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 :/ > > 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. 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.
