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From: Christian Brauner <christian.brauner@ubuntu.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Tejun Heo <tj@kernel.org>, Peter Xu <peterx@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	John Hubbard <jhubbard@nvidia.com>,
	Leon Romanovsky <leonro@nvidia.com>,
	Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"Maya B . Gokhale" <gokhale2@llnl.gov>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Marty Mcfadden <mcfadden8@llnl.gov>,
	Kirill Shutemov <kirill@shutemov.name>,
	Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
	Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Mon, 21 Sep 2020 17:04:50 +0200	[thread overview]
Message-ID: <20200921150450.3mjjb3p3jwgatn4v@wittgenstein> (raw)
In-Reply-To: <20200921145537.GM12990@dhcp22.suse.cz>

On Mon, Sep 21, 2020 at 04:55:37PM +0200, Michal Hocko wrote:
> On Mon 21-09-20 16:43:55, Christian Brauner wrote:
> > On Mon, Sep 21, 2020 at 10:38:47AM -0400, Tejun Heo wrote:
> > > Hello,
> > > 
> > > On Mon, Sep 21, 2020 at 04:28:34PM +0200, Michal Hocko wrote:
> > > > Fundamentaly CLONE_INTO_CGROUP is similar to regular fork + move to the
> > > > target cgroup after the child gets executed. So in principle there
> > > > shouldn't be any big difference. Except that the move has to be explicit
> > > > and the the child has to have enough privileges to move itself. I am not
> > > 
> > > Yeap, they're supposed to be the same operations. We've never clearly
> > > defined how the accounting gets split across moves because 1. it's
> > > inherently blurry and difficult 2. doesn't make any practical difference for
> > > the recommended and vast majority usage pattern which uses migration to seed
> > > the new cgroup. CLONE_INTO_CGROUP doesn't change any of that.
> > > 
> > > > completely sure about CLONE_INTO_CGROUP model though. According to man
> > > > clone(2) it seems that O_RDONLY for the target cgroup directory is
> > > > sufficient. That seems much more relaxed IIUC and it would allow to fork
> > > > into a different cgroup while keeping a lot of resources in the parent's
> > > > proper.
> > > 
> > > If the man page is documenting that, it's wrong. cgroup_css_set_fork() has
> > > an explicit cgroup_may_write() test on the destination cgroup.
> > > CLONE_INTO_CGROUP should follow exactly the same rules as regular
> > > migrations.
> > 
> > Indeed!
> > The O_RDONLY mention on the manpage doesn't make sense but it is
> > explained that the semantics are exactly the same for moving via the
> > filesystem:
> 
> OK, if the semantic is the same as for the task migration then I do not
> see any (new) problems. Care to point me where the actual check is
> enforced? For the migration you need a write access to cgroup.procs but
> if the API expects directory fd then I am not sure how that would expose
> the same behavior.

kernel/cgroup/cgroup.c:cgroup_csset_fork()

So there's which is the first check for inode_permission() essentially:

	/*
	 * Verify that we the target cgroup is writable for us. This is
	 * usually done by the vfs layer but since we're not going through
	 * the vfs layer here we need to do it "manually".
	 */
	ret = cgroup_may_write(dst_cgrp, sb);
	if (ret)
		goto err;

and what you're referring to is checked right after in:

	ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb,
					!(kargs->flags & CLONE_THREAD));
	if (ret)
		goto err;

which calls:

	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb);
	if (ret)
		return ret;

That should be what you're looking for. I've also added selftests as
always that verify this behavior under:

tools/testing/selftests/cgroup/

as soon as CLONE_INTO_CGROUP is detected on the kernel than all the
usual tests are exercised using CLONE_INTO_CGROUP so we should've seen
any regression hopefully.

Thanks!
Christian

  reply	other threads:[~2020-09-21 15:05 UTC|newest]

