[Intel-gfx] [PATCH 4/4] i915: fix remap_io_sg to verify the pgprot

[Thomas Hellström]

On 5/17/21 3:11 PM, Christoph Hellwig wrote:
> On Mon, May 17, 2021 at 04:09:42PM +0300, Serge Belyshev wrote:
>> Christoph Hellwig <hch at lst.de> writes:
>>
>>> As an ad-hoc experiment:  can you replace the call to remap_pfn_range
>>> with remap_pfn_range_notrack (and export it if you build i915 modular)
>>> in remap_io_sg and see if that makes any difference?
>> That worked, thanks -- no artifacts seen.
> Looks like it is caused by the validation failure then.  Which means the
> existing code is doing something wrong in its choice of the page
> protection bit.  I really need help from the i915 maintainers here..

Hmm,

Apart from the caching aliasing Mattew brought up, doesn't the 
remap_pfn_range_xxx() family require the mmap_sem held in write mode 
since it modifies the vma structure? remap_io_sg() is called from the 
fault handler with the mmap_sem held in read mode only.

/Thomas

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