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
