Re: [PATCH 2/2] Only check absolute watermarks for ALLOC_HIGH and ALLOC_HARDER allocations

[Christoph Lameter <clameter@sgi.com>]

> > few pages at those orders and some (where possible) are reserved for
> > PF_MEM tasks, for reclaim itself.  However, the reservation system takes
> > no account of higher orders, so we can always end up in a situation

Could we change the reservation system to take account of higher orders?

> > My understanding is all slabs within a slub slab cache have to be the
> > same order.  So we need to ensure that any slab that might be used from
> > the reclaim path must only use order-0 pages.  Also it seems that any
> > slab that is allocated from atomically will have to use order-0 pages in
> > order to remain reliable.  Christoph, do we have any facility to tag
> > caches to use a specific allocation order?

I would like to avoid adding such a flag. Forcing low orders on a 
slab limits its scalability. Higher orders mean less frequent taking of 
locks. We adding more special casing to the VM. Its better if we could 
handle the reserves in such a way that higher allocs are possible.

Another solution may be to make sure that we can tolerate failures of 
atomic allocs? GFP_ATOMIC has always had the stigma of being able to fail 
after all.

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