Quoting Guenter Roeck (2019-02-28 21:32:41)
> On Thu, Feb 28, 2019 at 11:12:49AM -0800, Guenter Roeck wrote:
> > Hi,
> >
> > On Thu, Feb 07, 2019 at 10:54:53AM +0200, Joonas Lahtinen wrote:
> > > Make sure the underlying VMA in the process address space is the
> > > same as it was during vm_mmap to avoid applying WC to wrong VMA.
> > >
> > > A more long-term solution would be to have vm_mmap_locked variant
> > > in linux/mmap.h for when caller wants to hold mmap_sem for an
> > > extended duration.
> > >
> >
> > It seems like we may have a regression due to this patch. I am still
> > debugging, but I have a question; please see below.
> >
> > Thanks,
> > Guenter
> >
> > > v2:
> > > - Refactor the compare function
> > >
> > > Fixes: 1816f9236303 ("drm/i915: Support creation of unbound wc user
> > > mappings for objects")
> > > Reported-by: Adam Zabrocki <[email protected]>
> > > Suggested-by: Linus Torvalds <[email protected]>
> > > Signed-off-by: Joonas Lahtinen <[email protected]>
> > > Cc: <[email protected]> # v4.0+
> > > Cc: Akash Goel <[email protected]>
> > > Cc: Chris Wilson <[email protected]>
> > > Cc: Tvrtko Ursulin <[email protected]>
> > > Cc: Adam Zabrocki <[email protected]>
> > > Reviewed-by: Chris Wilson <[email protected]>
> > > Reviewed-by: Tvrtko Ursulin <[email protected]> #v1
> > > ---
> > > drivers/gpu/drm/i915/i915_gem.c | 12 +++++++++++-
> > > 1 file changed, 11 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/i915_gem.c
> > > b/drivers/gpu/drm/i915/i915_gem.c
> > > index 05ce9176ac4e..52639f749908 100644
> > > --- a/drivers/gpu/drm/i915/i915_gem.c
> > > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > > @@ -1681,6 +1681,16 @@ i915_gem_sw_finish_ioctl(struct drm_device *dev,
> > > void *data,
> > > return 0;
> > > }
> > >
> > > +static inline bool
> > > +__vma_matches(struct vm_area_struct *vma, struct file *filp,
> > > + unsigned long addr, unsigned long size)
> > > +{
> > > + if (vma->vm_file != filp)
> > > + return false;
> > > +
> > > + return vma->vm_start == addr && (vma->vm_end - vma->vm_start) == size;
> >
> > Shouldn't this be:
> > return vma->vm_start == addr && (vma->vm_end - vma->vm_start + 1) ==
> > size;
> > instead ?
> >
>
> Answer is no .. because vm_end points to the first byte after the
> end address.
>
> The actual values are:
>
> start=7d288f7f9000 end=7d288f84d000 end-start=54000 size=53400
>
> meaning the size field passed in the ioctl is smaller than the total length
> of the area.
>
> Question is now: Is the request/ioctl indeed invalid, ie does the requested
> size have to match the vma size ?
Yes. The vma is page-aligned, your request isn't. What happens next is
undefined behaviour, and almost certainly not what you expect -- you
can't access the last bits of your framebuffer.
-Chris
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