linux-kernel - Re: [RFC] block: fix blk_queue

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Message-ID: <20160704082006.GN3239@soda.linbit>
Date:	Mon, 4 Jul 2016 10:20:07 +0200
From:	Lars Ellenberg <lars.ellenberg@...bit.com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	linux-block <linux-block@...r.kernel.org>,
	Roland Kammerer <roland.kammerer@...bit.com>,
	Jens Axboe <axboe@...nel.dk>, NeilBrown <neilb@...e.com>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Shaohua Li <shli@...nel.org>, Alasdair Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>,
	"open list:DEVICE-MAPPER (LVM)" <dm-devel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Takashi Iwai <tiwai@...e.de>, Jiri Kosina <jkosina@...e.cz>,
	Zheng Liu <gnehzuil.liu@...il.com>,
	Keith Busch <keith.busch@...el.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"open list:BCACHE (BLOCK LAYER CACHE)" <linux-bcache@...r.kernel.org>,
	"open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
	<linux-raid@...r.kernel.org>
Subject: Re: [RFC] block: fix blk_queue_split() resource exhaustion

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