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

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Date:   Mon, 19 Nov 2018 14:05:34 -0800 (PST)
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, 15 Nov 2018, Michal Hocko wrote:

> > The userspace had a single way to determine if thp had been disabled for a 
> > specific vma and that was broken with your commit.  We have since fixed 
> > it.  Modifying our software stack to start looking for some field 
> > somewhere else will not help anybody else that this has affected or will 
> > affect.  I'm interested in not breaking userspace, not trying a wait and 
> > see approach to see if anybody else complains once we start looking for 
> > some other field.  The risk outweighs the reward, it already broke us, and 
> > I'd prefer not to even open the possibility of breaking anybody else.
> 
> I very much agree on "do not break userspace" part but this is kind of
> gray area. VMA flags are a deep internal implementation detail and
> nobody should really depend on it for anything important. The original
> motivation for introducing it was CRIU where it is kind of
> understandable. I would argue they should find a different way but it is
> just too late for them.
> 
> For this particular case there was no other bug report except for yours
> and if it is possible to fix it on your end then I would really love to
> make the a sensible user interface to query the status. If we are going
> to change the semantic of the exported flag again then we risk yet
> another breakage.
> 
> Therefore I am asking whether changing your particular usecase to a new
> interface is possible because that would allow to have a longerm
> sensible user interface rather than another kludge which still doesn't
> cover all the usecases (e.g. there is no way to reliably query the
> madvise status after your patch).
> 

Providing another interface is great, I have no objection other than 
emitting another line for every vma on the system for smaps is probably 
overkill for something as rare as PR_SET_THP_DISABLE.

That said, I think the current handling of the "nh" flag being emitted in 
smaps is logical and ensures no further userspace breakage.  If that is to 
be removed, I consider it an unnecessary risk.  That would raised in code 
review.