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    ext4: Fix max file size and logical block counting of extent format file · f17722f9
    Lukas Czerner 提交于
    Kazuya Mio reported that he was able to hit BUG_ON(next == lblock)
    in ext4_ext_put_gap_in_cache() while creating a sparse file in extent
    format and fill the tail of file up to its end. We will hit the BUG_ON
    when we write the last block (2^32-1) into the sparse file.
    
    The root cause of the problem lies in the fact that we specifically set
    s_maxbytes so that block at s_maxbytes fit into on-disk extent format,
    which is 32 bit long. However, we are not storing start and end block
    number, but rather start block number and length in blocks. It means
    that in order to cover extent from 0 to EXT_MAX_BLOCK we need
    EXT_MAX_BLOCK+1 to fit into len (because we counting block 0 as well) -
    and it does not.
    
    The only way to fix it without changing the meaning of the struct
    ext4_extent members is, as Kazuya Mio suggested, to lower s_maxbytes
    by one fs block so we can cover the whole extent we can get by the
    on-disk extent format.
    
    Also in many places EXT_MAX_BLOCK is used as length instead of maximum
    logical block number as the name suggests, it is all a bit messy. So
    this commit renames it to EXT_MAX_BLOCKS and change its usage in some
    places to actually be maximum number of blocks in the extent.
    
    The bug which this commit fixes can be reproduced as follows:
    
     dd if=/dev/zero of=/mnt/mp1/file bs=<blocksize> count=1 seek=$((2**32-2))
     sync
     dd if=/dev/zero of=/mnt/mp1/file bs=<blocksize> count=1 seek=$((2**32-1))
    Reported-by: NKazuya Mio <k-mio@sx.jp.nec.com>
    Signed-off-by: NLukas Czerner <lczerner@redhat.com>
    Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
    f17722f9
move_extent.c 40.9 KB