• Kent Overstreet's avatar
    block: make generic_make_request handle arbitrarily sized bios · 54efd50b
    Kent Overstreet authored
    The way the block layer is currently written, it goes to great lengths
    to avoid having to split bios; upper layer code (such as bio_add_page())
    checks what the underlying device can handle and tries to always create
    bios that don't need to be split.
    But this approach becomes unwieldy and eventually breaks down with
    stacked devices and devices with dynamic limits, and it adds a lot of
    complexity. If the block layer could split bios as needed, we could
    eliminate a lot of complexity elsewhere - particularly in stacked
    drivers. Code that creates bios can then create whatever size bios are
    convenient, and more importantly stacked drivers don't have to deal with
    both their own bio size limitations and the limitations of the
    (potentially multiple) devices underneath them.  In the future this will
    let us delete merge_bvec_fn and a bunch of other code.
    We do this by adding calls to blk_queue_split() to the various
    make_request functions that need it - a few can already handle arbitrary
    size bios. Note that we add the call _after_ any call to
    blk_queue_bounce(); this means that blk_queue_split() and
    blk_recalc_rq_segments() don't need to be concerned with bouncing
    affecting segment merging.
    Some make_request_fn() callbacks were simple enough to audit and verify
    they don't need blk_queue_split() calls. The skipped ones are:
     * nfhd_make_request (arch/m68k/emu/nfblock.c)
     * axon_ram_make_request (arch/powerpc/sysdev/axonram.c)
     * simdisk_make_request (arch/xtensa/platforms/iss/simdisk.c)
     * brd_make_request (ramdisk - drivers/block/brd.c)
     * mtip_submit_request (drivers/block/mtip32xx/mtip32xx.c)
     * loop_make_request
     * null_queue_bio
     * bcache's make_request fns
    Some others are almost certainly safe to remove now, but will be left
    for future patches.
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Ming Lei <ming.lei@canonical.com>
    Cc: Neil Brown <neilb@suse.de>
    Cc: Alasdair Kergon <agk@redhat.com>
    Cc: Mike Snitzer <snitzer@redhat.com>
    Cc: dm-devel@redhat.com
    Cc: Lars Ellenberg <drbd-dev@lists.linbit.com>
    Cc: drbd-user@lists.linbit.com
    Cc: Jiri Kosina <jkosina@suse.cz>
    Cc: Geoff Levand <geoff@infradead.org>
    Cc: Jim Paris <jim@jtan.com>
    Cc: Philip Kelleher <pjk1939@linux.vnet.ibm.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Nitin Gupta <ngupta@vflare.org>
    Cc: Oleg Drokin <oleg.drokin@intel.com>
    Cc: Andreas Dilger <andreas.dilger@intel.com>
    Acked-by: NeilBrown <neilb@suse.de> (for the 'md/md.c' bits)
    Acked-by: default avatarMike Snitzer <snitzer@redhat.com>
    Reviewed-by: default avatarMartin K. Petersen <martin.petersen@oracle.com>
    Signed-off-by: default avatarKent Overstreet <kent.overstreet@gmail.com>
    [dpark: skip more mq-based drivers, resolve merge conflicts, etc.]
    Signed-off-by: default avatarDongsu Park <dpark@posteo.net>
    Signed-off-by: default avatarMing Lin <ming.l@ssi.samsung.com>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
blk-merge.c 17.7 KB