Hi Marek,
On Tue, 21 Aug 2012 17:01:08 +0200
Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> 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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