On Thu 15-02-18 17:02:41, Michal Hocko wrote: > 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. And just to clarify why I've mentioned vmalloc explicitly. vmalloc simply ignores the given gfp flags for pte allocations (those are hardcoded GFP_KERNEL) and that is why it is not generally suitable for atomic contexts. Maybe there are other obstacles (sleeping locks) but scope gfp would solve at least the pte allocation side. -- Michal Hocko SUSE Labs
