On Tue 09-10-18 10:33:26, Michal Hocko wrote: > On Thu 04-10-18 11:34:11, David Rientjes wrote: > > On Thu, 4 Oct 2018, Michal Hocko wrote: > > > > > > And prior to the offending commit, there were three ways to control thp > > > > but two ways to determine if a mapping was eligible for thp based on the > > > > implementation detail of one of those ways. > > > > > > Yes, it is really unfortunate that we have ever allowed to leak such an > > > internal stuff like VMA flags to userspace. > > > > > > > Right, I don't like userspace dependencies on VmFlags in smaps myself, but > > it's the only way we have available that shows whether a single mapping is > > eligible to be backed by thp :/ > > Which is not the case due to reasons mentioned earlier. It only speaks > about madvise status on the VMA. > > > > > If there are three ways to > > > > control thp, userspace is still in the dark wrt which takes precedence > > > > over the other: we have PR_SET_THP_DISABLE but globally sysfs has it set > > > > to "always", or we have MADV_HUGEPAGE set per smaps but PR_SET_THP_DISABLE > > > > shown in /proc/pid/status, etc. > > > > > > > > Which one is the ultimate authority? > > > > > > Isn't our documentation good enough? If not then we should document it > > > properly. > > > > > > > No, because the offending commit actually changed the precedence itself: > > PR_SET_THP_DISABLE used to be honored for future mappings and the commit > > changed that for all current mappings. > > Which is the actual and the full point of the fix as described in the > changelog. The original implementation was poor and inconsistent. > > > So as a result of the commit > > itself we would have had to change the documentation and userspace can't > > be expected to keep up with yet a fourth variable: kernel version. It > > really needs to be simpler, just a per-mapping specifier. > > As I've said, if you really need a per-vma granularity then make it a > dedicated line in the output with a clear semantic. Do not make VMA > flags even more confusing. Can we settle with something please? -- Michal Hocko SUSE Labs
