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    Subject[PATCH 1/2] mm, thp: lock filemap when truncating page cache
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    Transparent huge page has supported read-only non-shmem files. The file-
    backed THP is collapsed by khugepaged and truncated when written (for
    shared libraries).

    However, there is a race when multiple writers truncate the same page
    cache concurrently.

    In that case, subpage(s) of file THP can be revealed by find_get_entry
    in truncate_inode_pages_range, which will trigger PageTail BUG_ON in
    truncate_inode_page, as follows.

    page:000000009e420ff2 refcount:1 mapcount:0 mapping:0000000000000000 index:0x7ff pfn:0x50c3ff
    head:0000000075ff816d order:9 compound_mapcount:0 compound_pincount:0
    flags: 0x37fffe0000010815(locked|uptodate|lru|arch_1|head)
    raw: 37fffe0000000000 fffffe0013108001 dead000000000122 dead000000000400
    raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
    head: 37fffe0000010815 fffffe001066bd48 ffff000404183c20 0000000000000000
    head: 0000000000000600 0000000000000000 00000001ffffffff ffff000c0345a000
    page dumped because: VM_BUG_ON_PAGE(PageTail(page))
    ------------[ cut here ]------------
    kernel BUG at mm/truncate.c:213!
    Internal error: Oops - BUG: 0 [#1] SMP
    Modules linked in: xfs(E) libcrc32c(E) rfkill(E) ...
    CPU: 14 PID: 11394 Comm: check_madvise_d Kdump: ...
    Hardware name: ECS, BIOS 0.0.0 02/06/2015
    pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=--)
    pc : truncate_inode_page+0x64/0x70
    lr : truncate_inode_page+0x64/0x70
    sp : ffff80001b60b900
    x29: ffff80001b60b900 x28: 00000000000007ff
    x27: ffff80001b60b9a0 x26: 0000000000000000
    x25: 000000000000000f x24: ffff80001b60b9a0
    x23: ffff80001b60ba18 x22: ffff0001e0999ea8
    x21: ffff0000c21db300 x20: ffffffffffffffff
    x19: fffffe001310ffc0 x18: 0000000000000020
    x17: 0000000000000000 x16: 0000000000000000
    x15: ffff0000c21db960 x14: 3030306666666620
    x13: 6666666666666666 x12: 3130303030303030
    x11: ffff8000117b69b8 x10: 00000000ffff8000
    x9 : ffff80001012690c x8 : 0000000000000000
    x7 : ffff8000114f69b8 x6 : 0000000000017ffd
    x5 : ffff0007fffbcbc8 x4 : ffff80001b60b5c0
    x3 : 0000000000000001 x2 : 0000000000000000
    x1 : 0000000000000000 x0 : 0000000000000000
    Call trace:
    truncate_inode_page+0x64/0x70
    truncate_inode_pages_range+0x550/0x7e4
    truncate_pagecache+0x58/0x80
    do_dentry_open+0x1e4/0x3c0
    vfs_open+0x38/0x44
    do_open+0x1f0/0x310
    path_openat+0x114/0x1dc
    do_filp_open+0x84/0x134
    do_sys_openat2+0xbc/0x164
    __arm64_sys_openat+0x74/0xc0
    el0_svc_common.constprop.0+0x88/0x220
    do_el0_svc+0x30/0xa0
    el0_svc+0x20/0x30
    el0_sync_handler+0x1a4/0x1b0
    el0_sync+0x180/0x1c0
    Code: aa0103e0 900061e1 910ec021 9400d300 (d4210000)
    ---[ end trace f70cdb42cb7c2d42 ]---
    Kernel panic - not syncing: Oops - BUG: Fatal exception

    This patch mainly to lock filemap when one enter truncate_pagecache(),
    avoiding truncating the same page cache concurrently.

    Fixes: eb6ecbed0aa2 ("mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs")
    Cc: <stable@vger.kernel.org>
    Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    Tested-by: Song Liu <song@kernel.org>
    Signed-off-by: Xu Yu <xuyu@linux.alibaba.com>
    Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
    ---
    fs/open.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    diff --git a/fs/open.c b/fs/open.c
    index a7f6cab81267..e73bf88e5060 100644
    --- a/fs/open.c
    +++ b/fs/open.c
    @@ -856,8 +856,11 @@ static int do_dentry_open(struct file *f,
    * of THPs into the page cache will fail.
    */
    smp_mb();
    - if (filemap_nr_thps(inode->i_mapping))
    + if (filemap_nr_thps(inode->i_mapping)) {
    + filemap_invalidate_lock(inode->i_mapping);
    truncate_pagecache(inode, 0);
    + filemap_invalidate_unlock(inode->i_mapping);
    + }
    }

    return 0;
    --
    2.27.0
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