block: generic_make_request() recursive bios: process deepest levels first
By providing each q->make_request_fn() with an empty "recursion"
bio_list, then merging any recursively submitted bios to the
head of the "remainder" list, we can make the recursion-to-iteration
logic in generic_make_request() process deepest level bios first.
As suggested by Neil Brown while discussing
[RFC] block: fix blk_queue_split() resource exhaustion
https://lkml.org/lkml/2016/7/7/27
Stack: qA -> qB -> qC -> qD
=== Without this patch:
generic_make_request(bio_orig to qA)
recursion: empty, remainder: empty
qA->make_request_fn(bio_orig)
potential call to bio_queue_split()
result: bio_S, bio_R
recursion: empty, remainder: bio_R
bio_S
generic_make_request(bio_S to qB)
recursion: bio_S, remainder: bio_R
<- return
pop: recursion: empty, remainder: bio_R
qB->make_request_fn(bio_S)
remap, maybe many clones because of striping
generic_make_request(clones to qC)
recursion: bio_C1, bio_C2, bio_C3
remainder: bio_R
<- return
pop: recursion: bio_C2, bio_C3,
remainder: bio_R
qC->make_request_fn(bio_C1)
remap, ...
generic_make_request(clones to qD)
recursion: bio_C2, bio_C3, bio_D1_1, bio_D1_2
remainder: bio_R
<- return
pop: recursion: bio_C3, bio_D1_1, bio_D1_2
remainder: bio_R
qC->make_request_fn(bio_C2)
recursion: bio_C3, bio_D1_1, bio_D1_2, bio_D2_1, bio_D2_2
remainder: bio_R
<- return
pop: recursion: bio_D1_1, bio_D1_2, bio_D2_1, bio_D2_2
remainder: bio_R
qC->make_request_fn(bio_C3)
...
=== With this patch:
generic_make_request(bio_orig to qA)
recursion: empty, remainder: empty
qA->make_request_fn(bio_orig)
potential call to bio_queue_split()
result: bio_S, bio_R
recursion: empty, remainder: bio_R
bio_S
generic_make_request(bio_S to qB)
recursion: bio_S, remainder: bio_R
<- return
merge_head:
recursion: empty, remainder: bio_S, bio_R
pop: recursion: empty, remainder: bio_R
qB->make_request_fn(bio_S)
remap, maybe many clones because of striping
generic_make_request(clones to qC)
recursion: bio_C1, bio_C2, bio_C3
remainder: bio_R
<- return
merge_head:
recursion: empty
remainder: bio_C1, bio_C2, bio_C3, bio_R
pop: remainder: bio_C2, bio_C3, bio_R
qC->make_request_fn(bio_C1)
remap, ...
generic_make_request(clones to qD)
recursion: bio_D1_1, bio_D1_2
remainder: bio_C2, bio_C3, bio_R
<- return
merge_head:
recursion: empty
remainder: bio_D1_1, bio_D1_2, bio_C2, bio_C3, bio_R
pop
qC->make_request_fn(bio_D1_1)
remainder: bio_D1_2, bio_C2, bio_C3, bio_R
...
2 files changed
