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From: Michal Hocko <mhocko@kernel.org>
To: Jiri Kosina <jikos@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-api@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	Jann Horn <jannh@google.com>,
	Dominique Martinet <asmadeus@codewreck.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Chinner <david@fromorbit.com>,
	Kevin Easton <kevin@guarana.org>,
	Matthew Wilcox <willy@infradead.org>,
	Cyril Hrubis <chrubis@suse.cz>, Tejun Heo <tj@kernel.org>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Daniel Gruss <daniel@gruss.cc>, Josh Snyder <joshs@netflix.com>
Subject: Re: [PATCH 3/3] mm/mincore: provide mapped status when cached status is not allowed
Date: Tue, 12 Feb 2019 07:36:43 +0100
Message-ID: <20190212063643.GL15609@dhcp22.suse.cz> (raw)
In-Reply-To: <nycvar.YFH.7.76.1902120440430.11598@cbobk.fhfr.pm>

On Tue 12-02-19 04:44:30, Jiri Kosina wrote:
> On Fri, 1 Feb 2019, Vlastimil Babka wrote:
> 
> > >> After "mm/mincore: make mincore() more conservative" we sometimes restrict the
> > >> information about page cache residency, which we have to do without breaking
> > >> existing userspace, if possible. We thus fake the resulting values as 1, which
> > >> should be safer than faking them as 0, as there might theoretically exist code
> > >> that would try to fault in the page(s) until mincore() returns 1.
> > >>
> > >> Faking 1 however means that such code would not fault in a page even if it was
> > >> not in page cache, with unwanted performance implications. We can improve the
> > >> situation by revisting the approach of 574823bfab82 ("Change mincore() to count
> > >> "mapped" pages rather than "cached" pages") but only applying it to cases where
> > >> page cache residency check is restricted. Thus mincore() will return 0 for an
> > >> unmapped page (which may or may not be resident in a pagecache), and 1 after
> > >> the process faults it in.
> > >>
> > >> One potential downside is that mincore() will be again able to recognize when a
> > >> previously mapped page was reclaimed. While that might be useful for some
> > >> attack scenarios, it's not as crucial as recognizing that somebody else faulted
> > >> the page in, and there are also other ways to recognize reclaimed pages anyway.
> > > 
> > > Is this really worth it? Do we know about any specific usecase that
> > > would benefit from this change? TBH I would rather wait for the report
> > > than add a hard to evaluate side channel.
> > 
> > Well it's not that complicated IMHO. Linus said it's worth trying, so
> > let's see how he likes the result. The side channel exists anyway as
> > long as process can e.g. check if its rss shrinked, and I doubt we are
> > going to remove that possibility.
> 
> So, where do we go from here?
> 
> Either Linus and Andrew like the mincore() return value tweak, or this 
> could be further discussed (*). But in either of the cases, I think 
> patches 1 and 2 should be at least queued for 5.1.

I would go with patch 1 for 5.1. Patches 2 still sounds controversial or
incomplete to me. And patch 3, well I will leave the decision to
Andrew/Linus.

> (*) I'd personally include it as well, as I don't see how it would break 
>     anything, it's pretty straightforward, and brings back some sanity to
>     mincore() return value.

