Messages in this thread | | | Date | Mon, 28 Feb 2022 10:32:54 +0000 | Subject | Re: [PATCH] dma-mapping: remove CONFIG_DMA_REMAP | From | Robin Murphy <> |
| |
On 2022-02-27 14:35, Christoph Hellwig wrote: > CONFIG_DMA_REMAP is used to build a few helpers around the core > vmalloc code, and to use them in case there is a highmem page in > dma-direct, and to make dma coherent allocations be able to use > non-contiguous pages allocations for DMA allocations in the dma-iommu > layer. > > Right now it needs to be explicitly selected by architectures, and > is only done so by architectures that require remapping to deal > with devices that are not DMA coherent. Make it unconditional for > builds with CONFIG_MMU as it is very little extra code, but makes > it much more likely that large DMA allocations succeed on x86. > > This fixes hot plugging a NVMe thunderbolt SSD for me, which tries > to allocate a 1MB buffer that is otherwise hard to obtain due to > memory fragmentation on a heavily used laptop.
Simplifying the maze is most welcome, however one thing stands out...
[...] > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c > index 50f48e9e45987..fe1682fecdd57 100644 > --- a/kernel/dma/direct.c > +++ b/kernel/dma/direct.c > @@ -269,10 +269,10 @@ void *dma_direct_alloc(struct device *dev, size_t size, > /* > * Depending on the cma= arguments and per-arch setup, > * dma_alloc_contiguous could return highmem pages. > - * Without remapping there is no way to return them here, so > - * log an error and fail. > + * Without MMU-based remapping there is no way to return them > + * here, so log an error and fail. > */ > - if (!IS_ENABLED(CONFIG_DMA_REMAP)) { > + if (!IS_ENABLED(CONFIG_MMU)) { > dev_info(dev, "Rejecting highmem page from CMA.\n"); > goto out_free_pages; > }
Is it even possible to hit this case now? From a quick look, all the architectures defining HIGHMEM either have an explicit dependency on MMU or don't allow deselecting it anyway (plus I don't see how HIGHMEM && !MMU could work in general), so I'm pretty sure this whole chunk should go away now.
With that (or if there *is* some subtle wacky case where PageHighmem() can actually return true for !MMU, a comment to remind us in future),
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Cheers, Robin.
| |