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    SubjectRe: [RFC][PATCH -mm] hibernation: freeze swap at hibernation (Was Re: Memory corruption during hibernation since 2.6.31
    Date
    On Monday, August 02, 2010, KAMEZAWA Hiroyuki wrote:
    > On Fri, 30 Jul 2010 13:14:32 +0900
    > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    >
    > > On Thu, 29 Jul 2010 21:10:10 -0700
    > > Hugh Dickins <hughd@google.com> wrote:
    > >
    > > > On Thu, Jul 29, 2010 at 5:01 PM, KAMEZAWA Hiroyuki
    > > > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > >
    > > > > I think the best way is kexec(). But maybe rollback from hibernation failure
    > > > > will be difficult. Considering how crash-dump works well and under maintainance
    > > > > by many enterprise guys, hibernation-by-kexec is a choice. I think. It can make
    > > > > reuse of kdump code, ...or, hibernation-resume code can eat kdump image
    > > > > directly. Maybe the problem will be the speed of dump.
    > > >
    > > > I've no appetite for a total rework of hibernation, and I don't see
    > > > how that would
    > > > address the issue: I'm just looking for some protection against swap
    > > > reuse danger.
    > > >
    > > Okay ;) (And I forget that kexec has to prepare memory at boot time for 2nd kernel.
    > > It will be harmful for small device guys.)
    > >
    > > I'll prepare a routine not-quick-fix.
    >
    > Ok, here. Passed easy tests as
    > # echo disk > /sys/power/state
    >
    >
    > Looks like a big hammer ? But I think following kind of patch is required.
    > About swap-reuse, it's only possible when a page is added to swap cache
    > but try_to_unmap() fails and the page remains in memory. IIUC, most of this
    > kind of pages will be backed to swap by shrink_all_memory(). So, reuse-swap
    > happens only when the user unlucky. This patch ignores reuse-swap but freeze
    > swap_map[] for saving swap_map[] to the disk in consistent way.
    >
    > ==
    > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >
    > swap_map[] is a one of objects which can be update while hibernation. I.E,
    > usage counter in swap_map[] is updated while hibernation is making a copy
    > of memory image to the disk.
    >
    > At resume, hibenation code doesn't call swap_free() against the swaps they
    > used...So, the swap_map[] will turns to be an initial state.
    >
    > With small consideration, the question is how swap_map[] updated before
    > dumping to the disk is treated. In swap-system view, there are no guarantee
    > that swap_map[] are properly reloaded and there is no leak in swap count.
    >
    > This patch tries to freeze swap_map[] during hibernation.
    > By this, no updates will be happen to swap_map[] among save_image().
    > At load_image(), the swap_map[] has no record for swap entries used by
    > save_image(), we can simply forget it.
    >
    > Note: I'm not a specialist of hibernation...so, I'm not sure the hooks
    > to kernel/power/user.c is appropriate or not.
    > And this disables swap-out once hibernation starts saving.
    > Should we afraid of OOM ?

    OOM shoiuld not happen at that time, so no worry.

    I'll have a deeper look into the patch later today.

    Thanks,
    Rafael


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