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SubjectRe: [RFC] block: fix blk_queue_split() resource exhaustion
On Mon, Jul 4, 2016 at 4:20 PM, Lars Ellenberg
<lars.ellenberg@linbit.com> wrote:
> On Sat, Jul 02, 2016 at 06:28:29PM +0800, Ming Lei wrote:
>> The idea looks good, but not sure it can cover all cases of
>> dm over brbd or brbd over dm and maintaining two lists
>> becomes too complicated.
>>
>> 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.
>
> Consider DRBD on device-mapper on device-mapper on scsi,
> or insert multipath and / or md raid into the stack.
> The iterative call to q->make_request_fn() in the second iteration
> may queue bios after the remainder.

But this remainder is not the top remainder any more, I guess you mean
the following situation:

- drbd_make_request(bio)
->generic_make_request(bio)
->bio is added into current->bio_list
- bio is splitted as bio_a and bio_b in generic_make_request()
- dm_make_request(bio_a)
->generic_make_request(bio_a)
->bio_a is add into current_list
- bio_a is splitted as bio_a_a and bio_a_b in generic_make_request()
- dm_make_request(bio_a_a)
->.....
- bio_a_b is added into current->bio_list
- dm_make_request(bio_a_b)

But it is correct because bio_a depends on both bio_a_a and bio_a_b.
Or I understand you wrong?

>
> 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().

Thanks,
Ming

>
> You can achieve this by doing last-in-first-out on bio_list,
> or by using two lists, as I suggested in the original post.
>
> Lars
>
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