[RFC PATCH] mm, proc: report PR_SET_THP_DISABLE in proc

[Michal Hocko]

On Wed 26-09-18 08:06:24, Michal Hocko wrote:
> On Tue 25-09-18 15:04:06, Andrew Morton wrote:
> > On Tue, 25 Sep 2018 14:45:19 -0700 (PDT) David Rientjes <rientjes@xxxxxxxxxx> wrote:
> > 
> > > > > It is also used in 
> > > > > automated testing to ensure that vmas get disabled for thp appropriately 
> > > > > and we used "nh" since that is how PR_SET_THP_DISABLE previously enforced 
> > > > > this, and those tests now break.
> > > > 
> > > > This sounds like a bit of an abuse to me. It shows how an internal
> > > > implementation detail leaks out to the userspace which is something we
> > > > should try to avoid.
> > > > 
> > > 
> > > Well, it's already how this has worked for years before commit 
> > > 1860033237d4 broke it.  Changing the implementation in the kernel is fine 
> > > as long as you don't break userspace who relies on what is exported to it 
> > > and is the only way to determine if MADV_NOHUGEPAGE is preventing it from 
> > > being backed by hugepages.
> > 
> > 1860033237d4 was over a year ago so perhaps we don't need to be
> > too worried about restoring the old interface.  In which case
> > we have an opportunity to make improvements such as that suggested
> > by Michal?
> 
> Yeah, can we add a way to export PR_SET_THP_DISABLE to userspace
> somehow? E.g. /proc/<pid>/status. It is a process wide thing so
> reporting it per VMA sounds strange at best.

So how about this? (not tested yet but it should be pretty
straightforward)
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