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

[Takashi Iwai <tiwai@suse.de>]

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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


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>From 490d11cd440d169c8e1a234c1baee3022c2f3464 Mon Sep 17 00:00:00 2001
From: Takashi Iwai <tiwai@suse.de>
Date: Thu, 12 Apr 2018 09:46:12 +0200
Subject: [PATCH 1/2] dma-direct: Don't repeat allocation for no-op GFP_DMA

When an allocation with lower dma_coherent mask fails,
dma_direct_alloc() retries the allocation with GFP_DMA.  But, it's
useless for architectures that has no GFP_DMA, obviously.

This patch fixes it by adding the check of GFP_DMA before retrying the
allocation.

Fixes: 95f183916d4b ("dma-direct: retry allocations using GFP_DMA for small masks")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 lib/dma-direct.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/dma-direct.c b/lib/dma-direct.c
index c0bba30fef0a..191f96453419 100644
--- a/lib/dma-direct.c
+++ b/lib/dma-direct.c
@@ -84,7 +84,7 @@ void *dma_direct_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
 		__free_pages(page, page_order);
 		page = NULL;
 
-		if (dev->coherent_dma_mask < DMA_BIT_MASK(32) &&
+		if (GFP_DMA && dev->coherent_dma_mask < DMA_BIT_MASK(32) &&
 		    !(gfp & GFP_DMA)) {
 			gfp = (gfp & ~GFP_DMA32) | GFP_DMA;
 			goto again;
-- 
2.16.3


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>From 48ce0bb0a6b916f5615e24861cd9ba78c98ee33a Mon Sep 17 00:00:00 2001
From: Takashi Iwai <tiwai@suse.de>
Date: Thu, 12 Apr 2018 09:53:48 +0200
Subject: [PATCH 2/2] dma-direct: Try reallocation with GFP_DMA32 if possible

This patch adds a similar fallback reallocation with GFP_DMA32 as
we've done with GFP_DMA.  The condition is that the coherent mask is
smaller than 64bit (i.e. some address limitation), and neither GFP_DMA
nor GFP_DMA32 is set beforehand.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 lib/dma-direct.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/dma-direct.c b/lib/dma-direct.c
index 191f96453419..b3cabf06ce90 100644
--- a/lib/dma-direct.c
+++ b/lib/dma-direct.c
@@ -84,6 +84,12 @@ void *dma_direct_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
 		__free_pages(page, page_order);
 		page = NULL;
 
+		if (GFP_DMA32 && dev->coherent_dma_mask < DMA_BIT_MASK(64) &&
+		    !(gfp & (GFP_DMA32 | GFP_DMA))) {
+			gfp |= GFP_DMA32;
+			goto again;
+		}
+
 		if (GFP_DMA && dev->coherent_dma_mask < DMA_BIT_MASK(32) &&
 		    !(gfp & GFP_DMA)) {
 			gfp = (gfp & ~GFP_DMA32) | GFP_DMA;
-- 
2.16.3