Re: [RFC][PATCH] swsusp: support creating bigger images

[Rafael J. Wysocki]

Hi,

On Tuesday 25 April 2006 23:18, Nigel Cunningham wrote:
> On Wednesday 26 April 2006 07:12, Rafael J. Wysocki wrote:
> > On Tuesday 25 April 2006 22:32, Pavel Machek wrote:
> > > > >   > -unsigned int count_data_pages(void)
> > > > > >
> > > > > > +/**
> > > > > > + *	need_to_copy - determine if a page needs to be copied before
> > > > > > saving. + *	Returns false if the page can be saved without copying.
> > > > > > + */
> > > > > > +
> > > > > > +static int need_to_copy(struct page *page)
> > > > > > +{
> > > > > > +	if (!PageLRU(page) || PageCompound(page))
> > > > > > +		return 1;
> > > > > > +	if (page_mapped(page))
> > > > > > +		return page_mapped_by_current(page);
> > > > > > +
> > > > > > +	return 1;
> > > > > > +}
> > > > >
> > > > > I'd much rather VM internal type stuff get moved *out* of
> > > > > kernel/power :(
> > > >
> > > > Well, I kind of agree, but I don't know where to place it under mm/.
> > > >
> > > > > It needs more comments too. Also, how important is it for the page to
> > > > > be off the LRU?
> > > >
> > > > Hm, I'm not sure if that's what you're asking about, but the pages off
> > > > the LRU are handled in a usual way, ie. copied when snapshotting the
> > > > system.  The pages _on_ the LRU may be included in the snapshot image
> > > > without copying, but I require them additionally to be (a) mapped by
> > > > someone and (b) not mapped by the current task.
> > >
> > > Why do you _want_ them mapped by someone?
> >
> > Because this means they belong to a task that is frozen and won't touch
> > them (of course unless it's us).  The kernel has no reason to access them
> > either (even after we resume devices) except for reclaiming, but that's
> > handled explicitly.  Thus it's safe to include them in the image without
> > copying.
> >
> > As I said before, I think the page cache pages may be treated this way too.
> > It probably applies to all of the LRU pages, but there may be some corner
> > cases.  The mapped pages are just easy to single out.
> 
> It does apply to all of the LRU pages. This is what I've been doing for years 
> now. The only corner case I've come across is XFS. It still wants to write 
> data even when there's nothing to do and it's threads are frozen (IIRC - 
> haven't looked at it for a while). I got around that by freezing bdevs when 
> freezing processes.

This means if we freeze bdevs, we'll be able to save all of the LRU pages,
except for the pages mapped by the current task, without copying.  I think we
can try to do this, but we'll need a patch to freeze bdevs for this purpose. ;-)

Greetings,
Rafael
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