[dm-devel] [RFC] block: fix blk_queue_split() resource exhaustion

[Lars Ellenberg]

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.

Goal was to first process all "deeper level" bios
before processing the remainder.

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