Re: [Lsf-pc] [LSF/MM ATTEND] memory allocation scope

[Michal Hocko <mhocko@kernel.org>]

On Thu 15-02-18 07:57:37, James Bottomley wrote:
> On Thu, 2018-02-15 at 15:48 +0100, Michal Hocko wrote:
> > On Wed 14-02-18 16:51:53, Goldwyn Rodrigues wrote:
> > > 
> > > 
> > > Discussion with the memory folks towards scope based allocation
> > > I am working on converting some of the GFP_NOFS memory allocation
> > > calls to new scope API [1]. While other allocation types (noio,
> > > nofs, noreclaim) are covered. Are there plans for identifying scope
> > > of GFP_ATOMIC allocations? This should cover most (if not all) of
> > > the allocation scope.
> > 
> > There was no explicit request for that but I can see how some users
> > might want it. I would have to double check but maybe this would
> > allow vmalloc(GFP_ATOMIC). There were some users but most of them
> > could have been changed in some way so the motivation is not very
> > large.
> 
> We have to be careful about that: most GFP_ATOMIC allocations are in
> drivers and may be for DMA'able memory.  We can't currently use vmalloc
> memory for DMA to kernel via block because bio_map_kern() uses
> virt_to_page() which assumes offset mapping.  The latter is fixable,
> obviously, but is it worth fixing?  Very few GFP_ATOMIC allocations in
> drivers will be for large chunks.

Yes this might be not worth bothering. But from the conceptual POV
GFP_ATOMIC resp. GFP_NOWAIT is very often a scope context - IRQs,
preemption or RCU. So protecting all allocations from that context makes
some sense. Not sure this is really worth spending another context bit
though.
-- 
Michal Hocko
SUSE Labs

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