Re: [PATCH V6] Allow compaction of unevictable pages

[Eric B Munson <emunson@akamai.com>]

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On 03/19/2015 10:56 AM, Vlastimil Babka wrote:
> On 03/19/2015 02:57 PM, Eric B Munson wrote:
>> Currently, pages which are marked as unevictable are protected
>> from compaction, but not from other types of migration.  The
>> POSIX real time extension explicitly states that mlock() will
>> prevent a major page fault, but the spirit of is is that mlock()
>> should give a process the ability to control sources of latency,
>> including minor page faults. However, the mlock manpage only
>> explicitly says that a locked page will not be written to swap
>> and this can cause some confusion.  The compaction code today,
>> does not give a developer who wants to avoid swap but wants to
>> have large contiguous areas available any method to achieve this
>> state.  This patch introduces a sysctl for controlling
>> compaction behavoir with respect to the unevictable lru.  Users
>> that demand no page
> 
> behavior
> 
>> faults after a page is present can set compact_unevictable to 0
>> and
> 
> compact_unevictable_allowed
> 
>> users who need the large contiguous areas can enable compaction
>> on locked memory by leaving the default value of 1.
>> 
>> To illustrate this problem I wrote a quick test program that
>> mmaps a large number of 1MB files filled with random data.  These
>> maps are created locked and read only.  Then every other mmap is
>> unmapped and I attempt to allocate huge pages to the static huge
>> page pool.  When the compact_unevictable sysctl is 0, I cannot
>> allocate hugepages after
> 
> compact_unevictable_allowed
> 
>> fragmenting memory.  When the value is set to 1, allocations
>> succeed.
>> 
>> Signed-off-by: Eric B Munson <emunson@akamai.com> Cc: Vlastimil
>> Babka <vbabka@suse.cz>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 
> Thanks.
> 

Thanks, I have a version with the changelog fixed up to actually make
sense and can submit that if the patch is acceptable otherwise.

Eric

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