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Date: Wed, 3 Oct 2018 09:36:40 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [RFC PATCH] mm, proc: report PR_SET_THP_DISABLE in proc
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To: David Rientjes <rientjes@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Vlastimil Babka <vbabka@suse.cz>, Alexey Dobriyan <adobriyan@gmail.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org

On Tue 02-10-18 13:29:42, 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@google.com> 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)?

/me confused. I thought you want to query for the flag on a
_different_ process. 
-- 
Michal Hocko
SUSE Labs