Re: [PATCH] drm/vma-manager: Don't unmap COW'd pages when zapping bo ptes

[Thomas Hellstrom <thellstrom@vmware.com>]

On 11/20/2013 03:24 PM, Daniel Vetter wrote:
> On Wed, Nov 20, 2013 at 01:55:49AM -0800, Thomas Hellstrom wrote:
>> Not sure if there are any user-space users of private bo mappings, but
>> if there are, or will be, zapping the COW'd pages when, for example,
>> moving a bo would confuse the user immensely since the net effect for the
>> user would be that pages written to would lose their contents.
>>
>> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
> Presuming I'm not horribly confused about that all the vm slang in the
> kerneldoc means this changes is
>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>
> Now I still hold that userspace creating anynomous bo mappings is rather
> crazy, but meh ;-) I guess the real question is whether we have anyone
> relying on this out there (or planing to), in which case we either need to
> funnel this through stable kernels or whack a drm feature flag onto
> drivers/kernels with this fixed. But I seriously hope the answer is no.
>
> Cheers, Daniel
>
Thanks for reviewing. I don't think there's a need to take this through 
stable, since I don't know of anyone using private VMAs,

Actually I'll need to hold off on this for a while since on some archs 
this may cause
ptes of shared pages to not be zapped. If the arch doesn't have 
PTE_SPECIAL, shared pages on MIXEDMAP vmas will come through as
vm_normal_page, and since page->mapping is usually (un)set to NULL by 
our drivers, this will result in false positives for COW'ed pages.

So that test is buggy, or us not setting page->mapping to the mapping we 
use is buggy. It's too late in the day to decide which.

/Thomas