On Tue, May 02, 2017 at 01:42:24PM +1000, NeilBrown wrote: > A rescuing bioset is only useful if there might be bios from > that same bioset on the bio_list_on_stack queue at a time > when bio_alloc_bioset() is called. This never applies to > q->bio_split. > > Allocations from q->bio_split are only ever made from > blk_queue_split() which is only ever called early in each of > various make_request_fn()s. The original bio (call this A) > is then passed to generic_make_request() and is placed on > the bio_list_on_stack queue, and the bio that was allocated > from q->bio_split (B) is processed. > > The processing of this may cause other bios to be passed to > generic_make_request() or may even cause the bio B itself to > be passed, possible after some prefix has been split off > (using some other bioset). > > generic_make_request() now guarantees that all of these bios > (B and dependants) will be fully processed before the tail > of the original bio A gets handled. None of these early bios > can possible trigger an allocation from the original > q->bio_split as they are either too small to require > splitting or (more likely) are destined for a different queue. > > The next time that the original q->bio_split might be used > by this thread is when A is processed again, as it might > still be too big to handle directly. By this time there > cannot be any other bios allocated from q->bio_split in the > generic_make_request() queue. So no rescuing will ever be > needed. > > Reviewed-by: Christoph Hellwig <hch@xxxxxx> > Signed-off-by: NeilBrown <neilb@xxxxxxxx> > --- > block/blk-core.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/block/blk-core.c b/block/blk-core.c > index ae51f159a2ca..3797753f4085 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -730,7 +730,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id) > if (q->id < 0) > goto fail_q; > > - q->bio_split = bioset_create(BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS | BIOSET_NEED_RESCUER); > + q->bio_split = bioset_create(BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS); > if (!q->bio_split) > goto fail_id; > > > Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx> Thanks, Ming