-- 
Michal Hocko
SUSE Labs

  reply index

Thread overview: 161+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-05 17:27 [PATCH] mm/mincore: allow for making sys_mincore() privileged Jiri Kosina
2019-01-05 19:14 ` Vlastimil Babka
2019-01-05 19:24   ` Jiri Kosina
2019-01-05 19:38     ` Vlastimil Babka
2019-01-08  9:14       ` Bernd Petrovitsch
2019-01-08 11:37         ` Jiri Kosina
2019-01-08 13:53           ` Bernd Petrovitsch
2019-01-08 14:08             ` Kirill A. Shutemov
2019-01-05 19:44 ` kbuild test robot
2019-01-05 19:46 ` Linus Torvalds
2019-01-05 20:12   ` Jiri Kosina
2019-01-05 20:17     ` Linus Torvalds
2019-01-05 20:43       ` Jiri Kosina
2019-01-05 21:54         ` Linus Torvalds
2019-01-06 11:33           ` Kevin Easton
2019-01-08  8:50           ` Kevin Easton
2019-01-18 14:23           ` Tejun Heo
2019-01-05 20:13   ` Linus Torvalds
2019-01-05 19:56 ` kbuild test robot
2019-01-05 22:54 ` Jann Horn
2019-01-05 23:05   ` Linus Torvalds
2019-01-05 23:16     ` Linus Torvalds
2019-01-05 23:28       ` Linus Torvalds
2019-01-05 23:39       ` Linus Torvalds
2019-01-06  0:11         ` Matthew Wilcox
2019-01-06  0:22           ` Linus Torvalds
2019-01-06  1:50             ` Linus Torvalds
2019-01-06 21:46               ` Linus Torvalds
2019-01-08  4:43                 ` Dave Chinner
2019-01-08 17:57                   ` Linus Torvalds
2019-01-09  2:24                     ` Dave Chinner
2019-01-09  2:31                       ` Jiri Kosina
2019-01-09  4:39                         ` Dave Chinner
2019-01-09 10:08                           ` Jiri Kosina
2019-01-10  1:15                             ` Dave Chinner
2019-01-10  7:54                               ` Jiri Kosina
2019-01-09 18:25                           ` Linus Torvalds
2019-01-10  0:44                             ` Dave Chinner
2019-01-10  1:18                               ` Linus Torvalds
2019-01-10  5:26                                 ` Andy Lutomirski
2019-01-10 14:47                                   ` Matthew Wilcox
2019-01-10 21:44                                     ` Dave Chinner
2019-01-10 21:59                                       ` Linus Torvalds
2019-01-11  1:47                                   ` Dave Chinner
2019-01-10  7:03                                 ` Dave Chinner
2019-01-10 11:47                                   ` Linus Torvalds
2019-01-10 12:24                                     ` Dominique Martinet
2019-01-10 22:11                                       ` Linus Torvalds
2019-01-11  2:03                                         ` Dave Chinner
2019-01-11  2:18                                           ` Linus Torvalds
2019-01-11  4:04                                             ` Dave Chinner
2019-01-11  4:08                                               ` Andy Lutomirski
2019-01-11  7:20                                                 ` Dave Chinner
2019-01-11  7:08                                               ` Linus Torvalds
2019-01-11  7:36                                                 ` Dave Chinner
2019-01-11 16:26                                                   ` Linus Torvalds
2019-01-15 23:45                                                     ` Dave Chinner
2019-01-16  4:54                                                       ` Linus Torvalds
2019-01-16  5:49                                                         ` Linus Torvalds
2019-01-17  1:26                                                         ` Dave Chinner
2019-02-20 15:49                                                     ` Nicolai Stange
2019-01-11  4:57                                         ` Dominique Martinet
2019-01-11  7:11                                           ` Linus Torvalds
2019-01-11  7:32                                             ` Dominique Martinet
2019-01-16  0:42                                         ` Josh Snyder
2019-01-16  5:00                                           ` Linus Torvalds
2019-01-16  5:25                                             ` Andy Lutomirski
2019-01-16  5:34                                               ` Linus Torvalds
2019-01-16  5:46                                                 ` Dominique Martinet
2019-01-16  5:58                                                   ` Linus Torvalds
2019-01-16  6:34                                                     ` Dominique Martinet
2019-01-16  7:52                                                       ` Josh Snyder
2019-01-16 12:18                                                         ` Kevin Easton
2019-01-17 21:45                                                         ` Vlastimil Babka
2019-01-18  4:49                                                           ` Linus Torvalds
2019-01-18 18:58                                                             ` Vlastimil Babka
2019-01-16 16:12                                                     ` Jiri Kosina
2019-01-16 17:48                                                       ` Linus Torvalds
2019-01-16 20:23                                                         ` Jiri Kosina
