linux-kernel - RE: [PATCH 1/3] mm: Export vmf_insert_mixed

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Message-ID:
 <BN7PR02MB4148597D0495C631F6E3F8C0D4B42@BN7PR02MB4148.namprd02.prod.outlook.com>
Date: Wed, 9 Apr 2025 14:10:26 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Christoph Hellwig <hch@....de>
CC: "jayalk@...works.biz" <jayalk@...works.biz>, "simona@...ll.ch"
	<simona@...ll.ch>, "deller@....de" <deller@....de>, "haiyangz@...rosoft.com"
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	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
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	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: RE: [PATCH 1/3] mm: Export vmf_insert_mixed_mkwrite()

From: Christoph Hellwig <hch@....de> Sent: Wednesday, April 9, 2025 3:50 AM
> 
> On Tue, Apr 08, 2025 at 11:36:44AM -0700, mhkelley58@...il.com wrote:
> > From: Michael Kelley <mhklinux@...look.com>
> >
> > 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