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

[Nigel Cunningham <ncunningham@cyclades.com>]

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Hi Andrew et al.

On Thursday 11 May 2006 09:38, Andrew Morton wrote:
> "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).

I believe more than a sync is needed in at least some cases. I've seen XFS 
continue to submit I/O (presumably on the sb or such like) after everything 
else has been frozen and data has been synced. Freezing bdevs addressed this.

> 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.

Bdev freezing helps here too, right?

> >  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. :-)]

(For Rafael/Pavel): The swsusp version of the refrigerator signals these 
processes to enter the freezer too, just in case the uninterruptible task 
does continue, right?

Regards,

Nigel

> 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...
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