On Mon, Jul 06, 2020 at 04:09:36PM +0200, Nicolas Saenz Julienne wrote: > On Sun, 2020-07-05 at 16:41 -0700, David Rientjes wrote: > > On Fri, 3 Jul 2020, Robin Murphy wrote: > > > Or perhaps just get rid of atomic_pool_dma32 (and allocate atomic_pool_dma > > > from ZONE_DMA32 if !ZONE_DMA). That should make it fall pretty much back in > > > line while still preserving the potential benefit of the kernel pool for > > > non-address-constrained devices. > > > > > > > I assume it depends on how often we have devices where > > __dma_direct_alloc_pages() behavior is required, i.e. what requires the > > dma_coherent_ok() checks and altering of the gfp flags to get memory that > > works. > > > > Is the idea that getting rid of atomic_pool_dma32 would use GFP_KERNEL > > (and atomic_pool_kernel) as the default policy here? That doesn't do any > > dma_coherent_ok() checks so dma_direct_alloc_pages would return from > > ZONE_NORMAL without a < 3G check? > > IIUC this is not what Robin proposes. > > The idea is to only have one DMA pool, located in ZONE_DMA, if enabled, and > ZONE_DMA32 otherwise. This way you're always sure the memory is going to be > good enough for any device while maintaining the benefits of > atomic_pool_kernel. That is how I understood the proposal from Robin and I think it is the right thing to do. > > It *seems* like we want to check if dma_coherent_ok() succeeds for ret in > > dma_direct_alloc_pages() when allocating from the atomic pool and, based > > on criteria that allows fallback, just fall into > > __dma_direct_alloc_pages()? > > I suspect I don't have enough perspective here but, isn't that breaking the > point of having an atomic pool? Wouldn't that generate big latency spikes? I > can see how audio transfers over USB could be affected by this specifically, > IIRC those are allocated atomically and have timing constraints. > > That said, if Robin solution works for you, I don't mind having a go at it. We can't just fall back to __dma_direct_alloc_pages when allocation from the atomic pool fails, as the atomic pool exists for provide allocations that require sleeping actions for callers that can't sleep.
