Re: mm: page_alloc: avoid marking zones full prematurely after zone_reclaim()

[Mel Gorman <mgorman@suse.de>]

On Mon, Apr 01, 2013 at 02:13:24PM +0300, Dan Carpenter wrote:
> I still don't understand the code in gfp_to_alloc_flags().
> 
> 	int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
> 
> ORing with zero is odd.
> 

Thanks Dan for the report and thanks Michal for fixing it. I was offline
for last week which lead to my tardy response.

The odditiy is that ALLOC_WMARK_MIN is not treated as a flag but as an
offset within the zone->wmark so it starts as 0. It could have been
written as

	int alloc_flags = ALLOC_CPUSET;

but then it would be easy to forget that in this path we are using the
MIN watermark.

The "flag" is used as an offset because it eliminated a number of
branches in the page allocator and was a micro-optimisation at the time.

-- 
Mel Gorman
SUSE Labs

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