On Mon 19-11-18 14:05:34, David Rientjes wrote: > 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. Let me think about a full patch and see how it looks like. > > That said, I think the current handling of the "nh" flag being emitted in > smaps is logical and ensures no further userspace breakage. I have already expressed a concern that there is no way to query for MADV_NOHUGEPAGE if we overload the flag. So this is not a riskfree option. > If that is to > be removed, I consider it an unnecessary risk. That would raised in code > review. -- Michal Hocko SUSE Labs
