• Yasuaki Ishimatsu's avatar
    block: fix accounting bug on cross partition merges · 7681bfee
    Yasuaki Ishimatsu authored
    /proc/diskstats would display a strange output as follows.
    
    $ cat /proc/diskstats |grep sda
       8       0 sda 90524 7579 102154 20464 0 0 0 0 0 14096 20089
       8       1 sda1 19085 1352 21841 4209 0 0 0 0 4294967064 15689 4293424691
                                                    ~~~~~~~~~~
       8       2 sda2 71252 3624 74891 15950 0 0 0 0 232 23995 1562390
       8       3 sda3 54 487 2188 92 0 0 0 0 0 88 92
       8       4 sda4 4 0 8 0 0 0 0 0 0 0 0
       8       5 sda5 81 2027 2130 138 0 0 0 0 0 87 137
    
    Its reason is the wrong way of accounting hd_struct->in_flight. When a bio is
    merged into a request belongs to different partition by ELEVATOR_FRONT_MERGE.
    
    The detailed root cause is as follows.
    
    Assuming that there are two partition, sda1 and sda2.
    
    1. A request for sda2 is in request_queue. Hence sda1's hd_struct->in_flight
       is 0 and sda2's one is 1.
    
            | hd_struct->in_flight
       ---------------------------
       sda1 |          0
       sda2 |          1
       ---------------------------
    
    2. A bio belongs to sda1 is issued and is merged into the request mentioned on
       step1 by ELEVATOR_BACK_MERGE. The first sector of the request is changed
       from sda2 region to sda1 region. However the two partition's
       hd_struct->in_flight are not changed.
    
            | hd_struct->in_flight
       ---------------------------
       sda1 |          0
       sda2 |          1
       ---------------------------
    
    3. The request is finished and blk_account_io_done() is called. In this case,
       sda2's hd_struct->in_flight, not a sda1's one, is decremented.
    
            | hd_struct->in_flight
       ---------------------------
       sda1 |         -1
       sda2 |          1
       ---------------------------
    
    The patch fixes the problem by caching the partition lookup
    inside the request structure, hence making sure that the increment
    and decrement will always happen on the same partition struct. This
    also speeds up IO with accounting enabled, since it cuts down on
    the number of lookups we have to do.
    
    When reloading partition tables, quiesce IO to ensure that no
    request references to the partition struct exists. When it is safe
    to free the partition table, the IO for that device is restarted
    again.
    Signed-off-by: default avatarYasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
    Cc: stable@kernel.org
    Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>
    7681bfee
check.c 18.8 KB