[PATCH v1] ALSA: memalloc: Fix indefinite hang in non-iommu case

[Karthikeyan Ramasubramanian]

Before 9d8e536 ("ALSA: memalloc: Try dma_alloc_noncontiguous() at first")
the alsa non-contiguous allocator always called the alsa fallback
allocator in the non-iommu case. This allocated non-contig memory
consisting of progressively smaller contiguous chunks. Allocation was
fast due to the OR-ing in of __GFP_NORETRY.

After 9d8e536 ("ALSA: memalloc: Try dma_alloc_noncontiguous() at first")
the code tries the dma non-contig allocator first, then falls back to
the alsa fallback allocator. In the non-iommu case, the former supports
only a single contiguous chunk.

We have observed experimentally that under heavy memory fragmentation,
allocating a large-ish contiguous chunk with __GFP_RETRY_MAYFAIL
triggers an indefinite hang in the dma non-contig allocator. This has
high-impact, as an occurrence will trigger a device reboot, resulting in
loss of user state.

Fix the non-iommu path by letting dma_alloc_noncontiguous() fail quickly
so it does not get stuck looking for that elusive large contiguous chunk,
in which case we will fall back to the alsa fallback allocator.

Note that the iommu dma non-contiguous allocator is not affected. While
assembling an array of pages, it tries consecutively smaller contiguous
allocations, and lets higher-order chunk allocations fail quickly.

Suggested-by: Sven van Ashbrook <svenva@xxxxxxxxxxxx>
Suggested-by: Brian Geffon <bgeffon@xxxxxxxxxx>
Fixes: 9d8e536d36e7 ("ALSA: memalloc: Try dma_alloc_noncontiguous() at first")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Sven van Ashbrook <svenva@xxxxxxxxxxxx>
Cc: Brian Geffon <bgeffon@xxxxxxxxxx>
Cc: Curtis Malainey <cujomalainey@xxxxxxxxxxxx>
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@xxxxxxxxxxxx>
---

 sound/core/memalloc.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/sound/core/memalloc.c b/sound/core/memalloc.c
index f901504b5afc1..5f6526a0d731c 100644
--- a/sound/core/memalloc.c
+++ b/sound/core/memalloc.c
@@ -540,13 +540,18 @@ static void *snd_dma_noncontig_alloc(struct snd_dma_buffer *dmab, size_t size)
 {
 	struct sg_table *sgt;
 	void *p;
+	gfp_t gfp_flags = DEFAULT_GFP;
 
 #ifdef CONFIG_SND_DMA_SGBUF
 	if (cpu_feature_enabled(X86_FEATURE_XENPV))
 		return snd_dma_sg_fallback_alloc(dmab, size);
+
+	/* Non-IOMMU case: prevent allocator from searching forever */
+	if (!get_dma_ops(dmab->dev.dev))
+		gfp_flags |= __GFP_NORETRY;
 #endif
 	sgt = dma_alloc_noncontiguous(dmab->dev.dev, size, dmab->dev.dir,
-				      DEFAULT_GFP, 0);
+				      gfp_flags, 0);
 #ifdef CONFIG_SND_DMA_SGBUF
 	if (!sgt && !get_dma_ops(dmab->dev.dev))
 		return snd_dma_sg_fallback_alloc(dmab, size);
-- 
2.43.0.687.g38aa6559b0-goog