RFC mm modifications / helpers for emulated GPU coherent memory

Hi,
This is an early RFC to make sure I don't go too far in the wrong direction.

Non-coherent GPUs that can't directly see contents in CPU-visible memory, like VMWare's SVGA device, run into trouble when trying to implement coherent memory requirements of modern graphics APIs. Examples are Vulkan and OpenGL 4.4's ARB_buffer_storage.

To remedy, we need to emulate coherent memory. Typically when it's detected that a buffer object is about to be accessed by the GPU, we need to gather the ranges that have been dirtied by the CPU since the last operation, apply an operation to make the content visible to the GPU and clear the the dirty tracking.

Depending on the size of the buffer object and the access pattern there are two major possibilities:

1) Use page_mkwrite() and pfn_mkwrite(). (GPU buffer objects are backed either by PCI device memory or by driver-alloced pages).
The dirty-tracking needs to be reset by write-protecting the affected ptes and flush tlb. This has a complexity of O(num_dirty_pages), but the write page-fault is of course costly.

2) Use hardware dirty-flags in the ptes. The dirty-tracking needs to be reset by clearing the dirty bits and flush tlb. This has a complexity of O(num_buffer_object_pages) and dirty bits need to be scanned in full before each gpu-access.

So in practice the two methods need to be interleaved for best performance.

So to facilitate this, I propose two new helpers, apply_as_wrprotect() and apply_as_clean() ("as" stands for address-space) both inspired by unmap_mapping_range(). Users of these helpers are in the making, but needs some cleaning-up.

There's also a change to x_mkwrite() to allow dropping the mmap_sem while waiting.

Any comments or suggestions appreciated.

Thanks,
Thomas

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