linux-ext4 - Re: [RFC][PATCH] Set JBD2_FEATURE_INCOMPAT_64BIT on filesystems larger than 32-bit blocks (take 2).

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Date:	Tue, 05 Jun 2007 15:26:53 +0200
From:	Laurent Vivier <Laurent.Vivier@...l.net>
To:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
Cc:	"Jose R. Santos" <jrs@...ibm.com>, cmm@...ibm.com,
	Andreas Dilger <adilger@...sterfs.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [RFC][PATCH] Set JBD2_FEATURE_INCOMPAT_64BIT on filesystems	larger
 than 32-bit blocks (take 2).

Dave Kleikamp wrote:
> On Tue, 2007-06-05 at 06:41 -0500, Jose R. Santos wrote:
>> On Mon, 04 Jun 2007 16:01:45 -0700
>> Mingming Cao <cmm@...ibm.com> wrote:
>>
>>> On Mon, 2007-06-04 at 11:57 -0600, Andreas Dilger wrote:
>>>> On Jun 04, 2007  11:32 -0500, Jose R. Santos wrote:
>>>>> Set the journals JBD2_FEATURE_INCOMPAT_64BIT on devices with more
>>>>> than 32bit block sizes during mount time.  This ensure proper record
>>>>> lenth when writing to the journal.
>>>>>
>>>>> Signed-off-by: Jose R. Santos <jrs@...ibm.com>
>>>>> ---
>>>>>  fs/ext4/super.c |   11 +++++++++++
>>>>>  1 file changed, 11 insertions(+)
>>>>>
>>>>> Index: linux-2.6.22-rc3/fs/ext4/super.c
>>>>> ===================================================================
>>>>> --- linux-2.6.22-rc3.orig/fs/ext4/super.c	2007-06-04 11:01:20.028360650 -0500
>>>>> +++ linux-2.6.22-rc3/fs/ext4/super.c	2007-06-04 11:05:11.389126418 -0500
>>>>> @@ -1824,6 +1824,17 @@ static int ext4_fill_super (struct super
>>>>>  		goto failed_mount3;
>>>>>  	}
>>>>>  
>>>>> +        /*
>>>>> +	 * Make sure to set JBD2_FEATURE_INCOMPAT_64BIT on filesystems
>>>>> +	 * with more that 32-bit block counts
>>>>> +	 */
>>>>> +	if(es->s_blocks_count_hi &&
>>> This need to be le32_to_cpu(es->s_blocks_count_hi)
>> I'm curious,
>>
>> Why do we need to do an endian conversion to check for a non-zero value
>> in s_blocks_count_hi?  Seems unnecessary here.
> 
> Jose is right.  The endian conversion is unnecessary.
> 
> Shaggy

But by using le32_to_cpu(es->s_blocks_count_hi) you explicitly mark the variable
as a little-endian.
So if someone reads the code, he knows this is a little-endian value and this
allows to avoid errors if later variable must be tested for other value than 0.

For instance, you have :

if(es->s_blocks_count_hi)

and later the value should be compared to 10, how do you know easily you should use:

if (le32_to_cpu(es->s_blocks_count_hi) == 10)

instead of

if(es->s_blocks_count_hi == 10)

I think writing like Mingming asks should allow to avoid errors later.

(and code becomes really self-explicit...)

Regards,
Laurent
-- 
------------- Laurent.Vivier@...l.net  --------------
       "Any sufficiently advanced technology is
  indistinguishable from magic." - Arthur C. Clarke


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