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    SubjectRe: [ 110/180] ext4: dont let i_reserved_meta_blocks go negative
    On Tue, Oct 02, 2012 at 12:53:47AM +0200, Willy Tarreau wrote:
    > 2.6.32-longterm review patch. If anyone has any objections, please let me know.
    >
    > ------------------
    >
    > From: Brian Foster <bfoster@redhat.com>
    >
    > commit 97795d2a5b8d3c8dc4365d4bd3404191840453ba upstream.
    >
    > If we hit a condition where we have allocated metadata blocks that
    > were not appropriately reserved, we risk underflow of
    > ei->i_reserved_meta_blocks. In turn, this can throw
    > sbi->s_dirtyclusters_counter significantly out of whack and undermine
    > the nondelalloc fallback logic in ext4_nonda_switch(). Warn if this
    > occurs and set i_allocated_meta_blocks to avoid this problem.
    >
    > This condition is reproduced by xfstests 270 against ext2 with
    > delalloc enabled:
    >
    > Mar 28 08:58:02 localhost kernel: [ 171.526344] EXT4-fs (loop1): delayed block allocation failed for inode 14 at logical offset 64486 with max blocks 64 with error -28
    > Mar 28 08:58:02 localhost kernel: [ 171.526346] EXT4-fs (loop1): This should not happen!! Data will be lost
    >
    > 270 ultimately fails with an inconsistent filesystem and requires an
    > fsck to repair. The cause of the error is an underflow in
    > ext4_da_update_reserve_space() due to an unreserved meta block
    > allocation.
    [...]
    > + if (unlikely(ei->i_allocated_meta_blocks > ei->i_reserved_meta_blocks)) {
    > + ext4_msg(inode->i_sb, KERN_NOTICE, "%s: ino %lu, allocated %d "
    > + "with only %d reserved metadata blocks\n", __func__,
    > + inode->i_ino, ei->i_allocated_meta_blocks,
    > + ei->i_reserved_meta_blocks);
    > + WARN_ON(1);
    > + ei->i_allocated_meta_blocks = ei->i_reserved_meta_blocks;
    > + }
    [...]

    This seems to be working around a bug elsewhere. Has the underlying
    bug been fixed in mainline yet?

    Ben.

    --
    Ben Hutchings
    We get into the habit of living before acquiring the habit of thinking.
    - Albert Camus


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