mm: migrate: fix remove_migration_pte() for ksm pages

I found that calling page migration for ksm pages causes the following
bug:

    page:ffffea0004d51180 count:2 mapcount:2 mapping:ffff88013c785141 index:0x913
    flags: 0x57ffffc0040068(uptodate|lru|active|swapbacked)
    raw: 0057ffffc0040068 ffff88013c785141 0000000000000913 0000000200000001
    raw: ffffea0004d5f9e0 ffffea0004d53f60 0000000000000000 ffff88007d81b800
    page dumped because: VM_BUG_ON_PAGE(!PageLocked(page))
    page->mem_cgroup:ffff88007d81b800
    ------------[ cut here ]------------
    kernel BUG at /src/linux-dev/mm/rmap.c:1086!
    invalid opcode: 0000 [#1] SMP
    Modules linked in: ppdev parport_pc virtio_balloon i2c_piix4 pcspkr parport i2c_core acpi_cpufreq ip_tables xfs libcrc32c ata_generic pata_acpi ata_piix 8139too libata virtio_blk 8139cp crc32c_intel mii virtio_pci virtio_ring serio_raw virtio floppy dm_mirror dm_region_hash dm_log dm_mod
    CPU: 0 PID: 3162 Comm: bash Not tainted 4.11.0-rc2-mm1+ #1
    Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
    RIP: 0010:do_page_add_anon_rmap+0x1ba/0x260
    RSP: 0018:ffffc90002473b30 EFLAGS: 00010282
    RAX: 0000000000000021 RBX: ffffea0004d51180 RCX: 0000000000000006
    RDX: 0000000000000000 RSI: 0000000000000082 RDI: ffff88007dc0dfe0
    RBP: ffffc90002473b58 R08: 00000000fffffffe R09: 00000000000001c1
    R10: 0000000000000005 R11: 00000000000001c0 R12: ffff880139ab3d80
    R13: 0000000000000000 R14: 0000700000000200 R15: 0000160000000000
    FS:  00007f5195f50740(0000) GS:ffff88007dc00000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 00007fd450287000 CR3: 000000007a08e000 CR4: 00000000001406f0
    Call Trace:
     page_add_anon_rmap+0x18/0x20
     remove_migration_pte+0x220/0x2c0
     rmap_walk_ksm+0x143/0x220
     rmap_walk+0x55/0x60
     remove_migration_ptes+0x53/0x80
     migrate_pages+0x8ed/0xb60
     soft_offline_page+0x309/0x8d0
     store_soft_offline_page+0xaf/0xf0
     dev_attr_store+0x18/0x30
     sysfs_kf_write+0x3a/0x50
     kernfs_fop_write+0xff/0x180
     __vfs_write+0x37/0x160
     vfs_write+0xb2/0x1b0
     SyS_write+0x55/0xc0
     do_syscall_64+0x67/0x180
     entry_SYSCALL64_slow_path+0x25/0x25
    RIP: 0033:0x7f51956339e0
    RSP: 002b:00007ffcfa0dffc8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
    RAX: ffffffffffffffda RBX: 000000000000000c RCX: 00007f51956339e0
    RDX: 000000000000000c RSI: 00007f5195f53000 RDI: 0000000000000001
    RBP: 00007f5195f53000 R08: 000000000000000a R09: 00007f5195f50740
    R10: 000000000000000b R11: 0000000000000246 R12: 00007f5195907400
    R13: 000000000000000c R14: 0000000000000001 R15: 0000000000000000
    Code: fe ff ff 48 81 c2 00 02 00 00 48 89 55 d8 e8 2e c3 fd ff 48 8b 55 d8 e9 42 ff ff ff 48 c7 c6 e0 52 a1 81 48 89 df e8 46 ad fe ff <0f> 0b 48 83 e8 01 e9 7f fe ff ff 48 83 e8 01 e9 96 fe ff ff 48
    RIP: do_page_add_anon_rmap+0x1ba/0x260 RSP: ffffc90002473b30
    ---[ end trace a679d00f4af2df48 ]---
    Kernel panic - not syncing: Fatal exception
    Kernel Offset: disabled
    ---[ end Kernel panic - not syncing: Fatal exception

The problem is in the following lines:

    new = page - pvmw.page->index +
        linear_page_index(vma, pvmw.address);

The 'new' is calculated with 'page' which is given by the caller as a
destination page and some offset adjustment for thp.  But this doesn't
properly work for ksm pages because pvmw.page->index doesn't change for
each address but linear_page_index() changes, which means that 'new'
points to different pages for each addresses backed by the ksm page.  As
a result, we try to set totally unrelated pages as destination pages,
and that causes kernel crash.

This patch fixes the miscalculation and makes ksm page migration work
fine.

Fixes: 3fe87967 ("mm: convert remove_migration_pte() to use page_vma_mapped_walk()")
Link: http://lkml.kernel.org/r/1489717683-29905-1-git-send-email-n-horiguchi@ah.jp.nec.comSigned-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -209,6 +209,9 @@ static int remove_migration_pte(struct page *page, struct vm_area_struct *vma,
 
 	VM_BUG_ON_PAGE(PageTail(page), page);
 	while (page_vma_mapped_walk(&pvmw)) {
+		if (PageKsm(page))
+			new = page;
+		else
 			new = page - pvmw.page->index +
 				linear_page_index(vma, pvmw.address);