On Mon, Dec 22, 2014 at 7:48 PM, Dongsu Park <dongsu.park@xxxxxxxxxxxxxxxx> wrote: > From: Kent Overstreet <kmo@xxxxxxxxxxxxx> > > 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. Looks it is a very good idea to split bio in block. > > 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 I am wondering why the bio isn't splitted just before q->make_request_fn is called in generic_make_request()? By this way, drivers won't need to call blk_queue_split() at all. Is it because performance reason? or others? > 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 I guess the above drivers haven't max_sectors/max_segment limit. Thanks, Ming Lei -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
