow compaction of unevictable pages

On 03/13/2015 07:18 PM, David Rientjes wrote:
> On Fri, 13 Mar 2015, Eric B Munson wrote:
> 
>>>> --- a/mm/compaction.c
>>>> +++ b/mm/compaction.c
>>>> @@ -1046,6 +1046,8 @@ typedef enum {
>>>>  	ISOLATE_SUCCESS,	/* Pages isolated, migrate */
>>>>  } isolate_migrate_t;
>>>>  
>>>> +int sysctl_compact_unevictable;
>>>> +
>>>>  /*
>>>>   * Isolate all pages that can be migrated from the first suitable block,
>>>>   * starting at the block pointed to by the migrate scanner pfn within
>>>
>>> I suspect that the use cases where users absolutely do not want
>>> unevictable pages migrated are special cases, and it may make
>>> sense to enable sysctl_compact_unevictable by default.
>>
>> Given that sysctl_compact_unevictable=0 is the way the kernel behaves
>> now and the push back against always enabling compaction on unevictable
>> pages, I left the default to be the behavior as it is today.  I agree
>> that this is likely the minority case, but I'd really like Peter Z or
>> someone else from real time to say that they are okay with the default
>> changing.
>>
> 
> It would be really disappointing to not enable this by default for !rt 
> kernels.  We haven't migrated mlocked pages in the past by way of memory 
> compaction because it can theoretically result in consistent minor page 
> faults, but I haven't yet heard a !rt objection to enabling this.
> 
> If the rt patchset is going to carry a patch to disable this

It does not have to carry a patch to disable something that can be
disabled at run time.

The smaller the realtime patchset has to be, the better.

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