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Date:   Mon, 22 Jun 2020 14:46:47 +0200
From:   Joerg Roedel <joro@8bytes.org>
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Robin Murphy <robin.murphy@arm.com>, Roman Gushchin <guro@fb.com>,
        Yang Shi <shy828301@gmail.com>,
        iommu@lists.linux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Michal Hocko <mhocko@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Wei Yang <richardw.yang@linux.intel.com>
Subject: Re: kernel BUG at mm/huge_memory.c:2613!
Message-ID: <20200622124646.GI3701@8bytes.org>
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+ 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,