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    SubjectRe: [RFC][PATCH -mm] hibernation: freeze swap at hibernation (Was Re: Memory corruption during hibernation since 2.6.31
    On Mon, 2 Aug 2010 21:29:45 +0530
    Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

    > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-08-02 15:02:25]:
    >
    > > +/*
    > > + * Because updateing swap_map[] can make not-saved-status-change,
    > > + * we use our own easy allocator.
    > > + * Please see kernel/power/swap.c, Used swaps are recorded into
    > > + * RB-tree.
    > > + */
    > > +swp_entry_t get_swap_for_hibernation(int type)
    > > +{
    > > + pgoff_t off;
    > > + swp_entry_t val = {0};
    > > + struct swap_info_struct *si;
    > > +
    > > + spin_lock(&swap_lock);
    > > + /*
    > > + * Once hibernation starts to use swap, we freeze swap_map[]. Otherwise,
    > > + * saved swap_map[] image to the disk will be an incomplete because it's
    > > + * changing without synchronization with hibernation snap shot.
    > > + * At resume, we just make swap_for_hibernation=false. We can forget
    > > + * used maps easily.
    >
    > I don't understand the consequences of this action. Once swap_map is
    > fixed, we get additional swapping because we need more free memory,
    > what happens to the swapped out contents, since resume will never see
    > the changes?

    Sorry, I can't understand what you write. Why "we get additional swapping?"
    before starting hibernation, shrink_memory() is called and hibernation codes
    should have enough memory to work.

    This patch does

    1. set swap_for_hibernation = true
    => After this, kswapd/direct reclaim will make no swap.
    => But hibernation can make use of swap.
    2. this variable, swap_for_hibernation is saved to disk as it is.

    At resume
    3. swap_for_hibernation is loaded and it's value is "true"
    4. hibernation_thaw_swap() is called and set swap_for_hibernation=false.


    > How did this work before 2.6.31?
    >

    hmm? Are you talking about regression itself ?

    Before 2.6.31
    - At scan_swap_map(), free swap_map[] was used.
    After 2.6.31
    - At scan_swap_map(), if "swapcache-only" swap entry is found,
    it's reused by try_to_free_swapcache(). Because this happens
    during saving image of system memory, the snapshot will have inconsitency
    between swap_map <=> swap cache (I think mem_map is saved firstly)
    Then, memory corruption happens.
    After this patch.
    - scan_swap_map() is never called while saving snapshot to the disk.


    > > + */
    > > + if (!swap_for_hibernation)
    > > + hibernation_freeze_swap();
    > > +
    > > + si = swap_info[type];
    > > + if (!si || !(si->flags & SWP_WRITEOK))
    > > + goto done;
    > > +
    > > + for (off = hibernation_offset[type]; off < si->max; ++off) {
    > > + if (!si->swap_map[off])
    > > + break;
    >
    > So this is a linear scan for the first free entry, right?
    >
    yes. Maybe some clever code can be added but start from simple one.
    The result will not be very different because "write" time is long.
    Thanks,
    -Kame



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