Messages in this thread | | | Date | Fri, 30 Jul 2010 09:48:25 +0530 | From | Balbir Singh <> | Subject | Re: Memory corruption during hibernation since 2.6.31 |
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* Hugh Dickins <hughd@google.com> [2010-07-29 11:44:31]:
> 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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