[PATCH] ARM: DMA: Fix kzalloc flags in __iommu_alloc_buffer()

[Alexandre Courbot]

On Mon, Dec 8, 2014 at 7:24 PM, Arnd Bergmann <arnd at arndb.de> wrote:
> On Monday 08 December 2014 17:39:27 Alexandre Courbot wrote:
>> There doesn't seem to be any valid reason to allocate the pages array
>> with the same flags as the buffer itself. Doing so can eventually lead
>> to the following safeguard in mm/slab.c to be hit:
>>
>> BUG_ON(flags & GFP_SLAB_BUG_MASK);
>>
>> This happens when buffers are allocated with __GFP_DMA32 or
>> __GFP_HIGHMEM.
>>
>> Fix this by allocating the pages array with GFP_KERNEL to follow what is
>> done elsewhere in this file. Using GFP_KERNEL in __iommu_alloc_buffer()
>> is safe because atomic allocations are handled by __iommu_alloc_atomic().
>>
>
> I think you need to carry over the GFP_ATOMIC flag if that is set by the
> caller, but not the GFP_HIGHMEM or GFP_DMA32. Not sure if it's better
> to mask out flags from the caller mask, or to start with GFP_KERNEL
> and adding in extra bits.

I thought the issue of atomicity is already handled by
__iommu_alloc_buffer's caller (arm_iommu_alloc_attrs):

    if (!(gfp & __GFP_WAIT))
        return __iommu_alloc_atomic(dev, size, handle);
    ....
    pages = __iommu_alloc_buffer(dev, size, gfp, attrs);

Isn't the interesting property about GFP_ATOMIC that it does not
include __GFP_WAIT? I may very well misunderstand the issue, sorry if
that's the case.