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

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Message-ID: <58AD4147.20801@virtuozzo.com>
Date:   Wed, 22 Feb 2017 10:44:07 +0300
From:   Pavel Emelyanov <xemul@...tuozzo.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>,
        Andy Lutomirski <luto@...capital.net>
CC:     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 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.

> Look, here is how we build files tree in criu: we take inode^sdev^pos
> as a primary key and remember it inside rbtree while we're dumping files
> (note also that we don't keep files opened but rather dump them in
> chunks). Then once we find that two files have same primary key
> we use kcmp to build subtree. This really helps a lot. And I plan
> to do the same with target files from epolls:
> 
>  - they gonna be handled after all opened files of all processes
>    in container (or children processes if dumping single task),
>    thus the complete tree with primary key already will be built
> 
>  - on every target file I calculate primary key and then using
>    kcmp will find if file is exactly one matching
> .
>