linux-ext4 - Re: [RFC][PATCH 4/4] BIG_BG: block group descriptor modifications

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Date:	Thu, 30 Nov 2006 17:04:31 +0100
From:	Valerie Clement <valerie.clement@...l.net>
To:	Andreas Dilger <adilger@...sterfs.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [RFC][PATCH 4/4] BIG_BG: block group descriptor modifications

Andreas Dilger wrote:
> On Nov 24, 2006  17:48 +0100, Valerie Clement wrote:
>> @@ -133,6 +133,9 @@ struct ext4_group_desc
>>  	__le32	bg_block_bitmap_hi;	/* Blocks bitmap block MSB */
>>  	__le32	bg_inode_bitmap_hi;	/* Inodes bitmap block MSB */
>>  	__le32	bg_inode_table_hi;	/* Inodes table block MSB */
>> +	__le16	bg_free_blocks_count_hi;	/* Free blocks count MSB */
>> +	__le16	bg_free_inodes_count_hi;	/* Free inodes count MSB */
>> +	__le16	bg_used_dirs_count_hi;	/* Directories count MSB */
>>  };
> 
> Does the ext4 code already avoid using "sizeof(struct ext4_group_desc)"
> or "sizeof(*gdp)" everywhere?  Otherwise this is very dangerous.
Currently, the code doesn't use sizeof of the structure or of a group 
descriptor, but do you mean that adding padding to the structure is better?

> 
> Also note that the on-disk layout of this struct in e2fsprogs is a bit
> incorrect - it has the above 3 __u16, but then immediately __u32 bg_reserved
> so those fields are padded incorrectly.  I think it isn't a fatal problem,
> just something to be aware of and fix.
OK.

> 
>> +#define EXT4_READ_SPLIT_LE32(__sb, __field)	\
>> +	((__u32)le16_to_cpu(__field) +		\
>> +	 (EXT4_HAS_INCOMPAT_FEATURE((__sb), EXT4_FEATURE_INCOMPAT_64BIT) ? \
>> +	 (__u32)le16_to_cpu(__field##_hi) << 16 : 0))
> 
> Is it better to make this INCOMPAT_64BIT or s_desc_size? Does INCOMPAT_64BIT
> always imply s_desc_size > 32?  Hmm, I guess it does, or we have no place to
> store the _hi part of the block addresses for a group.
I prefer using INCOMPAT_64BIT rather than s_desc_size.
Thank you Andreas to point that out, I missed to update my e2fsprogs 
version to initialize s_desc_size with the right value in some cases.

   Valérie


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