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

[Takashi Iwai <tiwai@suse.de>]

On Thu, 15 Feb 2024 20:32:22 +0100,
Karthikeyan Ramasubramanian wrote:
> 
> On Thu, Feb 15, 2024 at 10:03 AM Takashi Iwai <tiwai@suse.de> wrote:
> >
> > On Thu, 15 Feb 2024 17:07:38 +0100,
> > Sven van Ashbrook wrote:
> > >
> > > Hi Takashi,
> > >
> > > On Thu, Feb 15, 2024 at 3:40 AM Takashi Iwai <tiwai@suse.de> wrote:
> > > >
> > > > Yes, the main problem is the indefinite hang from
> > > > dma_alloc_noncontiguous().
> > >
> > > We have a publicly-visible test [1] which readily triggers the
> > > indefinite hang on non-iommu Intel SoCs such as JasperLake.
> > > As noted in the commit message, iommu SoCs are not currently
> > > affected.
> > >
> > > > So, is the behavior more or less same even if you pass
> > > > __GFP_RETRY_MAYFAIL to dma_alloc_noncontiguous()?  Or is this flag
> > > > already implicitly set somewhere in the middle?  It shouldn't hang
> > > > indefinitely, but the other impact to the system like OOM-killer
> > > > kickoff may be seen.
> > >
> > > My incomplete understanding:
> > >
> > > Alsa specifies __GFP_RETRY_MAYFAIL because it wants to prevent triggering
> > > the OOM killer.
> >
> > Ah I forgot that we set that bit commonly in the flag.
> >
> > > This was __GFP_NORETRY in the not-too-distant past [2],
> > > but that failed too quickly, which resulted in permanent loss of audio due
> > > to failed firmware dma sg allocations.
> >
> > Hm, the change in commit a61c7d88d38c assumed that __GFP_RETRY_MAYFAIL
> > shouldn't have that big impact.  If the hang really happens with a
> > high order allocation, it's dangerous to use it in other code
> > allocations than noncontiguous case (i.e. SNDRV_DMA_TYPE_DEV and co).
> > In the tight memory situation, a similar problem can be triggered
> > quite easily, then.
> >
> > > In the iommu case, dma_alloc_noncontiguous() implements a backoff [3] loop
> > > which ORs in __GFP_NORETRY except for minimum order allocations. We observe
> > > experimentally that __GFP_RETRY_MAYFAIL does not get "stuck" on minimum order
> > > allocations. So the iommu case is not affected.
> > >
> > > In the non-iommu case however, dma_alloc_noncontiguous() actually becomes a
> > > contiguous allocator, with no backoff loop. The commit introducing it [4]
> > > states "This API is only properly implemented for dma-iommu and will simply
> > > return a contigious chunk as a fallback." In this case we observe the indefinite
> > > hang.
> > >
> > > The alsa fallback allocator is also not affected by the problem, as it does
> > > not specify __GFP_RETRY_MAYFAIL. Except in the XENPV case.
> >
> > So it sounds like that we should go back for __GFP_NORETRY in general
> > for non-zero order allocations, not only the call you changed, as
> > __GFP_RETRY_MAYFAIL doesn't guarantee the stuck.
> >
> > How about the changes like below?
> 
> We are concerned that the below proposed change might break the fix
> introduced by commit a61c7d88d38c ("FROMGIT: ALSA: memalloc: use
> __GFP_RETRY_MAYFAIL for DMA mem allocs"). More specifically in the
> IOMMU case with a large allocation, the fallback algorithm in the DMA
> IOMMU allocator[1] will not try really hard and hence may fail
> prematurely when it gets to minimum order allocations. This will
> result in no audio when there is significant load on physical memory.

