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 at 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 at 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. > 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
