Re: [RFC] arm64: swiotlb: cma_alloc error spew

From: Robin Murphy
Date: Tue Apr 23 2019 - 07:32:46 EST


On 17/04/2019 21:48, dann frazier wrote:
hey,
I'm seeing an issue on a couple of arm64 systems[*] where they spew
~10K "cma: cma_alloc: alloc failed" messages at boot. The errors are
non-fatal, and bumping up cma to a large enough size (~128M) gets rid
of them - but that seems suboptimal. Bisection shows that this started
after commit fafadcd16595 ("swiotlb: don't dip into swiotlb pool for
coherent allocations"). It looks like __dma_direct_alloc_pages()
is opportunistically using CMA memory but falls back to non-CMA if CMA
disabled or unavailable. I've demonstrated that this fallback is
indeed returning a valid pointer. So perhaps the issue is really just
the warning emission.

The CMA area being full isn't necessarily an ignorable non-problem, since it means you won't be able to allocate the kind of large buffers for which CMA was intended. The question is, is it actually filling up with allocations that deserve to be there, or is this the same as I've seen on a log from a ThunderX2 system where it's getting exhausted by thousands upon thousands of trivial single page allocations? If it's the latter (CONFIG_CMA_DEBUG should help shed some light if necessary), then that does lean towards spending a bit more effort on this idea:

https://lore.kernel.org/lkml/20190327080821.GB20336@xxxxxx/

Robin.

The following naive patch solves the problem for me - just silence the
cma errors, since it looks like a soft error. But is there a better
approach?

[*] APM X-Gene & HiSilicon Hi1620 w/ SMMU disabled

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 6310ad01f915b..0324aa606c173 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -112,7 +112,7 @@ struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
/* CMA can be used only in the context which permits sleeping */
if (gfpflags_allow_blocking(gfp)) {
page = dma_alloc_from_contiguous(dev, count, page_order,
- gfp & __GFP_NOWARN);
+ true);
if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
dma_release_from_contiguous(dev, page, count);
page = NULL;