Re: [Devel] Re: [PATCH 2.6.24-rc8-mm1 09/15] (RFC) IPC: new kernelAPI to change an ID

[Daniel Lezcano]

Pavel Emelyanov wrote:
> Kirill Korotaev wrote:
> > Cedric Le Goater wrote:
> > > Hello Kirill !
> > >
> > > Kirill Korotaev wrote:
> > > > Pierre,
> > > >
> > > > my point is that after you've added interface "set IPCID", you'll need
> > > > more and more for checkpointing:
> > > > - "create/setup conntrack" (otherwise connections get dropped),
> > > > - "set task start time" (needed for Oracle checkpointing BTW),
> > > > - "set some statistics counters (e.g. networking or taskstats)"
> > > > - "restore inotify"
> > > > and so on and so forth.
> > > right. we know that we will have to handle a lot of these
> > > and more and we will need an API for it :) so how should we handle it ?
> > > through a dedicated syscall that would be able to checkpoint and/or
> > > restart a process, an ipc object, an ipc namespace, a full container ?
> > > will it take a fd or a big binary blob ?  
> > > I personally really liked Pavel idea's of filesystem. but we dropped the
> > > thread.
> > Imho having a file system interface means having all its problems.
> > Imagine you have some information about tasks exported with a file system interface.
> > Obviously to collect the information you have to hold some spinlock like tasklist_lock or similar.
> > Obviously, you have to drop the lock between sys_read() syscalls.
> > So interface gets much more complicated - you have to rescan the objects and somehow find the place where
> > you stopped previous read. Or you have to to force reader to read everything at once.
>
> To remember the place when we stopped previous read we have a "pos" counter
> on the struct file.
>
> Actually, tar utility, that I propose to perform the most simple migration
> reads the directory contents with 4Kb buffer - that's enough for ~500 tasks.
>
> Besides, is this a real problem for a frozen container?

I like the idea of a C/R filesystem. Does it implies a specific user 
space program to orchestrate the checkpoint/restart of the different 
subsystems ? I mean the checkpoint is easy but what about the restart ? 
We must ensure, for example to restore a process before restoring the fd 
associated to it, or restore a deleted file before restoring the fd 
opened to it, no ?





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