On 11/21/2013 09:18 AM, Daniel Vetter wrote:
On Wed, Nov 20, 2013 at 11:35:31PM +0100, Thomas Hellstrom wrote:
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@xxxxxxxxxx>
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@xxxxxxxx>
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.
On second thought we also use this helper when zapping a bo when
deleting it or purging it's backing storage. And there I'd say it would be
consistent with normal semantics to also zap cow pages.
Agreed. That's consistent with truncation. Perhaps this was what David
meant.
Out of interest, why do you delete a bo or purge backing store before
all vmas are gone?
So I'm no longer
sure this is the right approach in general. Maybe we should instead wire
up the switch, but not sure whether that's indeed worth the effort.
I'll leave it as is. If I happen to sort out the non-cow zapping, I'll
post a new patch.
/Thomas
-Daniel
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