linux-kernel - Re: [PATCH 1/4] mm: Trial do_wp

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Message-ID: <20200921144355.mrzc66lina3dkfjq@wittgenstein>
Date:   Mon, 21 Sep 2020 16:43:55 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        Kirill Shutemov <kirill@...temov.name>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jan Kara <jack@...e.cz>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

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:

"In order to place the child process in a different cgroup, the caller
specifies CLONE_INTO_CGROUP in cl_args.flags and passes a file
descriptor  that  refers  to  a  version  2  cgroup  in  the
cl_args.cgroup  field.  (This file descriptor can be obtained by opening
a cgroup v2 directory using either the O_RDONLY or the O_PATH flag.)
Note that all of the usual restrictions (described in cgroups(7)) on
placing a process into a version 2 cgroup apply."

Christian