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

[Mike Rapoport]

On Tue, Oct 02, 2018 at 01:29:42PM -0700, David Rientjes wrote:
> On Tue, 2 Oct 2018, Michal Hocko wrote:
> 
> > 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)
> 
> Umm, prctl(PR_GET_THP_DISABLE)?
> 

~/git/linux$ git grep PR_GET_THP_DISABLE
include/uapi/linux/prctl.h:#define PR_GET_THP_DISABLE   42
kernel/sys.c:   case PR_GET_THP_DISABLE:
tools/include/uapi/linux/prctl.h:#define PR_GET_THP_DISABLE     42

-- 
Sincerely yours,
Mike.