On Wed, 2020-07-08 at 17:35 +0200, Christoph Hellwig wrote:
> On Tue, Jul 07, 2020 at 02:28:04PM +0200, Nicolas Saenz Julienne wrote:
> > When allocating atomic DMA memory for a device, the dma-pool core
> > queries __dma_direct_optimal_gfp_mask() to check which atomic pool to
> > use. It turns out the GFP flag returned is only an optimistic guess.
> > The pool selected might sometimes live in a zone higher than the
> > device's view of memory.
> >
> > As there isn't a way to grantee a mapping between a device's DMA
> > constraints and correct GFP flags this unifies both DMA atomic pools.
> > The resulting pool is allocated in the lower DMA zone available, if any,
> > so as for devices to always get accessible memory while having the
> > flexibility of using dma_pool_kernel for the non constrained ones.
> >
> > Fixes: c84dc6e68a1d ("dma-pool: add additional coherent pools to map to gfp
> > mask")
> > Reported-by: Jeremy Linton <jeremy.lin...@arm.com>
> > Suggested-by: Robin Murphy <robin.mur...@arm.com>
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulie...@suse.de>
>
> Hmm, this is not what I expected from the previous thread. I thought
> we'd just use one dma pool based on runtime available of the zones..I may be misunderstanding you, but isn't that going back to how things used to be before pulling in David Rientjes' work? The benefit of having a GFP_KERNEL pool is that non-address-constrained devices can get their atomic memory there, instead of consuming somewhat scarcer low memory.
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