Messages in this thread | | | Date | Wed, 22 Aug 2012 13:09:59 +0300 | From | Hiroshi Doyu <> | Subject | Re: [PATCHv6 2/2] ARM: dma-mapping: remove custom consistent dma region |
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Hi Marek,
On Tue, 21 Aug 2012 17:01:08 +0200 Marek Szyprowski <m.szyprowski@samsung.com> wrote:
> > > > -__iommu_alloc_remap(struct page **pages, size_t size, gfp_t gfp, pgprot_t prot) > > > > +__iommu_alloc_remap(struct page **pages, size_t size, gfp_t gfp, pgprot_t prot, > > > > + const void *caller) > > > > { > > > > - struct arm_vmregion *c; > > > > - size_t align; > > > > - size_t count = size >> PAGE_SHIFT; > > > > - int bit; > > > > + unsigned int i, nr_pages = PAGE_ALIGN(size) >> PAGE_SHIFT; > > > > + struct vm_struct *area; > > > > + unsigned long p; > > > > > > > > - if (!consistent_pte[0]) { > > > > - pr_err("%s: not initialised\n", __func__); > > > > - dump_stack(); > > > > + area = get_vm_area_caller(size, VM_ARM_DMA_CONSISTENT | VM_USERMAP, > > > > + caller); > > > > + if (!area) > > > > > > This patch replaced the custom "consistent_pte" with > > > get_vm_area_caller()", which breaks the compatibility with the > > > existing driver. This causes the following kernel oops(*1). That > > > driver has called dma_pool_alloc() to allocate memory from the > > > interrupt context, and it hits BUG_ON(in_interrpt()) in > > > "get_vm_area_caller()"(*2). Regardless of the badness of allocation > > > from interrupt handler in the driver, I have the following question. > > > > > > The following "__get_vm_area_node()" can take gfp_mask, it means that > > > this function is expected to be called from atomic context, but why > > > it's _NOT_ allowed _ONLY_ from interrupt context? > > > > > > According to the following definitions, "in_interrupt()" is in "in_atomic()". > > > > > > #define in_interrupt() (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK | NMI_MASK)) > > > #define in_atomic() ((preempt_count() & ~PREEMPT_ACTIVE) != 0) > > > > > > Does anyone know why BUG_ON(in_interrupt()) is set in __get_vm_area_node(*3)? > > > > For arm_dma_alloc(), it allocates from the pool if GFP_ATOMIC, but for > > arm_iommu_alloc_attrs() doesn't have pre-allocate pool at all, and it > > always call "get_vm_area_caller()". That's why it hits BUG(). But > > still I don't understand why it's not BUG_ON(in_atomic) as Russell > > already pointed out(*1). > > > > *1: http://article.gmane.org/gmane.linux.kernel.mm/76708 > > Ok, now I see the problem. I will try to find out a solution for your issue.
My explanation wasn't so good.
For a solution, I thought that, in order to allow IOMMU'able device drivers to allocate memory from atomic context/ISR, there were the following 2 solutions:
(1) To provide the pre-allocate area like arm_dma_alloc() does, or (2) __get_vm_area_node() can be called from ISR.
But (2) doesn't work because PGALLOC_GFP(GFP_KERNEL) is used to allocate a page table. This is called from:
arm_iommu_alloc_attrs() -> __iommu_alloc_remap() -> ioremap_page_range() -> ..... -> pte_alloc_one_kernel() -> pte = (pte_t *)__get_free_page(PGALLOC_GFP);
We always have to avoid changing a page table for atomic allocation. So for me, the only remaining solution is (1) pre-allocation. We can make use of the same atomic pool both for DMA and IOMMU. I'll send the patch.
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