Re: [PATCH 1/2] Don't merge different partition's IOs

[Vivek Goyal]

On Tue, Dec 07, 2010 at 04:18:27PM +0900, Satoru Takeuchi wrote:
> Hi Linus, Yasuaki,  and Jens
> 
> (2010/12/07 1:08), Linus Torvalds wrote:
> >2010/12/6 Yasuaki Ishimatsu<isimatu.yasuaki@xxxxxxxxxxxxxx>:
> >>
> >>The problem is caused by merging different partition's I/Os. So the patch
> >>check whether a merging bio or request is a same partition as a request or not
> >>by using a partition's start sector and size.
> >
> >I really think this is wrong.
> >
> >We should just carry the partition information around in the req and
> >the bio, and just compare the pointers, rather than compare the range.
> >No need to even dereference the pointers, you should be able to just
> >do
> >
> >    /* don't merge if not on the same partition */
> >    if (bio->part != req->part)
> >       return 0;
> >
> >or something.
> >
> >This is doubly true since the accounting already does that horrible
> >partition lookup: rather than look it up, we should just _set_ it in
> >__generic_make_request(), where I think we already know it since we do
> >that whole blk_partition_remap().
> >
> >So just something like the appended (TOTALLY UNTESTED) perhaps?
> >
> >Note that this should get it right even for overlapping partitions etc.
> >
> >                      Linus
> 
> The problem can occur even if your patches are applied. Think about a case
> like the following.
> 
>  1) There are 2 partition, sda1 and sda2, on sda.
>  2) Open sda and issue an IO to sda2's first sector. Then sda2's in_flight
>     is incremented though you open not sda2 but sda. It is because of
>     partition lookup method. It is based on which partition rq->__sector
>     sector belongs to.
>  3) Issue an IO to sda1's last sector and it merged to the IO issued in
>     step (2) because their part are both sda. In addition, rq->__sector
>     is modified to the sda1's region.
>  4) After completing the IO, sda1's in_flight is decremented and diskstat
>     is corrupted here.
> 
> I think fixing this case is difficult and would cause more complexity.
> 
> I hit on another approach. Although it doesn'tprevent any merge as Linus
> preferred, it can fix the problem anyway. In this idea, in_flight is
> incremented and decremented for the partition which the request belonged
> to in its creation. It has the following merits.
> 
>  - It can fix the problem which Yasuaki reported, including the cases which
>    I mentioned above.
>  - It only append one extra field to request.
> 
> Although it would causes a bit gap, it doesn't have most influences because
> merging requests beyond partitions is the rare case.
> 
> I confirmed the attached patch can be applied to 2.6.37-rc4 and succeeded
> to compile. If you can accept this idea, I'll test it soon.
> 
> ---
>  block/blk-core.c       |   12 +++++++-----
>  block/blk-merge.c      |    2 +-
>  include/linux/blkdev.h |    6 ++++++
>  3 files changed, 14 insertions(+), 6 deletions(-)
> 
> Index: linux-2.6.37-rc4/block/blk-core.c
> ===================================================================
> --- linux-2.6.37-rc4.orig/block/blk-core.c	2010-11-30 13:42:04.000000000 +0900
> +++ linux-2.6.37-rc4/block/blk-core.c	2010-12-07 14:31:55.000000000 +0900
> @@ -64,11 +64,13 @@ static void drive_stat_acct(struct reque
>  		return;
>  	cpu = part_stat_lock();
> -	part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> -	if (!new_io)
> +	if (!new_io) {
> +		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_init_pos(rq));
>  		part_stat_inc(cpu, part, merges[rw]);
> -	else {
> +	} else {
> +		rq->__initial_sector = rq->__sector;
> +		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_init_pos(rq));
>  		part_round_stats(cpu, part);
>  		part_inc_in_flight(part, rw);

Ok, so idea seems to be that lets keep track of the sector number against
which we do the accounting. Even if we are doing merging later, accounting
sector of the request will not change so that in_flight will not go out
of the sync.

The only thing is that by allowing merging across partitions, one request
can have IO from multiple partitions and all of it being accounted to
only one partition. So it is more of little accounting error. Though I am
not sure how big a issue that is.

This sounds reasonable to me.

>  	}
> @@ -1776,7 +1778,7 @@ static void blk_account_io_completion(st
>  		int cpu;
>  		cpu = part_stat_lock();
> -		part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
> +		part = disk_map_sector_rcu(req->rq_disk, blk_rq_init_pos(req));
>  		part_stat_add(cpu, part, sectors[rw], bytes >> 9);
>  		part_stat_unlock();
>  	}
> @@ -1796,7 +1798,7 @@ static void blk_account_io_done(struct r
>  		int cpu;
>  		cpu = part_stat_lock();
> -		part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
> +		part = disk_map_sector_rcu(req->rq_disk, blk_rq_init_pos(req));
>  		part_stat_inc(cpu, part, ios[rw]);
>  		part_stat_add(cpu, part, ticks[rw], duration);
> Index: linux-2.6.37-rc4/block/blk-merge.c
> ===================================================================
> --- linux-2.6.37-rc4.orig/block/blk-merge.c	2010-11-30 13:42:04.000000000 +0900
> +++ linux-2.6.37-rc4/block/blk-merge.c	2010-12-07 14:14:55.000000000 +0900
> @@ -351,7 +351,7 @@ static void blk_account_io_merge(struct
>  		int cpu;
>  		cpu = part_stat_lock();
> -		part = disk_map_sector_rcu(req->rq_disk, blk_rq_pos(req));
> +		part = disk_map_sector_rcu(req->rq_disk, blk_rq_init_pos(req));
>  		part_round_stats(cpu, part);
>  		part_dec_in_flight(part, rq_data_dir(req));
> Index: linux-2.6.37-rc4/include/linux/blkdev.h
> ===================================================================
> --- linux-2.6.37-rc4.orig/include/linux/blkdev.h	2010-11-30 13:42:04.000000000 +0900
> +++ linux-2.6.37-rc4/include/linux/blkdev.h	2010-12-07 14:13:11.000000000 +0900
> @@ -91,6 +91,7 @@ struct request {
>  	/* the following two fields are internal, NEVER access directly */
>  	unsigned int __data_len;	/* total data len */
>  	sector_t __sector;		/* sector cursor */
> +	sector_t __initial_sector;

Would "acct_sector" be a better name. It just means this is the sector
which we will be using for accounting purposes of this rq.

Thanks
Vivek
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