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

From: Pavel Emelyanov
Date: Wed Feb 22 2017 - 02:44:40 EST

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
> .