Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent() failures

[Robin Murphy]

On 12/04/18 09:27, Takashi Iwai wrote:
> On Thu, 12 Apr 2018 10:19:05 +0200,
> Takashi Iwai wrote:
> > On Thu, 12 Apr 2018 10:03:56 +0200,
> > Takashi Iwai wrote:
> > > 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.
> >
> > Gah, scratch this, it doesn't work.  A different check is needed...
>
> The v2 patches are below, replaced with IS_ENABLED(CONFIG_ZONE_DMA).

That looks pretty reasonable to me - I'd be tempted to try a factoring 
out a helper function to avoid the "goto again" logic, but that's hardly 
crucial.

What I'd really love to see is something like the alloc_pages_mask() 
proposal from years ago to come back such that GFP_DMA32 could die 
entirely, but I don't know anywhere near enough about the mm layer to 
consider taking that on myself :(

Robin.