Hi,
I've got an USB FAT32-formatted external drive that developed bad blocks
in the area of the 0th FAT. So, I've developed a quick patch for
that adds support for non-mirrored FATs in case of FAT32 (as per FS
spec). If FAT mirroring is on, the behavior is unchanged: read/write the
0th FAT, and mirror changes to the remaining FATs.
I've tested this patch on the said external drive as well as on some
loop devices.
Best regards,
Roma
--- cut here ---
>From 869fb1b0ed81396c0a0d1a42f557ea5be2c39797 Mon Sep 17 00:00:00 2001
From: Roman Dubtsov <dubtsov@xxxxxxxxx>
Date: Sat, 6 Nov 2010 12:21:49 +0100
Subject: [PATCH] fat: add support for FAT32 with non-mirrored FATs
Use BPB_ExtFlags member of the FAT32 boot sector to detect volumes with only a
single active FAT.
Signed-off-by: Roman Dubtsov <dubtsov@xxxxxxxxx>
---
fs/fat/fat.h | 6 ++++++
fs/fat/fatent.c | 6 +++++-
fs/fat/inode.c | 17 +++++++++++++++++
3 files changed, 28 insertions(+), 1 deletions(-)
diff --git a/fs/fat/fat.h b/fs/fat/fat.h
index d75a77f..d09191a 100644
--- a/fs/fat/fat.h
+++ b/fs/fat/fat.h
@@ -52,6 +52,9 @@ struct fat_mount_options {
#define FAT_HASH_BITS 8
#define FAT_HASH_SIZE (1UL << FAT_HASH_BITS)
+#define FAT_FLAG_DONT_MIRROR 0x0080
+#define FAT_FLAG_USE_FAT_NR 0x000F
+
/*
* MS-DOS file system in-core superblock data
*/
@@ -87,6 +90,9 @@ struct msdos_sb_info {
spinlock_t inode_hash_lock;
struct hlist_head inode_hashtable[FAT_HASH_SIZE];
+
+ int active_fat; /* Active FAT number (FAT32 only) */
+ int mirror_fat; /* Fat mirroring flag (FAT32 only) */
};
#define FAT_CACHE_VALID 0 /* special case for valid cache */
diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
index b47d2c9..7932921 100644
--- a/fs/fat/fatent.c
+++ b/fs/fat/fatent.c
@@ -36,9 +36,10 @@ static void fat_ent_blocknr(struct super_block *sb, int entry,
{
struct msdos_sb_info *sbi = MSDOS_SB(sb);
int bytes = (entry << sbi->fatent_shift);
+ int fat_start = sbi->fat_start + sbi->active_fat * sbi->fat_length;
WARN_ON(entry < FAT_START_ENT || sbi->max_cluster <= entry);
*offset = bytes & (sb->s_blocksize - 1);
- *blocknr = sbi->fat_start + (bytes >> sb->s_blocksize_bits);
+ *blocknr = fat_start + (bytes >> sb->s_blocksize_bits);
}
static void fat12_ent_set_ptr(struct fat_entry *fatent, int offset)
@@ -372,6 +373,9 @@ static int fat_mirror_bhs(struct super_block *sb, struct buffer_head **bhs,
struct buffer_head *c_bh;
int err, n, copy;
+ if (!sbi->mirror_fat)
+ return 0;
+
err = 0;
for (copy = 1; copy < sbi->fats; copy++) {
sector_t backup_fat = sbi->fat_length * copy;
diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index ad6998a..a3401f8 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -1361,10 +1361,13 @@ int fat_fill_super(struct super_block *sb, void *data, int silent,
sbi->free_clusters = -1; /* Don't know yet */
sbi->free_clus_valid = 0;
sbi->prev_free = FAT_START_ENT;
+ sbi->mirror_fat = 1; /* FAT mirroring is ON by default, */
+ sbi->active_fat = 0; /* and the default active FAT # is 0 */
if (!sbi->fat_length && b->fat32_length) {
struct fat_boot_fsinfo *fsinfo;
struct buffer_head *fsinfo_bh;
+ u16 flags;
/* Must be FAT32 */
sbi->fat_bits = 32;
@@ -1378,6 +1381,20 @@ int fat_fill_super(struct super_block *sb, void *data, int silent,
if (sbi->fsinfo_sector == 0)
sbi->fsinfo_sector = 1;
+ flags = le16_to_cpu(b->flags);
+ if (flags & FAT_FLAG_DONT_MIRROR) {
+ sbi->active_fat = flags & FAT_FLAG_USE_FAT_NR;
+ sbi->mirror_fat = 0;
+ printk(KERN_INFO "FAT: FAT mirroring is off."
+ " Using FAT #%d\n",
+ sbi->active_fat);
+ if (sbi->active_fat >= sbi->fats) {
+ printk(KERN_ERR "FAT: invalid active fat number\n");
+ brelse(bh);
+ goto out_fail;
+ }
+ }
+
fsinfo_bh = sb_bread(sb, sbi->fsinfo_sector);
if (fsinfo_bh == NULL) {
printk(KERN_ERR "FAT: bread failed, FSINFO block"
--
1.7.3.2
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