Hm, then we may give __GFP_RETRY_MAYFAIL specially for iommu case,
too?  Something like v2 patch below.


thanks,

Takashi

--- a/sound/core/memalloc.c
+++ b/sound/core/memalloc.c
@@ -21,7 +21,11 @@
 
 #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 */
+#define DEFAULT_GFP_RETRY \
+	(GFP_KERNEL | \
+	 __GFP_RETRY_MAYFAIL | /* try a bit harder */ \
 	 __GFP_NOWARN)   /* no stack trace print - this call is non-critical */
 
 static const struct snd_malloc_ops *snd_dma_get_ops(struct snd_dma_buffer *dmab);
@@ -30,6 +34,13 @@ static const struct snd_malloc_ops *snd_dma_get_ops(struct snd_dma_buffer *dmab)
 static void *snd_dma_sg_fallback_alloc(struct snd_dma_buffer *dmab, size_t size);
 #endif
 
+/* default GFP bits for our allocations */
+static gfp_t default_gfp(size_t size)
+{
+	/* don't allocate intensively for high-order pages */
+	return (size > PAGE_SIZE) ? DEFAULT_GFP : DEFAULT_GFP_RETRY;
+}
+
 static void *__snd_dma_alloc_pages(struct snd_dma_buffer *dmab, size_t size)
 {
 	const struct snd_malloc_ops *ops = snd_dma_get_ops(dmab);
@@ -281,7 +292,7 @@ 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(size);
 
  again:
 	p = alloc_pages_exact(size, gfp);
@@ -466,7 +477,7 @@ static const struct snd_malloc_ops snd_dma_iram_ops = {
  */
 static void *snd_dma_dev_alloc(struct snd_dma_buffer *dmab, size_t size)
 {
-	return dma_alloc_coherent(dmab->dev.dev, size, &dmab->addr, DEFAULT_GFP);
+	return dma_alloc_coherent(dmab->dev.dev, size, &dmab->addr, default_gfp(size));
 }
 
 static void snd_dma_dev_free(struct snd_dma_buffer *dmab)
@@ -511,7 +522,7 @@ static int snd_dma_wc_mmap(struct snd_dma_buffer *dmab,
 #else
 static void *snd_dma_wc_alloc(struct snd_dma_buffer *dmab, size_t size)
 {
-	return dma_alloc_wc(dmab->dev.dev, size, &dmab->addr, DEFAULT_GFP);
+	return dma_alloc_wc(dmab->dev.dev, size, &dmab->addr, default_gfp(size));
 }
 
 static void snd_dma_wc_free(struct snd_dma_buffer *dmab)
@@ -539,14 +550,20 @@ 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(size);
 	void *p;
 
 #ifdef CONFIG_SND_DMA_SGBUF
 	if (cpu_feature_enabled(X86_FEATURE_XENPV))
 		return snd_dma_sg_fallback_alloc(dmab, size);
+	/* dma_alloc_noncontiguous() with IOMMU is safe to pass
+	 * __GFP_RETRY_MAYFAIL option for more intensive allocations
+	 */
+	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);
@@ -783,7 +800,7 @@ static void *snd_dma_sg_fallback_alloc(struct snd_dma_buffer *dmab, size_t size)
 	while (size > 0) {
 		chunk = min(size, chunk);
 		if (sgbuf->use_dma_alloc_coherent)
-			p = dma_alloc_coherent(dmab->dev.dev, chunk, &addr, DEFAULT_GFP);
+			p = dma_alloc_coherent(dmab->dev.dev, chunk, &addr, default_gfp(size));
 		else
 			p = do_alloc_pages(dmab->dev.dev, chunk, &addr, false);
 		if (!p) {
@@ -871,7 +888,7 @@ static void *snd_dma_noncoherent_alloc(struct snd_dma_buffer *dmab, size_t size)
 	void *p;
 
 	p = dma_alloc_noncoherent(dmab->dev.dev, size, &dmab->addr,
-				  dmab->dev.dir, DEFAULT_GFP);
+				  dmab->dev.dir, default_gfp(size));
 	if (p)
 		dmab->dev.need_sync = dma_need_sync(dmab->dev.dev, dmab->addr);
 	return p;