Re: [Q] Why does dma_alloc_coherent() of ia64 GFP_DMA?

[FUJITA Tomonori]

On Mon, 10 Nov 2008 13:47:51 +0900
Yasunori Goto <y-goto@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
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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