Re: [patch v2] mm, thp: add new defer+madvise defrag option

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Jan 10, 2017 at 04:15:27PM -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, "defer+madvise", 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.
> 
> A second proposal to allow both "defer" and "madvise" to be selected at
> the same time was also offered: http://marc.info/?t=148357345300001.
> This is possible, but there was a concern that it might break existing
> userspaces the parse the output of the defrag mode, so the fifth option
> was introduced instead.
> 
> 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>
> ---
>  v2: uses new naming suggested by Vlastimil
>      (defer+madvise order looks better in
>       "... defer defer+madvise madvise ...")
> 
>  v1 was acked by Mel, and it probably could have been preserved but it was
>  removed in case there is an issue with the name change.
> 

There isn't

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

Thanks.

-- 
Mel Gorman
SUSE Labs

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