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    Subject[PATCH][RFC] fix kernel BUG at mm/migrate.c:719! in 2.6.26-rc5-mm3
    Hi.

    I got this bug while migrating pages only a few times
    via memory_migrate of cpuset.

    Unfortunately, even if this patch is applied,
    I got bad_page problem after hundreds times of page migration
    (I'll report it in another mail).
    But I believe something like this patch is needed anyway.

    ------------[ cut here ]------------
    kernel BUG at mm/migrate.c:719!
    invalid opcode: 0000 [1] SMP
    last sysfs file: /sys/devices/system/cpu/cpu3/cache/index1/shared_cpu_map
    CPU 0
    Modules linked in: ipv6 autofs4 hidp rfcomm l2cap bluetooth sunrpc dm_mirror dm_log dm_multipath dm_mod sbs sbshc button battery acpi_memhotplug ac parport_pc lp parport floppy serio_raw rtc_cmos rtc_core rtc_lib 8139too pcspkr 8139cp mii ata_piix libata sd_mod scsi_mod ext3 jbd ehci_hcd ohci_hcd uhci_hcd [last unloaded: microcode]
    Pid: 3096, comm: switch.sh Not tainted 2.6.26-rc5-mm3 #1
    RIP: 0010:[<ffffffff8029bb85>] [<ffffffff8029bb85>] migrate_pages+0x33e/0x49f
    RSP: 0018:ffff81002f463bb8 EFLAGS: 00010202
    RAX: 0000000000000000 RBX: ffffe20000c17500 RCX: 0000000000000034
    RDX: ffffe20000c17500 RSI: ffffe200010003c0 RDI: ffffe20000c17528
    RBP: ffffe200010003c0 R08: 8000000000000000 R09: 304605894800282f
    R10: 282f87058b480028 R11: 0028304005894800 R12: ffff81003f90a5d8
    R13: 0000000000000000 R14: ffffe20000bf4cc0 R15: ffff81002f463c88
    FS: 00007ff9386576f0(0000) GS:ffffffff8061d800(0000) knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
    CR2: 00007ff938669000 CR3: 000000002f458000 CR4: 00000000000006e0
    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
    Process switch.sh (pid: 3096, threadinfo ffff81002f462000, task ffff81003e99cf10)
    Stack: 0000000000000001 ffffffff80290777 0000000000000000 0000000000000000
    ffff81002f463c88 ffff81000000ea18 ffff81002f463c88 000000000000000c
    ffff81002f463ca8 00007ffffffff000 00007fff649f6000 0000000000000004
    Call Trace:
    [<ffffffff80290777>] ? new_node_page+0x0/0x2f
    [<ffffffff80291611>] ? do_migrate_pages+0x19b/0x1e7
    [<ffffffff802315c7>] ? set_cpus_allowed_ptr+0xe6/0xf3
    [<ffffffff8025c827>] ? cpuset_migrate_mm+0x58/0x8f
    [<ffffffff8025d0fd>] ? cpuset_attach+0x8b/0x9e
    [<ffffffff8025a3e1>] ? cgroup_attach_task+0x3a3/0x3f5
    [<ffffffff80276cb5>] ? __alloc_pages_internal+0xe2/0x3d1
    [<ffffffff8025af06>] ? cgroup_common_file_write+0x150/0x1dd
    [<ffffffff8025aaf4>] ? cgroup_file_write+0x54/0x150
    [<ffffffff8029f839>] ? vfs_write+0xad/0x136
    [<ffffffff8029fd76>] ? sys_write+0x45/0x6e
    [<ffffffff8020bef2>] ? tracesys+0xd5/0xda


    Code: 4c 48 8d 7b 28 e8 cc 87 09 00 48 83 7b 18 00 75 30 48 8b 03 48 89 da 25 00 40 00 00 48 85 c0 74 04 48 8b 53 10 83 7a 08 01 74 04 <0f> 0b eb fe 48 89 df e8 5e 50 fd ff 48 89 df e8 7d d6 fd ff eb
    RIP [<ffffffff8029bb85>] migrate_pages+0x33e/0x49f
    RSP <ffff81002f463bb8>
    Clocksource tsc unstable (delta = 438246251 ns)
    ---[ end trace ce4e6053f7b9bba1 ]---


    This bug is caused by VM_BUG_ON() in unmap_and_move().

    unmap_and_move()
    710 if (rc != -EAGAIN) {
    711 /*
    712 * A page that has been migrated has all references
    713 * removed and will be freed. A page that has not been
    714 * migrated will have kepts its references and be
    715 * restored.
    716 */
    717 list_del(&page->lru);
    718 if (!page->mapping) {
    719 VM_BUG_ON(page_count(page) != 1);
    720 unlock_page(page);
    721 put_page(page); /* just free the old page */
    722 goto end_migration;
    723 } else
    724 unlock = putback_lru_page(page);
    725 }

    I think the page count is not necessarily 1 here, because
    migration_entry_wait increases page count and waits for the
    page to be unlocked.
    So, if the old page is accessed between migrate_page_move_mapping,
    which checks the page count, and remove_migration_ptes, page count
    would not be 1 here.

    Actually, just commenting out get/put_page from migration_entry_wait
    works well in my environment(succeeded in hundreds times of page migration),
    but modifying migration_entry_wait this way is not good, I think.


    This patch depends on Lee Schermerhorn's fix for double unlock_page.

    This patch also fixes a race between migrate_entry_wait and
    page_freeze_refs in migrate_page_move_mapping.


    Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

    ---
    diff -uprN linux-2.6.26-rc5-mm3/mm/migrate.c linux-2.6.26-rc5-mm3-test/mm/migrate.c
    --- linux-2.6.26-rc5-mm3/mm/migrate.c 2008-06-17 15:31:23.000000000 +0900
    +++ linux-2.6.26-rc5-mm3-test/mm/migrate.c 2008-06-17 13:59:15.000000000 +0900
    @@ -232,6 +232,7 @@ void migration_entry_wait(struct mm_stru
    swp_entry_t entry;
    struct page *page;

    +retry:
    ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
    pte = *ptep;
    if (!is_swap_pte(pte))
    @@ -243,11 +244,20 @@ void migration_entry_wait(struct mm_stru

    page = migration_entry_to_page(entry);

    - get_page(page);
    - pte_unmap_unlock(ptep, ptl);
    - wait_on_page_locked(page);
    - put_page(page);
    - return;
    + /*
    + * page count might be set to zero by page_freeze_refs()
    + * in migrate_page_move_mapping().
    + */
    + if (get_page_unless_zero(page)) {
    + pte_unmap_unlock(ptep, ptl);
    + wait_on_page_locked(page);
    + put_page(page);
    + return;
    + } else {
    + pte_unmap_unlock(ptep, ptl);
    + goto retry;
    + }
    +
    out:
    pte_unmap_unlock(ptep, ptl);
    }
    @@ -715,13 +725,7 @@ unlock:
    * restored.
    */
    list_del(&page->lru);
    - if (!page->mapping) {
    - VM_BUG_ON(page_count(page) != 1);
    - unlock_page(page);
    - put_page(page); /* just free the old page */
    - goto end_migration;
    - } else
    - unlock = putback_lru_page(page);
    + unlock = putback_lru_page(page);
    }

    if (unlock)

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