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    SubjectRe: [PATCH] ext4: implement error handling of ext4_mb_new_preallocation()
    On Mon, May 06, 2013 at 12:51:15AM +0400, Alexey Khoroshilov wrote:
    > If memory allocation in ext4_mb_new_group_pa() is failed,
    > it returns error code, ext4_mb_new_preallocation() propages it,
    > but ext4_mb_new_blocks() ignores it.
    >
    > An observed result was:
    > - allocation fail means ext4_mb_new_group_pa() does not update ext4_allocation_context;
    > - ext4_mb_new_blocks() sets ext4_allocation_request->len (ar->len = ac->ac_b_ex.fe_len;)
    > to number of blocks preallocated (512) instead of number of blocks requested (1);
    > - that activates update cycle in ext4_splice_branch():
    > for (i = 1; i < blks; i++) <-- blks is 512 instead of 1 here
    > *(where->p + i) = cpu_to_le32(current_block++);
    > - it iterates 511 times and corrupts a chunk of memory including inode structure;
    > - page fault happens at EXT4_SB(inode->i_sb) in ext4_mark_inode_dirty();
    > - system hangs with 'scheduling while atomic' BUG.
    >
    > The patch implements a check for ext4_mb_new_preallocation() error code
    > and handles its failure as if ext4_mb_regular_allocator() fails.
    >
    > Found by Linux File System Verification project (linuxtesting.org).
    >
    > Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>

    Hi Alexey,

    Thanks for reporting this bug, and proposing a fix. I've restructured
    the patch slightly to make the flow of control slightly easier to
    follow, and more consistent with the coding style in ext4.

    Thanks again!

    - Ted

    From fa4f073ab981d4aabb61f9262405af53072a0d8d Mon Sep 17 00:00:00 2001
    From: Alexey Khoroshilov <khoroshilov@ispras.ru>
    Date: Mon, 17 Jun 2013 09:15:34 -0400
    Subject: [PATCH] ext4: implement error handling of
    ext4_mb_new_preallocation()

    If memory allocation in ext4_mb_new_group_pa() is failed,
    it returns error code, ext4_mb_new_preallocation() propages it,
    but ext4_mb_new_blocks() ignores it.

    An observed result was:

    - allocation fail means ext4_mb_new_group_pa() does not update
    ext4_allocation_context;

    - ext4_mb_new_blocks() sets ext4_allocation_request->len (ar->len =
    ac->ac_b_ex.fe_len;) to number of blocks preallocated (512) instead
    of number of blocks requested (1);

    - that activates update cycle in ext4_splice_branch():
    for (i = 1; i < blks; i++) <-- blks is 512 instead of 1 here
    *(where->p + i) = cpu_to_le32(current_block++);

    - it iterates 511 times and corrupts a chunk of memory including inode
    structure;

    - page fault happens at EXT4_SB(inode->i_sb) in ext4_mark_inode_dirty();

    - system hangs with 'scheduling while atomic' BUG.

    The patch implements a check for ext4_mb_new_preallocation() error
    code and handles its failure as if ext4_mb_regular_allocator() fails.

    Found by Linux File System Verification project (linuxtesting.org).

    [ Patch restructed by tytso to make the flow of control easier to follow. ]

    Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
    ---
    fs/ext4/mballoc.c | 17 ++++++++++-------
    1 file changed, 10 insertions(+), 7 deletions(-)

    diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
    index 1a9c22b..a9ff5e5 100644
    --- a/fs/ext4/mballoc.c
    +++ b/fs/ext4/mballoc.c
    @@ -4406,17 +4406,20 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
    repeat:
    /* allocate space in core */
    *errp = ext4_mb_regular_allocator(ac);
    - if (*errp) {
    - ext4_discard_allocated_blocks(ac);
    - goto errout;
    - }
    + if (*errp)
    + goto discard_and_exit;

    /* as we've just preallocated more space than
    - * user requested orinally, we store allocated
    + * user requested originally, we store allocated
    * space in a special descriptor */
    if (ac->ac_status == AC_STATUS_FOUND &&
    - ac->ac_o_ex.fe_len < ac->ac_b_ex.fe_len)
    - ext4_mb_new_preallocation(ac);
    + ac->ac_o_ex.fe_len < ac->ac_b_ex.fe_len)
    + *errp = ext4_mb_new_preallocation(ac);
    + if (*errp) {
    + discard_and_exit:
    + ext4_discard_allocated_blocks(ac);
    + goto errout;
    + }
    }
    if (likely(ac->ac_status == AC_STATUS_FOUND)) {
    *errp = ext4_mb_mark_diskspace_used(ac, handle, reserv_clstrs);
    --
    1.7.12.rc0.22.gcdd159b


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