[lkml]   [2018]   [Oct]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC PATCH] mm, proc: report PR_SET_THP_DISABLE in proc
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.

Michal Hocko

 \ /
  Last update: 2018-10-09 10:34    [W:0.111 / U:27.588 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site