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

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Date:   Thu, 4 Oct 2018 11:34:11 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
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, 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 :/

> > 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.  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.