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

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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@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: NeilBrown <neilb@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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[PATCH 00/13] block: assorted cleanup for bio splitting and cloning. NeilBrown <neilb@suse.com> - 2017-05-02 05:50 +0200
  [PATCH 04/13] blk: use non-rescuing bioset for q->bio_split. NeilBrown <neilb@suse.com> - 2017-05-02 05:50 +0200
    Re: [PATCH 04/13] blk: use non-rescuing bioset for q->bio_split. Ming Lei <ming.lei@redhat.com> - 2017-05-02 14:00 +0200
    [PATCH 04/13 V2] blk: use non-rescuing bioset for q->bio_split. NeilBrown <neilb@suse.com> - 2017-05-03 01:30 +0200
  [PATCH 06/13] rbd: use bio_clone_fast() instead of bio_clone() NeilBrown <neilb@suse.com> - 2017-05-02 05:50 +0200
  [PATCH 10/13] xen-blkfront: remove bio splitting. NeilBrown <neilb@suse.com> - 2017-05-02 05:50 +0200
    Re: [PATCH 10/13] xen-blkfront: remove bio splitting. Christoph Hellwig <hch@infradead.org> - 2017-05-02 10:20 +0200
  [PATCH 07/13] drbd: use bio_clone_fast() instead of bio_clone() NeilBrown <neilb@suse.com> - 2017-05-02 05:50 +0200
    [PATCH 07/13 V2] drbd: use bio_clone_fast() instead of bio_clone() NeilBrown <neilb@suse.com> - 2017-05-03 01:30 +0200
  [PATCH 13/13] block: don't check for BIO_MAX_PAGES in  blk_bio_segment_split() NeilBrown <neilb@suse.com> - 2017-05-02 05:50 +0200
    Re: [PATCH 13/13] block: don't check for BIO_MAX_PAGES in  blk_bio_segment_split() Christoph Hellwig <hch@infradead.org> - 2017-05-02 10:20 +0200
    Re: [PATCH 13/13] block: don't check for BIO_MAX_PAGES in  blk_bio_segment_split() Ming Lei <ming.lei@redhat.com> - 2017-05-02 12:30 +0200
      Re: [PATCH 13/13] block: don't check for BIO_MAX_PAGES in blk_bio_segment_split() NeilBrown <neilb@suse.com> - 2017-05-03 01:00 +0200
        Re: [PATCH 13/13] block: don't check for BIO_MAX_PAGES in  blk_bio_segment_split() Ming Lei <ming.lei@redhat.com> - 2017-05-03 02:00 +0200
  [PATCH 12/13] block: remove bio_clone() and all references. NeilBrown <neilb@suse.com> - 2017-05-02 05:50 +0200
  Re: [PATCH 00/13] block: assorted cleanup for bio splitting and cloning. NeilBrown <neilb@suse.com> - 2017-05-11 03:00 +0200

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