[PATCH 04/13 V2] blk: use non-rescuing bioset for q->bio_split.

[NeilBrown]

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 <h...@lst.de>
Reviewed-by: Ming Lei <ming....@redhat.com>
Signed-off-by: NeilBrown <ne...@suse.com>
---

This patch needed to be refreshed after previous patch
had long lines wrapped.
Also added Ming's Reviewed-by.

Thanks,
NeilBrown



 block/blk-core.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index bc66cd595bef..3797753f4085 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -730,8 +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;
 
-- 
2.12.2

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