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    Subject[patch 28/86] mm: compaction: check pfn_valid when entering a new MAX_ORDER_NR_PAGES block during isolation for migration
    3.2-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Mel Gorman <mgorman@suse.de>

    commit 0bf380bc70ecba68cb4d74dc656cc2fa8c4d801a upstream.

    When isolating for migration, migration starts at the start of a zone
    which is not necessarily pageblock aligned. Further, it stops isolating
    when COMPACT_CLUSTER_MAX pages are isolated so migrate_pfn is generally
    not aligned. This allows isolate_migratepages() to call pfn_to_page() on
    an invalid PFN which can result in a crash. This was originally reported
    against a 3.0-based kernel with the following trace in a crash dump.

    PID: 9902 TASK: d47aecd0 CPU: 0 COMMAND: "memcg_process_s"
    #0 [d72d3ad0] crash_kexec at c028cfdb
    #1 [d72d3b24] oops_end at c05c5322
    #2 [d72d3b38] __bad_area_nosemaphore at c0227e60
    #3 [d72d3bec] bad_area at c0227fb6
    #4 [d72d3c00] do_page_fault at c05c72ec
    #5 [d72d3c80] error_code (via page_fault) at c05c47a4
    EAX: 00000000 EBX: 000c0000 ECX: 00000001 EDX: 00000807 EBP: 000c0000
    DS: 007b ESI: 00000001 ES: 007b EDI: f3000a80 GS: 6f50
    CS: 0060 EIP: c030b15a ERR: ffffffff EFLAGS: 00010002
    #6 [d72d3cb4] isolate_migratepages at c030b15a
    #7 [d72d3d14] zone_watermark_ok at c02d26cb
    #8 [d72d3d2c] compact_zone at c030b8de
    #9 [d72d3d68] compact_zone_order at c030bba1
    #10 [d72d3db4] try_to_compact_pages at c030bc84
    #11 [d72d3ddc] __alloc_pages_direct_compact at c02d61e7
    #12 [d72d3e08] __alloc_pages_slowpath at c02d66c7
    #13 [d72d3e78] __alloc_pages_nodemask at c02d6a97
    #14 [d72d3eb8] alloc_pages_vma at c030a845
    #15 [d72d3ed4] do_huge_pmd_anonymous_page at c03178eb
    #16 [d72d3f00] handle_mm_fault at c02f36c6
    #17 [d72d3f30] do_page_fault at c05c70ed
    #18 [d72d3fb0] error_code (via page_fault) at c05c47a4
    EAX: b71ff000 EBX: 00000001 ECX: 00001600 EDX: 00000431
    DS: 007b ESI: 08048950 ES: 007b EDI: bfaa3788
    SS: 007b ESP: bfaa36e0 EBP: bfaa3828 GS: 6f50
    CS: 0073 EIP: 080487c8 ERR: ffffffff EFLAGS: 00010202

    It was also reported by Herbert van den Bergh against 3.1-based kernel
    with the following snippet from the console log.

    BUG: unable to handle kernel paging request at 01c00008
    IP: [<c0522399>] isolate_migratepages+0x119/0x390
    *pdpt = 000000002f7ce001 *pde = 0000000000000000

    It is expected that it also affects 3.2.x and current mainline.

    The problem is that pfn_valid is only called on the first PFN being
    checked and that PFN is not necessarily aligned. Lets say we have a case
    like this

    H = MAX_ORDER_NR_PAGES boundary
    | = pageblock boundary
    m = cc->migrate_pfn
    f = cc->free_pfn
    o = memory hole

    H------|------H------|----m-Hoooooo|ooooooH-f----|------H

    The migrate_pfn is just below a memory hole and the free scanner is beyond
    the hole. When isolate_migratepages started, it scans from migrate_pfn to
    migrate_pfn+pageblock_nr_pages which is now in a memory hole. It checks
    pfn_valid() on the first PFN but then scans into the hole where there are
    not necessarily valid struct pages.

    This patch ensures that isolate_migratepages calls pfn_valid when
    necessary.

    Reported-by: Herbert van den Bergh <herbert.van.den.bergh@oracle.com>
    Tested-by: Herbert van den Bergh <herbert.van.den.bergh@oracle.com>
    Signed-off-by: Mel Gorman <mgorman@suse.de>
    Acked-by: Michal Nazarewicz <mina86@mina86.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/compaction.c | 13 +++++++++++++
    1 file changed, 13 insertions(+)

    --- a/mm/compaction.c
    +++ b/mm/compaction.c
    @@ -313,6 +313,19 @@ static isolate_migrate_t isolate_migrate
    } else if (!locked)
    spin_lock_irq(&zone->lru_lock);

    + /*
    + * migrate_pfn does not necessarily start aligned to a
    + * pageblock. Ensure that pfn_valid is called when moving
    + * into a new MAX_ORDER_NR_PAGES range in case of large
    + * memory holes within the zone
    + */
    + if ((low_pfn & (MAX_ORDER_NR_PAGES - 1)) == 0) {
    + if (!pfn_valid(low_pfn)) {
    + low_pfn += MAX_ORDER_NR_PAGES - 1;
    + continue;
    + }
    + }
    +
    if (!pfn_valid_within(low_pfn))
    continue;
    nr_scanned++;



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