From: Michal Hocko <mhocko@kernel.org>
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
Subject: Re: [RFC PATCH] mm, proc: report PR_SET_THP_DISABLE in proc
Date: Tue, 20 Nov 2018 08:48:00 +0100 [thread overview]
Message-ID: <20181120074759.GB22247@dhcp22.suse.cz> (raw)
In-Reply-To: <alpine.DEB.2.21.1811191404030.150313@chino.kir.corp.google.com>
On Mon 19-11-18 14:05:34, David Rientjes wrote:
> 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.
Let me think about a full patch and see how it looks like.
>
> That said, I think the current handling of the "nh" flag being emitted in
> smaps is logical and ensures no further userspace breakage.
I have already expressed a concern that there is no way to query for
MADV_NOHUGEPAGE if we overload the flag. So this is not a riskfree
option.
> If that is to
> be removed, I consider it an unnecessary risk. That would raised in code
> review.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2018-11-20 7:48 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-24 17:55 [patch] mm, thp: always specify ineligible vmas as nh in smaps David Rientjes
2018-09-24 18:25 ` Vlastimil Babka
2018-09-24 19:17 ` David Rientjes
2018-09-24 19:30 ` [patch v2] " David Rientjes
2018-09-24 19:56 ` Michal Hocko
2018-09-24 20:02 ` Michal Hocko
2018-09-24 20:43 ` Vlastimil Babka
2018-09-25 5:50 ` Michal Hocko
2018-09-25 19:52 ` David Rientjes
2018-09-25 20:29 ` Michal Hocko
2018-09-25 21:45 ` David Rientjes
2018-09-25 22:04 ` Andrew Morton
2018-09-26 0:55 ` David Rientjes
2018-09-26 6:06 ` Michal Hocko
2018-10-02 11:28 ` [RFC PATCH] mm, proc: report PR_SET_THP_DISABLE in proc Michal Hocko
2018-10-02 20:29 ` David Rientjes
2018-10-03 7:36 ` Michal Hocko
2018-10-03 22:51 ` David Rientjes
2018-10-04 5:58 ` Michal Hocko
2018-10-04 9:15 ` David Rientjes
2018-10-04 9:46 ` Michal Hocko
2018-10-04 18:34 ` David Rientjes
2018-10-09 8:33 ` Michal Hocko
2018-10-15 15:03 ` Michal Hocko
2018-10-15 22:25 ` David Rientjes
2018-10-16 10:48 ` Michal Hocko
2018-10-16 21:24 ` David Rientjes
2018-10-17 7:05 ` Michal Hocko
2018-10-17 19:59 ` David Rientjes
2018-10-18 7:00 ` Michal Hocko
2018-11-14 13:23 ` Michal Hocko
2018-11-14 21:41 ` David Rientjes
2018-11-15 9:02 ` Michal Hocko
2018-11-15 9:22 ` Michal Hocko
2018-11-19 22:05 ` David Rientjes
2018-11-20 7:48 ` Michal Hocko [this message]
2018-10-03 17:33 ` Mike Rapoport
2018-09-25 21:50 ` [patch v3] mm, thp: always specify disabled vmas as nh in smaps David Rientjes
2018-09-26 6:12 ` Michal Hocko
2018-09-26 7:17 ` Michal Hocko
2018-09-26 8:40 ` Vlastimil Babka
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