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

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Date:	Thu, 23 Dec 2010 18:04:11 +0100
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Vivek Goyal <vgoyal@...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 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.

Jerome

> 
> Thanks
> Vivek
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