linux-kernel - Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc

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Date:   Thu, 12 Apr 2018 10:03:56 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Takashi Iwai <tiwai@...e.de>, Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Christian König <christian.koenig@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent()   failures

On Thu, 12 Apr 2018 08:02:27 +0200,
Christoph Hellwig wrote:
> 
> On Wed, Apr 11, 2018 at 09:28:54AM +0200, Takashi Iwai wrote:
> > > But we should try a GFP_DMA32 allocation first, so this is a bit
> > > surprising.
> > 
> > Hm, do we really try that?
> > Through a quick glance, dma_alloc_coherent_gfp_flags() gives GFP_DMA32
> > only when coherent mask <= DMA_BIT_MASK(32); in the case of iwlwifi,
> > it's 36bit, so GFP_DMA isn't set.
> 
> Oh, yes - it is using an odd dma mask, and amdgpu seems to use an
> just as odd 40-bit dma mask.
> 
> > We had a fallback allocation with GFP_DMA32 in the past, but this
> > seems gone long time ago along with cleanups (commit c647c3bb2d16).
> > 
> > But I haven't followed about this topic for long time, so I might have
> > missed obviously...
> 
> I think a fallback would be much better here rather than relying on the
> limited swiotlb buffer bool.  dma_direct_alloc (which in 4.17 is also
> used for x86) already has a GFP_DMA fallback, so extending this for
> GFP_DMA32 as well would seem reasonable.
> 
> Any volunteers?

Below is a quick attempt, totally untested.  Actually the retry with
GFP_DMA is superfluous for archs without it, so the first patch
corrects it.  The second patch adds the retry with GFP_DMA32.

I'll resubmit properly if these are OK (and better if anyone could
test them :)


thanks,

Takashi


Download attachment "0001-dma-direct-Don-t-repeat-allocation-for-no-op-GFP_DMA.patch" of type "application/octet-stream" (1187 bytes)

Download attachment "0002-dma-direct-Try-reallocation-with-GFP_DMA32-if-possib.patch" of type "application/octet-stream" (1155 bytes)