linux-kernel - Re: [RFC PATCH] mm, proc: report PR_SET_THP

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Date:   Tue, 9 Oct 2018 10:33:26 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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: [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
SUSE Labs