Re: Swap

[Mike Fedyk <mfedyk@matchmail.com>]

On Tue, Nov 20, 2001 at 10:05:37PM +0100, Steffen Persvold wrote:
> Christopher Friesen wrote:
> > 
> > "Richard B. Johnson" wrote:
> > >
> > > On Tue, 20 Nov 2001, Wolfgang Rohdewald wrote:
> > >
> > > > On Tuesday 20 November 2001 15:51, J.A. Magallon wrote:
> > > > > When a page is deleted for one executable (because we can re-read it from
> > > > > on-disk binary), it is discarded, not paged out.
> > > >
> > > > What happens if the on-disk binary has changed since loading the program?
> > > > -
> > >
> > > It can't. That's the reason for `install` and other methods of changing
> > > execututable files (mv exe-file exe-file.old ; cp newfile exe-file).
> > > The currently open, and possibly mapped file can be re-named, but it
> > > can't be overwritten.
> > 
> > Actually, with NFS (and probably others) it can.  Suppose I change the file on
> > the server, and it's swapped out on a client that has it mounted.  When it swaps
> > back in, it can get the new information.
> > 
> 
> This sounds really dangerous... What about shared libraries ??
> 

IIRC (if wrong flame...)

When you delete an open file, the entry is removed from the directory, but
not unlinked until the file is closed.  This is a standard UNIX semantic.

Now, if you have a set of processes with shared memory, and one closes, and
another is created to replace, the new process will get the new libraries,
or even new version of the process.  This could/will bring down the entire
set of processes.

Apps like samba come to mind...

Mike