Messages in this thread | | | Date | Tue, 11 Nov 2008 15:21:47 +0900 | From | Yasunori Goto <> | Subject | Re: [Q] Why does dma_alloc_coherent() of ia64 GFP_DMA? |
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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 *
Wao! Seems reasonable! Ack Ack!
Acked-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Thanks a lot!
-- Yasunori Goto
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