Re: [PATCH V6] Allow compaction of unevictable pages

[Vlastimil Babka]

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@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Thanks.

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