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

[Takashi Iwai <tiwai@suse.de>]

On Fri, 16 Feb 2024 13:19:54 +0100,
Kai Vehmanen wrote:
> 
> Hi,
> 
> On Fri, 16 Feb 2024, Takashi Iwai wrote:
> 
> > On Fri, 16 Feb 2024 09:35:32 +0100, Takashi Iwai wrote:
> > > The fact that we have to drop __GFP_RETRY_MAYFAIL indicates that the
> > > handling there doesn't suffice -- at least for the audio operation.
> > 
> > Reconsidering on this again, I wonder keeping __GFP_RETRY_MAYFAIL
> > makes sense.  We did have __GFP_NORETRY for avoiding OOM-killer.
> > But it's been over ages, and the memory allocation core became smart
> > enough.
> > 
> > The side-effect of __GFP_RETRY_MAYFAIL is that the page reclaim and
> > compaction happens even for high-order allocations, and that must be
> 
> for the original problem that led to "ALSA: memalloc: use 
> __GFP_RETRY_MAYFAIL for DMA mem allocs", reclaim for low-order case
> would be enough. I.e. the case was:
> 
> > OTOH, a slight concern with the drop of __GFP_RETRY_MAYFAIL is whether
> > allowing OOM-killer for low order allocations is acceptable or not.
> > 
> > There are two patterns of calling allocators:
> [..]
> > 3. SNDRV_DMA_TYPE_NONCONTIG for large size:
> >    this is called often, once per stream open, since the driver
> >    doesn't keep the buffer.
> 
> So with SOF we have additional case where we do an allocation for the DSP 
> firmware (snd_dma_alloc_pages(SNDRV_DMA_TYPE_DEV_SG, ...)) and this is 
> called at system resume. With s/__GPF_RETRY_MAYFAIL/__GFP_NORETRY/, these 
> allocations failed (on a iommu enabled Chromebook) at system resume in a 
> case where system was not really running out of memory (reclaim was 
> possible). A failed allocation means there's no audio in the system after 
> resume, so we want to try harder.
> 
> But yeah, I think the proposed handling for (3) category would work. If 
> needed, we can further specialize the DSP firmware case with some hint 
> to snd_dma_alloc_pages().

OK, then how about the one like below?

This changes:
- Back to __GFP_NORETRY as default
- Use __GFP_RETRY_MAYFAIL for SNDRV_DMA_TYPE_NONCONTIG with IOMMU;
  this should cover the commit a61c7d88d38c
- Also use __GFP_RETRY_MAYFAIL for the SG-fallback allocations of the
  minimal order, just like IOMMU allocator does.

This should be less destructive, while still allowing more aggressive
allocations for SG buffers.


thanks,

Takashi

--- a/sound/core/memalloc.c
+++ b/sound/core/memalloc.c
@@ -21,9 +21,13 @@
 
 #define DEFAULT_GFP \
 	(GFP_KERNEL | \
-	 __GFP_RETRY_MAYFAIL | /* don't trigger OOM-killer */ \
+	 __GFP_NORETRY | /* don't trigger OOM-killer */ \
 	 __GFP_NOWARN)   /* no stack trace print - this call is non-critical */
 
+/* GFP flags to be used for low order pages, allowing reclaim and compaction */
+#define DEFAULT_GFP_RETRY \
+	(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN)
+
 static const struct snd_malloc_ops *snd_dma_get_ops(struct snd_dma_buffer *dmab);
 
 #ifdef CONFIG_SND_DMA_SGBUF
@@ -281,7 +285,11 @@ static void *do_alloc_pages(struct device *dev, size_t size, dma_addr_t *addr,
 			    bool wc)
 {
 	void *p;
-	gfp_t gfp = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
+	gfp_t gfp = DEFAULT_GFP;
+
+	/* allow reclaim and compaction for low order pages */
+	if (size <= PAGE_SIZE)
+		gfp = DEFAULT_GFP_RETRY;
 
  again:
 	p = alloc_pages_exact(size, gfp);
@@ -539,14 +547,18 @@ static const struct snd_malloc_ops snd_dma_wc_ops = {
 static void *snd_dma_noncontig_alloc(struct snd_dma_buffer *dmab, size_t size)
 {
 	struct sg_table *sgt;
+	gfp_t gfp = DEFAULT_GFP;
 	void *p;
 
 #ifdef CONFIG_SND_DMA_SGBUF
 	if (cpu_feature_enabled(X86_FEATURE_XENPV))
 		return snd_dma_sg_fallback_alloc(dmab, size);
+	/* with IOMMU, it's safe to pass __GFP_RETRY_MAYFAIL with high order */
+	if (get_dma_ops(dmab->dev.dev))
+		gfp = DEFAULT_GFP_RETRY;
 #endif
 	sgt = dma_alloc_noncontiguous(dmab->dev.dev, size, dmab->dev.dir,
-				      DEFAULT_GFP, 0);
+				      gfp, 0);
 #ifdef CONFIG_SND_DMA_SGBUF
 	if (!sgt && !get_dma_ops(dmab->dev.dev))
 		return snd_dma_sg_fallback_alloc(dmab, size);