linux-kernel - Re: mmu_notifier and i915_gem

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Message-ID: <20140625221859.GD7732@8bytes.org>
Date:	Thu, 26 Jun 2014 00:18:59 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	Daniel Vetter <daniel.vetter@...ll.ch>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	David Airlie <airlied@...ux.ie>,
	intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: mmu_notifier and i915_gem_userptr.c

On Fri, Jun 20, 2014 at 01:43:50PM +0200, Joerg Roedel wrote:
> Change_pte is also called when the underlying page of an address
> changes in the kernel which would matter for DMA. But that can only
> happen in KSM and uprobes code which is probably not of interest for the
> i915 driver.
> 
> The other case where I think it matters is the do_wp_page() path for
> COW. The code works by calling invalidate_range_start -> change_pte ->
> invalidate_range_end. Your driver would react to this by unbinding the
> vma from itself internally (after a fork for example).
> 
> But I have to check whether this really matters here.

Okay, I think it does not matter for the i915 driver. The code-paths
which map pages read-only for COW invoke invalidate_range_start/end on
the page-ranges which causes the driver to unbind the pages.

When get_user_pages() is called again later it will do the COW by
itself, so the driver doesn't need to care.

So I tend to say that the i915 driver does not need a change_pte()
call-back at all. But probably someone should double-check to make sure
I didn't miss something.


	Joerg


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