Re: [PATCH 1/2] btrfs-progs: Fix a bug in __btrfs_map_block() always maps wrong stripe for DUP/RAID1

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

-------- Original Message  --------
Subject: Re: [PATCH 1/2] btrfs-progs: Fix a bug in __btrfs_map_block() 
always maps wrong stripe for DUP/RAID1
From: David Sterba <dsterba@suse.cz>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>
Date: 2015年05月22日 00:06

> On Mon, May 11, 2015 at 08:45:47AM +0800, Qu Wenruo wrote:
>> In __btrfs_map_block(), stripe_index is always wrong if mirror_num is
>> given.
>>
>> For DUP/RAID1 case, if mirror_num is given, e.g. 1, it should return the
>> second stripe.
>> But codes consider the mirror_num is start from 1 and always minus 1,
>> causing even mirror number is given as 1, __btrfs_map_block() will still
>> map the first stripe not the second.
>>
>> Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
>> ---
>>   volumes.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/volumes.c b/volumes.c
>> index 16dbf64..c66c02a 100644
>> --- a/volumes.c
>> +++ b/volumes.c
>> @@ -1397,7 +1397,7 @@ again:
>> -			stripe_index = mirror_num - 1;
>> +			stripe_index = mirror_num;
>> @@ -1416,7 +1416,7 @@ again:
>> -			stripe_index = mirror_num - 1;
>> +			stripe_index = mirror_num;
>
> The original code is same in kernel, is the logic different in the
> userspace code? If yes I don't see it.
>
It seems that I was confused about mirror_num of btrfs_map_block function.

When mirror mirror_num 0, it will map all stripes and if larger than 0,
it will map the <mirror_num>th stripe.

So when mirror_num is given 1, it should return the first stripe.
And it's the correct behavior.
If I am wrong again, please correct me.

Please ignore the patch.

Thanks,
Qu