Thread overview: 98+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-21 23:49 [PATCH 0/4] mm: Simplfy cow handling Peter Xu
2020-08-21 23:49 ` [PATCH 1/4] mm: Trial do_wp_page() simplification Peter Xu
2020-08-24  8:36   ` Kirill Tkhai
2020-08-24 14:30     ` Jan Kara
2020-08-24 15:37       ` Kirill Tkhai
2020-08-24 18:22         ` Linus Torvalds
2020-09-01  7:01           ` Hugh Dickins
2020-09-14 14:38   ` Jason Gunthorpe
2020-09-14 17:32     ` Linus Torvalds
2020-09-14 18:34       ` Peter Xu
2020-09-14 21:15         ` Peter Xu
2020-09-14 22:55           ` Jason Gunthorpe
2020-09-14 22:59             ` Linus Torvalds
2020-09-14 23:28               ` Jason Gunthorpe
2020-09-15  0:19                 ` Linus Torvalds
2020-09-15 14:50                 ` Peter Xu
2020-09-15 15:17                   ` Peter Xu
2020-09-15 16:05                   ` Jason Gunthorpe
2020-09-15 18:29                     ` Jason Gunthorpe
2020-09-15 19:13                       ` Peter Xu
2020-09-15 19:38                         ` Jason Gunthorpe
2020-09-15 21:33                           ` Peter Xu
2020-09-15 23:22                             ` Jason Gunthorpe
2020-09-16  1:50                               ` John Hubbard
2020-09-16 17:48                                 ` Jason Gunthorpe
2020-09-16 18:46                                   ` Peter Xu
2020-09-17 11:25                                     ` Jason Gunthorpe
2020-09-17 18:11                                       ` Linus Torvalds
2020-09-17 19:38                                         ` Jason Gunthorpe
2020-09-17 19:51                                           ` Linus Torvalds
2020-09-18 16:40                                             ` Peter Xu
2020-09-18 17:16                                               ` Linus Torvalds
2020-09-18 19:57                                                 ` Peter Xu
2020-09-18 17:32                                               ` Jason Gunthorpe
2020-09-18 20:40                                                 ` Peter Xu
2020-09-18 20:59                                                   ` Linus Torvalds
2020-09-19  0:28                                                     ` Jason Gunthorpe
2020-09-18 21:06                                                   ` John Hubbard
2020-09-19  0:01                                                     ` Jason Gunthorpe
2020-09-21  8:35                                                       ` Jan Kara
2020-09-21 12:03                                                         ` Jason Gunthorpe
2022-02-16 16:59                                                           ` Oded Gabbay
2022-02-16 17:24                                                             ` Oded Gabbay
2022-02-16 19:04                                                             ` Linus Torvalds
2022-02-16 19:20                                                               ` Oded Gabbay
2022-02-16 19:24                                                               ` David Hildenbrand
2020-09-21 13:42                                               ` Michal Hocko
2020-09-21 14:18                                                 ` Peter Xu
2020-09-21 14:28                                                   ` Michal Hocko
2020-09-21 14:38                                                     ` Tejun Heo
2020-09-21 14:43                                                       ` Christian Brauner
2020-09-21 14:55                                                         ` Michal Hocko
2020-09-21 15:04                                                           ` Christian Brauner [this message]
2020-09-21 16:06                                                             ` Michal Hocko
2020-09-23  7:53                                                               ` Michal Hocko
2020-09-21 14:41                                                 ` Christian Brauner
2020-09-21 14:57                                                   ` Michal Hocko
2020-09-21 16:31                                                     ` Peter Xu
2020-09-17 18:14                                       ` Peter Xu
2020-09-17 18:26                                         ` Linus Torvalds
2020-09-17 19:03                                           ` Peter Xu
2020-09-17 19:42                                             ` Linus Torvalds
2020-09-17 19:55                                               ` John Hubbard
2020-09-17 20:06                                               ` Jason Gunthorpe
2020-09-17 20:19                                                 ` John Hubbard
2020-09-17 20:25                                                   ` Jason Gunthorpe
2020-09-17 20:35                                                 ` Linus Torvalds
2020-09-17 21:40                                                   ` Peter Xu
2020-09-17 22:09                                                     ` Jason Gunthorpe
2020-09-17 22:25                                                       ` Linus Torvalds
2020-09-17 22:48                                                       ` Ira Weiny
2020-09-18  9:36                                                         ` Jan Kara
2020-09-18  9:44                                                       ` Jan Kara
2020-09-18 16:19                                             ` Jason Gunthorpe
2020-09-15 10:23           ` Leon Romanovsky
2020-09-15 15:56           ` Jason Gunthorpe
2020-09-15 15:03   ` Oleg Nesterov
2020-09-15 16:18     ` Peter Xu
2020-08-21 23:49 ` [PATCH 2/4] mm/ksm: Remove reuse_ksm_page() Peter Xu
2020-08-21 23:49 ` [PATCH 3/4] mm/gup: Remove enfornced COW mechanism Peter Xu
2020-09-14 14:27   ` Oleg Nesterov
2020-09-14 17:59     ` Peter Xu
2020-09-14 19:03       ` Linus Torvalds
2020-08-21 23:49 ` [PATCH 4/4] mm: Add PGREUSE counter Peter Xu
2020-08-22 16:14   ` Linus Torvalds
2020-08-24  0:24     ` Peter Xu
2020-08-22 16:05 ` [PATCH 0/4] mm: Simplfy cow handling Linus Torvalds
2020-08-23 23:58   ` Peter Xu
2020-08-24  8:38 ` Kirill Tkhai
2020-08-27 14:15 ` Peter Xu
2021-02-02 14:40 [PATCH 1/4] mm: Trial do_wp_page() simplification Gal Pressman
2021-02-02 16:31 ` Peter Xu
2021-02-02 16:44   ` Jason Gunthorpe
2021-02-02 17:05     ` Peter Xu
2021-02-02 17:13       ` Jason Gunthorpe
2021-02-03 12:43         ` Gal Pressman
2021-02-03 14:00           ` Jason Gunthorpe
2021-02-03 14:47             ` Gal Pressman

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