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SubjectRe: [RFC] block: fix blk_queue_split() resource exhaustion
On Wed, Jul 6, 2016 at 8:38 PM, Lars Ellenberg
<lars.ellenberg@linbit.com> wrote:
> On Mon, Jul 04, 2016 at 06:47:29PM +0800, Ming Lei wrote:
>> >> One clean solution may be to convert the loop of generic_make_request()
>> >> into the following way:
>> >>
>> >> do {
>> >> struct bio *splitted, *remainder = NULL;
>> >> struct request_queue *q = bdev_get_queue(bio->bi_bdev);
>> >>
>> >> blk_queue_split(q, &bio, &remainder, q->bio_split);
>> >>
>> >> ret = q->make_request_fn(q, bio);
>> >>
>> >> if (remainder)
>> >> bio_list_add(current->bio_list, remainder);
>> >> bio = bio_list_pop(current->bio_list);
>> >> } while (bio)
>> >
>> > Not good enough.
>
>
>> > Goal was to first process all "deeper level" bios
>> > before processing the remainder.
>>
>> For the reported bio splitting issue, I think the goal is to make sure all
>> BIOs generated from 'bio' inside .make_request_fn(bio) are queued
>> before the 'current' remainder. Cause it is the issue introduced by
>> blk_split_bio().

I believe the above description is correct, but my previous solution
is wrong, looks what we need is a binary tree, in which left child is
the splitted bio and the remainder bio is the right child, and the tree
need to be traversed in pre-order to make sure all BIOs generated
from 'bio' inside .make_request_fn(bio) are queued before the
remainder of bio splitting.

Another property of the tree is that if one node has only one child,
the child has to be left one.

I will think about it further to see if it is doable to implement.

>
> Stacking:
> qA (limitting layer)
> -> qB (remapping)
> -> qC (remapping)
> -> qD (hardware)
>
> [in fact you don't even need four layers,
> this is just to clarify that the stack may be more complex than you
> assume it would be]
>
> Columns below:
> 1) argument to generic_make_request() and its target queue.
> 2) current->bio_list
> 3) "in-flight" counter of qA.
>
> ==== In your new picture, iiuc:
>
> generic_make_request(bio_orig)
> NULL in-flight=0
> bio_orig empty
> blk_queue_split()
> result:
> bio_s, bio_r
> qA->make_request_fn(bio_s)
> in-flight=1
> bio_c = bio_clone(bio_s)
> generic_make_request(bio_c to qB)
> bio_c
> <-return
> bio_list_add(bio_r)
> bio_c, bio_r
> bio_list_pop()
> bio_r
> qB->make_request_fn(bio_c)
> (Assume it does not clone, but only remap.
> But it may also be a striping layer,
> and queue more than one bio here.)
> generic_make_request(bio_c to qC)
> bio_r, bio_c
> <-return
> bio_list_pop()
> bio_c
> qA->make_request_fn(bio_r) in-flight still 1
>
> BLOCKS, because bio_c has not been processed to its final
> destination qD yet, and not dispatched to hardware.

Yes, you are right.

>
>
> ==== my suggestion
>
> generic_make_request(bio_orig)
> NULL in-flight=0
> bio_orig empty in-flight=0
> qA->make_request_fn(bio_orig)
> blk_queue_split()
> result:
> bio_s, and bio_r stuffed away to head of remainder list.
> in-flight=1
> bio_c = bio_clone(bio_s)
> generic_make_request(bio_c to qB)
> bio_c
> <-return
> bio_c
> bio_list_pop()
> empty
> qB->make_request_fn(bio_c)
> (Assume it does not clone, but only remap.
> But it may also be a striping layer,
> and queue more than one bio here.)
> generic_make_request(bio_c to qC)
> bio_c
> <-return
> bio_list_pop()
> empty
> qC->make_request_fn(bio_c)
> generic_make_request(bio_c to qD)
> bio_c
> <-return
> bio_list_pop()
> empty
> qD->make_request_fn(bio_c)
> dispatches to hardware
> <-return
> empty
> bio_list_pop()
> NULL, great, lets pop from remainder list
> qA->make_request_fn(bio_r) in-flight=?
>
> May block, but only until completion of bio_c.
> Which may already have happened.
>
> *makes progress*

I admit your solution is smart, but it isn't easy to prove it as correct
in theory. But if the traversal can be mapped into pre-order traversal
of the above binary tree, it may be correct.

Thanks,
Ming

>
> Thanks,
>
> Lars
>
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