* Hugh Dickins <hughd@google.com> [2010-07-29 11:44:31]:

quoted text
> On Thu, 29 Jul 2010, KAMEZAWA Hiroyuki wrote:
> > On Thu, 29 Jul 2010 14:23:33 +0900 (JST)
> > KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> > 
> > > Can you please add explicit commenting in the code?
> > > 
> > How about this ?
> > ==
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > 
> > At hibernation, all pages-should-be-saved are written into a image (here, swap).
> > Then, swap_map[], memmap etcs are also saved into disks.
> > 
> > But, swap allocation happens one by one. So, the final image of swap_map[] is
> > different from saved one and the commit c9e444103b5e7a5a3519f9913f59767f92e33baf
> > changes page's state while assiging swap. Because memory can be modified in
> > hibernation is only not-to-be-save memory. it's a breakage.
> > 
> > This patch fixes it by disabling swap entry reuse at hibernation.
> > 
> > 
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > ---
> >  mm/swapfile.c |    4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > Index: linux-2.6.34.org/mm/swapfile.c
> > ===================================================================
> > --- linux-2.6.34.org.orig/mm/swapfile.c
> > +++ linux-2.6.34.org/mm/swapfile.c
> > @@ -315,8 +315,15 @@ checks:
> >  	if (offset > si->highest_bit)
> >  		scan_base = offset = si->lowest_bit;
> >  
> > -	/* reuse swap entry of cache-only swap if not busy. */
> > -	if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
> > +	/*
> > + 	 * reuse swap entry of cache-only swap if not busy &&
> > + 	 * when we're called via pageout(). At hibernation, swap-reuse
> > + 	 * is harmful because it changes memory status...which may
> > + 	 * be saved already.
> > + 	 */
> > +	if (vm_swap_full()
> > +		&& usage == SWAP_HAS_CACHE
> > +		&& si->swap_map[offset] == SWAP_HAS_CACHE) {
> >  		int swap_was_freed;
> >  		spin_unlock(&swap_lock);
> >  		swap_was_freed = __try_to_reclaim_swap(si, offset);
> > 
> > --
> 
> KAMEZAWA-San, that is a brilliant realization, I salute you.
> 
> So, in between snapshotting the image and actually hibernating, we have
> two parallel universes, one frozen in the image, the other writing that
> out to swap: with the danger that the latter (which is about to die)
> will introduce fatal inconsistencies in the former by placing pages in
> swap locations innocently reallocated from it.  (Excuse me while I go
> write the movie script.)
> 
> I'd never got to think about that before.
> 
> Your fix is neat though hacky (it's somewhat of a coincidence that the
> usage arg happens to distinguish the hibernation case), and should be
> enough to fix "your" regression, but is it really enough?
> 
> I'm worrying about the try_to_free_swap() calls in shrink_page_list():
> can't those get called from direct reclaim (even if GFP_NOIO), and can't
> direct reclaim get invoked from somewhere in the I/O path below
> swap_writepage(), used for writing out the hibernation image?
> 
> Direct reclaim because kswapd does set_freezable(), so should itself
> be out of the picture.  But we cannot freeze writing the hibernation
> image, and its occasional need for memory, so maybe a different approach
> is required.
> 
> I've CC'ed Andrea because we were having an offline conversation about
> whether ksmd (and his khugepaged) need to set_freezable(); and I wonder
> if this swap bug underlies his interest, though he was mainly worrying
> about I/O in progress.
> 
> Despite reading Documentation/power/freezing-of-tasks.txt, I have no
> clear idea of what really needs freezing, and whether freezing can
> fully handle the issues.  Rafael, please can you advise?
>

Couldn't we reuse PF_* flags to differentiate between the paths, if
that is what it eventually boils down to? On an unrelated note, I was
looking at shrink_all_memory() and wondering if swappiness really
mattered there. 

-- 
	Three Cheers,
	Balbir
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