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    SubjectRe: Memory corruption during hibernation since 2.6.31
    * 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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