Re: [PATCH 1/3] mm: Export vmf_insert_mixed_mkwrite()

[Christoph Hellwig <hch@lst.de>]

On Wed, Apr 09, 2025 at 02:10:26PM +0000, Michael Kelley wrote:
> Hmmm. What's the reference to "as told last time"? I don't think I've had
> this conversation before.

Hmm, there was a conversation about deferred I/O, and I remember the
drm folks even defending their abuse of vmalloc_to_page on dma coherent
memory against the documentation in the most silly way.  Maybe that was
a different discussion of the same thing.

> 
> For the hyperv_fb driver, the memory in question is allocated with a direct call
> to alloc_pages(), not via dma_alloc_coherent(). There's no DMA in this scenario.
> The memory is shared with the Hyper-V host and designated as the memory
> for the virtual framebuffer device. It is then mapped into user space using the
> mmap() system call against /dev/fb0. User space writes to the memory are
> eventually (and I omit the details) picked up by the Hyper-V host and displayed.

Oh, great.

> Is your point that memory dma_alloc_coherent() memory must be treated as
> a black box, and can't be deconstructed into individual pages? If so, that makes
> sense to me.

Yes.

> But must the same treatment be applied to memory from
> alloc_pages()? This is where I need some education.

No, that's just fine.