Messages in this thread Patch in this message | | | Date | Tue, 11 Nov 2008 14:41:45 +0900 | Subject | Re: [Q] Why does dma_alloc_coherent() of ia64 GFP_DMA? | From | FUJITA Tomonori <> |
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On Mon, 10 Nov 2008 13:47:51 +0900 Yasunori Goto <y-goto@jp.fujitsu.com> wrote:
> I have a (may be dumb) question about dma_alloc_coherent() for ia64. > > > Why does dma_alloc_coherent() of ia64 force GFP_DMA yet? > And why is swiotlb_dma_alloc_coherent() default routine of > platform_dma_alloc_coherent()? > > ------- > #define dma_alloc_coherent(dev, size, handle, gfp) \ > platform_dma_alloc_coherent(dev, size, handle, (gfp) | GFP_DMA) > -------- > > Even if a device allows over 4G access and the driver doesn't specify > GFP_DMA, dma_alloc_coherent() returns under 4G area. > I guess many people think this is not so big issue because drivers require > very small memory generally. > > However, I think this has the possibility of a finishing blow of OOM. > For example, > > 1) Page caches occupy normal zone, and DMA zone is free. > 2) A user's application requires a few GB memory and mlock it. > All DMA zone is occupied by it. > 3) A device which allows over 4GB is hot-added. > But dma_alloc_coherent() try to allocate DMA zone. > Then OOM occurs because there is no freeable pages. > > I heard there are some users who require a few GB mlock. > There are similar trouble in past. > > > If GFP_DMA is removed from above definition of dma_alloc_coherent(), > what will happen?
Probably, it breaks swiotlb with devices that don't have DMA_64BIT_MASK coherent_dma_mask.
> If it is not allowed, how is followings? > > dma_alloc_coherent() > -> platform_dma_alloc_coherent() > -> normal_alloc_coherent() > { > if (dma_mask allow over 4G) > ret = __get_free_pages(); > : > (check validation of returned address) > : > else > swiotlb_alloc_coherent(); > } > } > > If I'm something misunderstanding, please let me know.
Hmm, platform_dma_alloc_coherent is supposed to handle multiple dma ops, swiotlb, hardware IOMMUs like VT-d and sba, etc?
In IA64, the gfp zone flag matters for only swiotlb, I think.
= From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> Subject: [PATCH] IA64: use GFP_DMA in dma_alloc_coherent only when necessary
For swiotlb, we need to set GFP_DMA if a device doesn't have DMA_64BIT_MASK coherent_dma_mask. hardware IOMMUs like VT-d and sba should ignore gfp zone flag.
diff --git a/arch/ia64/include/asm/dma-mapping.h b/arch/ia64/include/asm/dma-mapping.h index bbab7e2..d4de41b 100644 --- a/arch/ia64/include/asm/dma-mapping.h +++ b/arch/ia64/include/asm/dma-mapping.h @@ -52,7 +52,9 @@ extern struct ia64_machine_vector ia64_mv; extern void set_iommu_machvec(void); #define dma_alloc_coherent(dev, size, handle, gfp) \ - platform_dma_alloc_coherent(dev, size, handle, (gfp) | GFP_DMA) + platform_dma_alloc_coherent(dev, size, handle, \ + (dev)->coherent_dma_mask != DMA_64BIT_MASK ? \ + (gfp) | GFP_DMA : gfp) /* coherent mem. is cheap */ static inline void *
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