On Thu, Jun 12, 2025 at 12:42:42PM -0300, Jason Gunthorpe wrote:
> On Thu, Jun 12, 2025 at 05:23:01PM +0200, Thomas Weißschuh wrote:
> > On Thu, Jun 12, 2025 at 11:58:01AM -0300, Jason Gunthorpe wrote:
> > > On Thu, Jun 12, 2025 at 04:27:41PM +0200, Thomas Weißschuh wrote:
> > >
> > > > If the assumption is that this is most likely a kernel bug,
> > > > shouldn't it be fixed properly rather than worked around?
> > > > After all the job of a selftest is to detect bugs to be fixed.
> > >
> > > I investigated the history for a bit and it seems likely we cannot
> > > change the kernel here. Call it an undocumented "feature".
> >
> > I looked a bit and it seems to be mentioned in mmap(2):
> >
> > For mmap(), offset must be a multiple of the underlying huge page size.
> > The system automatically aligns length to be a multiple of the
> > underlying huge page size.
>
> Oh there you go then :) Horrible design. No way for userspace to know
> what the rounded up length actually was and thus no way for
> userspace to unmap it.
OK. I think we would have to skip those cases then.
> > > MAP_HUGETLBFS rounds up the length to some value, userspace has to
> > > figure that out and not pass incorrect lengths. The selftest is doing
> > > that wrong.
> >
> > The selftest would be more robust if MAP_FIXED is replaced by
> > MAP_FIXED_NOREPLACE. Even with the new explicit skip logic it should
> > make debugging easier if something goes wrong.
>
> The point is to replace something that is already mapped there, though
> I no longer remember why it is working like this.
By replacing MAP_FIXED with MAP_FIXED_NOREPLACE, at the existing
two places, the selftest crashed at early setup_sizes...:
iommufd: iommufd.c:53: setup_sizes: Assertion `vrc == buffer' failed.
/nicolinc/iommufd_selftest.sh: line 19: 21487 Aborted
(core dumped) tools/testing/selftests/iommu/iommufd
strace:
mmap(0xffff80000000, 1048576, PROT_READ|PROT_WRITE,
MAP_SHARED|MAP_ANONYMOUS|MAP_FIXED_NOREPLACE, -1, 0) = -1 EEXIST (File exists)
This one doesn't MAP_HUGETLBFS btw...
Thanks
Nicolin
