Re: [PATCH -mm] swsusp: support creating bigger images (rev. 2)

[Andrew Morton <akpm@osdl.org>]

"Rafael J. Wysocki" <rjw@sisk.pl> wrote:
>
> On Wednesday 10 May 2006 00:27, Andrew Morton wrote:
> > "Rafael J. Wysocki" <rjw@sisk.pl> wrote:
> > >
> > > Now if the mapped pages that are not mapped by the
> > >  current task are considered, it turns out that they would change only if they
> > >  were reclaimed by try_to_free_pages().  Thus if we take them out of reach
> > >  of try_to_free_pages(), for example by (temporarily) moving them out of their
> > >  respective LRU lists after creating the image, we will be able to include them
> > >  in the image without copying.
> > 
> > I'm a bit curious about how this is true.  There are all sorts of way in
> > which there could be activity against these pages - interrupt-time
> > asynchronous network Tx completion, async interrupt-time direct-io
> > completion, tasklets, schedule_work(), etc, etc.
> 
> AFAIK, many of these things are waited for uninterruptibly, and uninterruptible
> tasks cannot be frozen.

There can be situations where we won't be waiting on this IO at all. 
Network zero-copy transmit, for example.

Or maybe there's some async writeback going on against pagecache - we'll
end up looking at the page's LRU state within interrupt context at IO
completion.  (A sync would prevent this from happening).

One possibly problematic scenario is where task A is doing a direct-IO read
and task B truncates the same file - here, the page will be actually
removed from the LRU and freed in interrupt context.  The direct-IO read
process will be waiting on the IO in D state though.  It it was a
synchronous read - if it was an AIO read then it won't be waiting on the
IO.  Something else might save us here, but it's fragile.

>  Theoretically we may have a problem if there's an
> interruptible task that waits for the completion of an operation that gets
> finished after snapshotting the system.  However that would have to survive the
> syncing of filesystems, freezing of kernel threads, freeing of memory as well
> as suspending and resuming all devices.  [In which case it would be starving
> to death. :-)]

hm.  It's all a bit of a worry.  I don't understand what swsusp is trying
to do here sufficiently well to be able to advise, sorry.  I was rather
surprised to learn that it's presently taking copies of all these pages...