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SubjectRe: [PATCH] Freeze bdevs when freezing processes.

On Mon, 2006-10-23 at 16:07 +0200, Rafael J. Wysocki wrote:
> On Monday, 23 October 2006 14:09, Nigel Cunningham wrote:
> > Hi.
> >
> > On Mon, 2006-10-23 at 12:36 +0200, Rafael J. Wysocki wrote:
> > > On Monday, 23 October 2006 06:12, Nigel Cunningham wrote:
> > > > XFS can continue to submit I/O from a timer routine, even after
> > > > freezeable kernel and userspace threads are frozen. This doesn't seem to
> > > > be an issue for current swsusp code,
> > >
> > > So it doesn't look like we need the patch _now_.
> >
> > I'm trying to prepare the patches to make swsusp into suspend2.
> Oh, I see. Please don't do that.

(Yes, echoing Andrew..) Why not?

> > Please assume it's part of a stream of patches that belong together, rather
> > than an isolated modification. Yes, I'd really rather just rm -f swap.c
> > user.c swsusp.c,
> And I'm against that, sorry. Also I don't think you can remove user.c just
> like that.

Are you saying you're against a stream of patches that belong together?
Well, I'm sorry but that's the way things work sometimes. I know
everyone loves this evolutionary model, but it just doesn't work in
practice. We see sets of patches going through all the time in which
[2/4] prepares the foundations for [3/4] and so on. That's what I'm
doing, I just don't yet know how big the n is in [2/n].

Regarding removing user.c, I said I'd rather, not that I'm going to
submit a patch to do it :)

> > but I'm trying to do the incremental modification thing for you.
> >
> > > > but is definitely an issue for Suspend2, where the pages being written could
> > > > be overwritten by Suspend2's atomic copy.
> > >
> > > And IMO that's a good reason why we shouldn't use RCU pages for storing the
> > > image. XFS is one known example that breaks things if we do so and
> > > there may be more such things that we don't know of. The fact that they
> > > haven't appeared in testing so far doesn't mean they don't exist and
> > > moreover some things like that may appear in the future.
> >
> > Ok. We can modify the selection of pages to be overwritten. I agree that
> > absence of proof isn't proof of absence, but on the later part, we can't
> > rule some modification out now because something in the future might
> > break it.

(s/RCU/LRU applied to avoid confusion)

> This case is a bit special. I don't think it would be right to require every
> device driver writer to avoid modifying LRU pages from the interrupt
> context, because that would break the suspend to disk ...

I'm not arguing for that.

> Besides, if there is an LRU page that we _know_ we can use to store the image
> in it, we can just include this page in the image without copying. This
> already gives us one extra free page for the rest of the image and we can
> _avoid_ creating two images which suspend2 does and which adds a _lot_ of
> complexity to the code.

Do you have a number for '_lot_'? It's easy to say, but it would be
better to be able to say "I counted n lines of code you could remove if
you didn't save a full image of memory".

> > > > We can address this issue by freezing bdevs after stopping userspace
> > > > threads, and thawing them prior to thawing userspace.
> > >
> > > This will only address the issues related to filesystems, but the RCU pages
> > > can also be modified from interrupt context by other code, AFAICT.
> >
> > Agreed. We can find some other way to address that.
> I think, to stat with, we should look for pages that we're deadly sure will
> _not_ be overwritten from the interrupt context (for example, the pages that
> are mapped read-only by the user space look promising). Then, we should
> estimate the number of these pages and consider if dealing with them in
> a special way is worth the effort.

I was thinking of coming at it from the other direction, since I expect
the pages that might be modified from an interrupt context to be
minimal. The fact that Suspend2 has worked for years now using this
algorithm seems to confirm that.



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