Re: [PATCH V3] Allow compaction of unevictable pages

From: Rik van Riel
Date: Mon Mar 09 2015 - 16:52:36 EST

On 03/09/2015 04:48 PM, Eric B Munson wrote:
Currently, pages which are marked as unevictable are protected from
compaction, but not from other types of migration. The mlock
desctription does not promise that all page faults will be avoided, only
major ones so this protection is not necessary. This extra protection
can cause problems for applications that are using mlock to avoid
swapping pages out, but require order > 0 allocations to continue to
succeed in a fragmented environment. This patch removes the
ISOLATE_UNEVICTABLE mode and the check for it in __isolate_lru_page().
Removing this check allows the removal of the isolate_mode argument from
isolate_migratepages_block() because it can compute the required mode
from the compact_control structure.

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. Without
this patch I am unable to allocate any huge pages after fragmenting
memory. With it, I can allocate almost all the space freed by unmapping
as huge pages.

Signed-off-by: Eric B Munson <emunson@xxxxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx

Acked-by: Rik van Riel <riel@xxxxxxxxxx>

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