Re: [PATCH v4 09/14] selftests: harness: Move teardown conditional into test metadata

[Jason Gunthorpe]

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.

> > 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.

Jason