On Thu, 15 Nov 2018, Michal Hocko wrote: > > The userspace had a single way to determine if thp had been disabled for a > > specific vma and that was broken with your commit. We have since fixed > > it. Modifying our software stack to start looking for some field > > somewhere else will not help anybody else that this has affected or will > > affect. I'm interested in not breaking userspace, not trying a wait and > > see approach to see if anybody else complains once we start looking for > > some other field. The risk outweighs the reward, it already broke us, and > > I'd prefer not to even open the possibility of breaking anybody else. > > I very much agree on "do not break userspace" part but this is kind of > gray area. VMA flags are a deep internal implementation detail and > nobody should really depend on it for anything important. The original > motivation for introducing it was CRIU where it is kind of > understandable. I would argue they should find a different way but it is > just too late for them. > > For this particular case there was no other bug report except for yours > and if it is possible to fix it on your end then I would really love to > make the a sensible user interface to query the status. If we are going > to change the semantic of the exported flag again then we risk yet > another breakage. > > Therefore I am asking whether changing your particular usecase to a new > interface is possible because that would allow to have a longerm > sensible user interface rather than another kludge which still doesn't > cover all the usecases (e.g. there is no way to reliably query the > madvise status after your patch). > Providing another interface is great, I have no objection other than emitting another line for every vma on the system for smaps is probably overkill for something as rare as PR_SET_THP_DISABLE. That said, I think the current handling of the "nh" flag being emitted in smaps is logical and ensures no further userspace breakage. If that is to be removed, I consider it an unnecessary risk. That would raised in code review.
