Re: [PATCH 11/12] mm, page_alloc: Reserve pageblocks for high-order atomic allocations on demand

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Sep 18, 2015 at 03:38:35PM +0900, Joonsoo Kim wrote:
> > > And, there is some mismatch that check atomic high-order allocation.
> > > In some place, you checked __GFP_ATOMIC, but some other places,
> > > you checked ALLOC_HARDER. It is better to use unified one.
> > > Introducing helper function may be a good choice.
> > > 
> > 
> > Which cases specifically? In the zone_watermark check, it's because
> > there is no GFP flags in that context. They could be passed in but then
> > every caller needs to be updated accordingly and overall it gains
> > nothing.
> 
> You use __GFP_ATOMIC in rmqueue() to allow highatomic reserve.
> ALLOC_HARDER is used in watermark check and to reserve highatomic
> pageblock after allocation.
> 
> ALLOC_HARDER is set if (__GFP_ATOMIC && !__GFP_NOMEMALLOC) *or*
> (rt_task && !in_interrupt()). So, later case could pass watermark
> check but cannot use HIGHATOMIC reserve. And, it will reserve
> highatomic pageblock. When it try to allocate again, it can't use
> this reserved pageblock due to GFP flags and this could happens
> repeatedly.
> And, first case also has a problem. If user requests memory
> with __GFP_NOMEMALLOC, it's intend doesn't touch reserved mem,
> but, in current patch, it can use highatomic pageblock.
> 
> I'm not sure these causes real trouble but unifying it as much as
> possible is preferable solution.
> 

Ok, that makes sense. Thanks

-- 
Mel Gorman
SUSE Labs