linux-kernel - Re: [RFC 1/3] procfs: fdinfo -- Extend information about epoll target files

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Date:   Wed, 22 Feb 2017 10:54:38 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Pavel Emelyanov <xemul@...tuozzo.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Andrew Vagin <avagin@...tuozzo.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Kirill Kolyshkin <kir@...nvz.org>,
        Jason Baron <jbaron@...mai.com>,
        Andrey Vagin <avagin@...nvz.org>
Subject: Re: [RFC 1/3] procfs: fdinfo -- Extend information about epoll
 target files

On Wed, Feb 22, 2017 at 10:44:07AM +0300, Pavel Emelyanov wrote:
> On 02/21/2017 10:16 PM, Cyrill Gorcunov wrote:
> > On Tue, Feb 21, 2017 at 10:41:12AM -0800, Andy Lutomirski wrote:
> >>> Thus lets add file position, inode and device number where
> >>> this target lays. This three fields can be used as a primary
> >>> key for sorting, and together with kcmp help CRIU can find
> >>> out an exact file target (from the whole set of processes
> >>> being checkpointed).
> >>
> >> I have no problem with this, but I'm wondering whether kcmp's ordered
> >> comparisons could also be used for this purpose.
> > 
> > Yes it can, but it would increas number of kcmp calls signisicantly.
> 
> Actually it shouldn't. If you extend the kcmp argument to accept the
> epollfd:epollslot pair, this would be effectively the same as if you
> had all your epoll-ed files injected into your fdtable with "strange"
> fd numbers. We already have two-level rbtree for this in criu, adding
> extended ("strange") fd to it should be OK.

Nope. Pavel, I guess you forget how we handle file tree in criu currently.
We call for kcmp only if we have to -- when primary key for two entries
is the same.