2019-01-16 21:37                                                           ` Matthew Wilcox
2019-01-16 21:41                                                             ` Jiri Kosina
2019-01-17  9:52                                                               ` Cyril Hrubis
2019-01-28 13:49                                                               ` Cyril Hrubis
2019-01-17  4:51                                                             ` Linus Torvalds
2019-01-18  4:54                                                               ` Linus Torvalds
2019-01-17  1:49                                                           ` Dominique Martinet
2019-01-23 20:27                                                           ` Linus Torvalds
2019-01-23 20:35                                                             ` Linus Torvalds
2019-01-23 23:12                                                               ` Jiri Kosina
2019-01-24  0:20                                                                 ` Linus Torvalds
2019-01-24  0:24                                                             ` Dominique Martinet
2019-01-24 12:45                                                               ` Dominique Martinet
2019-01-24 14:25                                                                 ` Jiri Kosina
2019-01-27 22:35                                                                   ` Jiri Kosina
2019-01-28  0:05                                                                     ` Dominique Martinet
2019-01-29 23:52                                                                       ` Jiri Kosina
2019-01-30  9:09                                                                         ` Michal Hocko
2019-01-30 12:29                                                                           ` Jiri Kosina
2019-01-16 12:36                                             ` Matthew Wilcox
2019-01-10 14:50                               ` Matthew Wilcox
2019-01-11  7:36                               ` Jiri Kosina
2019-01-17  2:22                                 ` Dave Chinner
2019-01-17  8:18                                   ` Jiri Kosina
2019-01-17 21:06                                     ` Dave Chinner
2019-01-07  4:32             ` Dominique Martinet
2019-01-07 10:33               ` Vlastimil Babka
2019-01-07 11:08                 ` Dominique Martinet
2019-01-07 11:59                   ` Vlastimil Babka
2019-01-07 13:29                   ` Daniel Gruss
2019-01-07 10:10         ` Michael Ellerman
2019-01-05 23:09   ` Jiri Kosina
2019-01-30 12:44 ` [PATCH 0/3] mincore() and IOCB_NOWAIT adjustments Vlastimil Babka
2019-01-30 12:44   ` [PATCH 1/3] mm/mincore: make mincore() more conservative Vlastimil Babka
2019-01-31  9:43     ` Michal Hocko
2019-01-31  9:51       ` Dominique Martinet
2019-01-31 17:46       ` Josh Snyder
2019-02-01  8:56     ` Vlastimil Babka
2019-03-06 23:13     ` Andrew Morton
2019-03-07  0:01       ` Jiri Kosina
2019-03-07  0:40         ` Dominique Martinet
2019-03-07  5:46           ` Jiri Kosina
2019-01-30 12:44   ` [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O Vlastimil Babka
2019-01-30 15:04     ` Florian Weimer
2019-01-30 15:15       ` Jiri Kosina
2019-01-31 10:47         ` Florian Weimer
2019-01-31 11:34           ` Jiri Kosina
2019-01-31  9:56     ` Michal Hocko
2019-01-31 10:15       ` Jiri Kosina
2019-01-31 10:23         ` Michal Hocko
2019-01-31 10:30           ` Jiri Kosina
2019-01-31 11:32             ` Michal Hocko
2019-01-31 17:54           ` Linus Torvalds
2019-02-01  5:13             ` Dave Chinner
2019-02-01  7:05               ` Linus Torvalds
2019-02-01  7:21                 ` Linus Torvalds
2019-02-01  1:44       ` Dave Chinner
2019-02-12 15:48         ` Jiri Kosina
2019-01-31 12:04     ` Daniel Gruss
2019-01-31 12:06       ` Vlastimil Babka
2019-01-31 12:08       ` Jiri Kosina
2019-01-31 12:57         ` Daniel Gruss
2019-01-30 12:44   ` [PATCH 3/3] mm/mincore: provide mapped status when cached status is not allowed Vlastimil Babka
2019-01-31 10:09     ` Michal Hocko
2019-02-01  9:04       ` Vlastimil Babka
2019-02-01  9:11         ` Michal Hocko
2019-02-01  9:27           ` Vlastimil Babka
2019-02-06 20:14             ` Jiri Kosina
2019-02-12  3:44         ` Jiri Kosina
2019-02-12  6:36           ` Michal Hocko [this message]
2019-02-12 13:09             ` Jiri Kosina
2019-02-12 14:01               ` Michal Hocko
2019-03-06 12:11   ` [PATCH 0/3] mincore() and IOCB_NOWAIT adjustments Jiri Kosina
2019-03-06 22:35     ` Andrew Morton
2019-03-06 22:48       ` Jiri Kosina
2019-03-06 23:23         ` Andrew Morton
2019-03-06 23:32           ` Dominique Martinet
2019-03-06 23:38             ` Andrew Morton
2019-03-09 16:53               ` Linus Torvalds
2019-03-12 14:17   ` [PATCH v2 0/2] prevent mincore() page cache leaks Vlastimil Babka
2019-03-12 14:17     ` [PATCH v2 1/2] mm/mincore: make mincore() more conservative Vlastimil Babka
2019-03-12 14:17     ` [PATCH v2 2/2] mm/mincore: provide mapped status when cached status is not allowed Vlastimil Babka

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