Re: [PATCH V3] Allow compaction of unevictable pages

[Rik van Riel]

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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