linux-ext4 - [RFC] split 32-bit and 64-bit words

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Message-ID: <20070828202915.GY5377@schatzie.adilger.int>
Date:	Tue, 28 Aug 2007 14:29:15 -0600
From:	Andreas Dilger <adilger@...sterfs.com>
To:	linux-ext4@...r.kernel.org
Subject: [RFC] split 32-bit and 64-bit words

There are now several places in the on-disk format that we are accessing
split 64-bit and 32-bit words with helper functions (e.g.
ext4_free_{blocks,inodes}_count()), but there is no easy way to ensure
that we are accessing them correctly.

There were several cases in the recently-ported uninit-groups patch that
were missing these checks, and there may be others already in the code.
I propose that we do something akin to the __le32 and __be32 types to
detect cases where these are improperly accessed.

Something like __ll32 and __lh32 for the low and high parts of the word
(similarly __ll16 and __lh16) that are cast to __le32 and __le16 respectively
in the helper function/macro before being passed to le32_to_cpu() and friends.

The alternative is to rename the "low" parts of the field explicitly, so any
incorrect code gets a compile error instead of silently working for any
filesystem < 16TB in size.

I can find the following split fields in ext4_fs.h:

__le16  l_i_file_acl_high;
__le16  l_i_uid_high;   /* these 2 fields    */
__le16  l_i_gid_high;   /* were reserved2[0] */
#define i_size_high   i_dir_acl
__le32  s_blocks_count_hi;      /* Blocks count */
__le32  s_r_blocks_count_hi;    /* Reserved blocks count */
__le32  s_free_blocks_count_hi; /* Free blocks count */
__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 */

We should just rename i_dir_acl to i_size_high permanently, and allow
directories to use i_size_high as is done for regular files.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

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