Re: [PATCH 08/11] mm, compaction: Simplify __alloc_pages_direct_compact feedback interface

[Vlastimil Babka]

On 04/05/2016 01:25 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> __alloc_pages_direct_compact communicates potential back off by two
> variables:
> 	- deferred_compaction tells that the compaction returned
> 	  COMPACT_DEFERRED
> 	- contended_compaction is set when there is a contention on
> 	  zone->lock resp. zone->lru_lock locks
>
> __alloc_pages_slowpath then backs of for THP allocation requests to
> prevent from long stalls. This is rather messy and it would be much
> cleaner to return a single compact result value and hide all the nasty
> details into __alloc_pages_direct_compact.

On the other hand, the nasty subtle details of THP allocation handling 
are now split between __alloc_pages_direct_compact and 
__alloc_pages_slowpath(). Lesser evil, I guess. I wish we could get rid 
of these special cases, now that latency of THP direct allocations is 
reduced by Mel's new defaults.

> This patch shouldn't introduce any functional changes.
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>