linux-kernel - Re: [patch v2] mm, thp: always specify ineligible vmas as nh in smaps

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Date:   Wed, 26 Sep 2018 08:06:24 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org
Subject: Re: [patch v2] mm, thp: always specify ineligible vmas as nh in smaps

On Tue 25-09-18 15:04:06, Andrew Morton wrote:
> On Tue, 25 Sep 2018 14:45:19 -0700 (PDT) David Rientjes <rientjes@...gle.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.

This would also keep a sane (and currently documented) semantic for
the smaps flag to be really
    hg  - huge page advise flag
    nh  - no-huge page advise flag
-- 
Michal Hocko
SUSE Labs