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 16:03:44 +0200
From:	Laurent Vivier <Laurent.Vivier@...l.net>
To:	"Jose R. Santos" <jrs@...ibm.com>
Cc:	Dave Kleikamp <shaggy@...ux.vnet.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).

Jose R. Santos wrote:
> On Tue, 05 Jun 2007 15:26:53 +0200 Laurent Vivier <Laurent.Vivier@...l.net>
> wrote:
> 
>> Dave Kleikamp wrote:
>>> 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
> 
> Hi Laurent,
> 
> In this particular case though, the value of s_blocks_count_hi should not be
> uses on its own.  The correct way would be to use ext4_blocks_count() which
> already does the endian conversion.  If you think the code could confuse
> people as to how to access the data in s_blocks_count_hi, wouldn't hiding it
> through the use of a macro make more sense than doing an unnecessary endian
> conversion?
> 

Yes, I think the code could confuse people, but I don't think defining "Yet
Another Macro" is a good choice (IMHO).

I think we can resolve this (non-)issue by two ways:
- using le32_to_cpu() (but I agree it does an unnecessary endian conversion on
big-endian systems)
- put a comment on the line (but are we allowed to put comments in kernel source
code... ;-) )

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


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