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

[Michael Kelley]

From: Christoph Hellwig <hch@xxxxxx> Sent: Wednesday, April 9, 2025 3:50 AM
> 
> On Tue, Apr 08, 2025 at 11:36:44AM -0700, mhkelley58@xxxxxxxxx wrote:
> > From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> >
> > Export vmf_insert_mixed_mkwrite() for use by fbdev deferred I/O code,
> 
> But they are using this on dma coherent memory, where you can't legally
> get at the page.  As told last time you need to fix that first before
> hacking around that code.

Hmmm. What's the reference to "as told last time"? I don't think I've had
this conversation before.

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.

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

> 
> > which can be built as a module. For consistency with the related function
> > vmf_insert_mixed(), export without the GPL qualifier.
> 
> No.  All advanced new Linux functionality must be _GPL.  Don't try to
> sneak around that.

No problem. Just trying to be consistent, not sneak around anything.

Thanks,

Michael