• Milan Broz's avatar
    block: fix setting of max_segment_size and seg_boundary mask · 0e435ac2
    Milan Broz authored
    Fix setting of max_segment_size and seg_boundary mask for stacked md/dm
    devices.
    
    When stacking devices (LVM over MD over SCSI) some of the request queue
    parameters are not set up correctly in some cases by default, namely
    max_segment_size and and seg_boundary mask.
    
    If you create MD device over SCSI, these attributes are zeroed.
    
    Problem become when there is over this mapping next device-mapper mapping
    - queue attributes are set in DM this way:
    
    request_queue   max_segment_size  seg_boundary_mask
    SCSI                65536             0xffffffff
    MD RAID1                0                      0
    LVM                 65536                 -1 (64bit)
    
    Unfortunately bio_add_page (resp.  bio_phys_segments) calculates number of
    physical segments according to these parameters.
    
    During the generic_make_request() is segment cout recalculated and can
    increase bio->bi_phys_segments count over the allowed limit.  (After
    bio_clone() in stack operation.)
    
    Thi is specially problem in CCISS driver, where it produce OOPS here
    
        BUG_ON(creq->nr_phys_segments > MAXSGENTRIES);
    
    (MAXSEGENTRIES is 31 by default.)
    
    Sometimes even this command is enough to cause oops:
    
      dd iflag=direct if=/dev/<vg>/<lv> of=/dev/null bs=128000 count=10
    
    This command generates bios with 250 sectors, allocated in 32 4k-pages
    (last page uses only 1024 bytes).
    
    For LVM layer, it allocates bio with 31 segments (still OK for CCISS),
    unfortunatelly on lower layer it is recalculated to 32 segments and this
    violates CCISS restriction and triggers BUG_ON().
    
    The patch tries to fix it by:
    
     * initializing attributes above in queue request constructor
       blk_queue_make_request()
    
     * make sure that blk_queue_stack_limits() inherits setting
    
     (DM uses its own function to set the limits because it
     blk_queue_stack_limits() was introduced later.  It should probably switch
     to use generic stack limit function too.)
    
     * sets the default seg_boundary value in one place (blkdev.h)
    
     * use this mask as default in DM (instead of -1, which differs in 64bit)
    
    Bugs related to this:
    https://bugzilla.redhat.com/show_bug.cgi?id=471639
    http://bugzilla.kernel.org/show_bug.cgi?id=8672Signed-off-by: default avatarMilan Broz <mbroz@redhat.com>
    Reviewed-by: default avatarAlasdair G Kergon <agk@redhat.com>
    Cc: Neil Brown <neilb@suse.de>
    Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
    Cc: Tejun Heo <htejun@gmail.com>
    Cc: Mike Miller <mike.miller@hp.com>
    Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
    0e435ac2
blk-settings.c 14.7 KB