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

[Rafael J. Wysocki]

Hi,

On Thursday 11 May 2006 02:11, Nigel Cunningham wrote:
> Hi Andrew et al.
> 
> On Thursday 11 May 2006 09:38, Andrew Morton wrote:
> > "Rafael J. Wysocki" <rjw@xxxxxxx> wrote:
> > > On Wednesday 10 May 2006 00:27, Andrew Morton wrote:
> > > > "Rafael J. Wysocki" <rjw@xxxxxxx> 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(). ÂThus 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?

Well, I'm not sure.  How exactly?

> > >  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?

Uninterruptible tasks are not freezable with the swsusp's freezer at all.
The other tasks are signaled to enter the refrigerator - first user space,
then we sync filesystems and finally we freeze kernel threads.

Greetings,
Rafael
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