+ Robin
Robin, any idea on this?
On Thu, Jun 18, 2020 at 10:40:26PM -0400, Andrea Arcangeli wrote:
> Hello,
>
> On Thu, Jun 18, 2020 at 06:14:49PM -0700, Roman Gushchin wrote:
> > I agree. The whole
> >
> > page = alloc_pages_node(nid, alloc_flags, order);
> > if (!page)
> > continue;
> > if (!order)
> > break;
> > if (!PageCompound(page)) {
> > split_page(page, order);
> > break;
> > } else if (!split_huge_page(page)) {
> > break;
> > }
> >
> > looks very suspicious to me.
> > My wild guess is that gfp flags changed somewhere above, so we hit
> > the branch which was never hit before.
>
> Right to be suspicious about the above: split_huge_page on a regular
> page allocated by a driver was never meant to work.
>
> The PageLocked BUG_ON is just a symptom of a bigger issue, basically
> split_huge_page it may survive, but it'll stay compound and in turn it
> must be freed as compound.
>
> The respective free method doesn't even contemplate freeing compound
> pages, the only way the free method can survive, is by removing
> __GFP_COMP forcefully in the allocation that was perhaps set here
> (there are that many __GFP_COMP in that directory):
>
> static void snd_malloc_dev_pages(struct snd_dma_buffer *dmab, size_t size)
> {
> gfp_t gfp_flags;
>
> gfp_flags = GFP_KERNEL
> | __GFP_COMP /* compound page lets parts be mapped */
>
> And I'm not sure what the comment means here, compound or non compound
> doesn't make a difference when you map it, it's not a THP, the
> mappings must be handled manually so nothing should check PG_compound
> anyway in the mapping code.
>
> Something like this may improve things, it's an untested quick hack,
> but this assumes it's always a bug to setup a compound page for these
> DMA allocations and given the API it's probably a correct
> assumption.. Compound is slower, unless you need it, you can avoid it
> and then split_page will give contiguous memory page granular. Ideally
> the code shouldn't call split_page at all and it should free it all at
> once by keeping track of the order and by returning the order to the
> caller, something the API can't do right now as it returns a plain
> array that can only represent individual small pages.
>
> Once this is resolved, you may want to check your config, iommu passthrough
> sounds more optimal for a soundcard.
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index f68a62c3c32b..3dfbc010fa83 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -499,6 +499,10 @@ static struct page **__iommu_dma_alloc_pages(struct device *dev,
>
> /* IOMMU can map any pages, so himem can also be used here */
> gfp |= __GFP_NOWARN | __GFP_HIGHMEM;
> + if (unlikely(gfp & __GFP_COMP)) {
> + WARN();
> + gfp &= ~__GFP_COMP;
> + }
>
> while (count) {
> struct page *page = NULL;
> @@ -522,13 +526,8 @@ static struct page **__iommu_dma_alloc_pages(struct device *dev,
> continue;
> if (!order)
> break;
> - if (!PageCompound(page)) {
> - split_page(page, order);
> - break;
> - } else if (!split_huge_page(page)) {
> - break;
> - }
> - __free_pages(page, order);
> + split_page(page, order);
> + break;
> }
> if (!page) {
> __iommu_dma_free_pages(pages, i);
> diff --git a/sound/core/memalloc.c b/sound/core/memalloc.c
> index 6850d13aa98c..378f5a36ec5f 100644
> --- a/sound/core/memalloc.c
> +++ b/sound/core/memalloc.c
> @@ -28,7 +28,6 @@ static void snd_malloc_dev_pages(struct snd_dma_buffer *dmab, size_t size)
> gfp_t gfp_flags;
>
> gfp_flags = GFP_KERNEL
> - | __GFP_COMP /* compound page lets parts be mapped */
> | __GFP_NORETRY /* don't trigger OOM-killer */
> | __GFP_NOWARN; /* no stack trace print - this call is non-critical */
> dmab->area = dma_alloc_coherent(dmab->dev.dev, size, &dmab->addr,
