linux-kernel - Re: [PATCH] block: fix accounting bug on cross partition merges

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Date:	Fri, 24 Dec 2010 14:29:16 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: fix accounting bug on cross partition merges

On Thu, Dec 23, 2010 at 06:04:11PM +0100, Jerome Marchand wrote:
> On 12/23/2010 04:39 PM, Vivek Goyal wrote:
> >> diff --git a/block/blk-core.c b/block/blk-core.c
> >> index 4ce953f..72d12d2 100644
> >> --- a/block/blk-core.c
> >> +++ b/block/blk-core.c
> >> @@ -64,13 +64,21 @@ static void drive_stat_acct(struct request *rq, int new_io)
> >>  		return;
> >>  
> >>  	cpu = part_stat_lock();
> >> -	part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> >>  
> >> -	if (!new_io)
> >> +	if (!new_io) {
> >> +		part = rq->part;
> >>  		part_stat_inc(cpu, part, merges[rw]);
> >> -	else {
> >> +	} else {
> >> +		part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq));
> >> +		if(part->partno && !kref_test_and_get(&part->ref))
> >> +			/*
> > 
> > Do we have to check this part->partno both while taking and releasing
> > reference. Can't we take one extra reference for disk->part0, at
> > alloc_disk_node() time so that it is never freed and only freed when
> > disk is going away and gendisk is being freed.
> > 
> > That way, you don't have to differentiate between disk->part0 and rest
> > of the partitions while taking or dropping references.
> 
> We could do it in any way, as long as we don't end up trying to free
> disk->part0. I choose not to touch part0->ref at all, but we could also
> drop all the part->partno test, and get a reference on part0 when we use
> it as a backup. I have no strong opinion about a way or an other.

Ok, I personally like taking an extra reference to disk->part0 and put
a proper comment there so that at rest of the places we don't try to
differentiate between part0 and other partitions. 

Having said that I am not too concerned about it. It is just one of the
minor details. So post the new version of patch after testing.

Thanks
Vivek
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