Re: [patch] mm, thp: add new background defrag option

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Jan 04, 2017 at 03:41:59PM -0800, David Rientjes wrote:
> There is no thp defrag option that currently allows MADV_HUGEPAGE regions 
> to do direct compaction and reclaim while all other thp allocations simply 
> trigger kswapd and kcompactd in the background and fail immediately.
> 
> The "defer" setting simply triggers background reclaim and compaction for 
> all regions, regardless of MADV_HUGEPAGE, which makes it unusable for our 
> userspace where MADV_HUGEPAGE is being used to indicate the application is 
> willing to wait for work for thp memory to be available.
> 
> The "madvise" setting will do direct compaction and reclaim for these
> MADV_HUGEPAGE regions, but does not trigger kswapd and kcompactd in the 
> background for anybody else.
> 
> For reasonable usage, there needs to be a mesh between the two options.  
> This patch introduces a fifth mode, "background", that will do direct 
> reclaim and compaction for MADV_HUGEPAGE regions and trigger background 
> reclaim and compaction for everybody else so that hugepages may be 
> available in the near future.
> 
> A proposal to allow direct reclaim and compaction for MADV_HUGEPAGE 
> regions as part of the "defer" mode, making it a very powerful setting and 
> avoids breaking userspace, was offered: 
> http://marc.info/?t=148236612700003.  This additional mode is a 
> compromise.
> 
> This patch also cleans up the helper function for storing to "enabled" 
> and "defrag" since the former supports three modes while the latter 
> supports five and triple_flag_store() was getting unnecessarily messy.
> 
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  I don't understand Mel's suggestion of "defer-fault" as option naming.
> 

defer-fault was intended to reflect "defer faults but not anything else"
with the only sensible alternative being madvise requests. While not a
major fan of the background name, I don't have a better suggestion either
other than defer-fault.

There are likely to be objections based on how this should be specified
and investigating alternative proposals such as fine-grained control of
how background compaction should be done but I hadn't proposed them and
hadn't intended to work on such patches. This patch appears to give the
semantics you want and I said I would ack such a configuration option so;

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs

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