fat: introduce special inode for managing the FSINFO block

This is patchset makes fatfs stop using the VFS '->write_super()' method
for writing out the FSINFO block.

The final goal is to get rid of the 'sync_supers()' kernel thread.  This
kernel thread wakes up every 5 seconds (by default) and calls
'->write_super()' for all mounted file-systems.  And the bad thing is that
this is done even if all the superblocks are clean.  Moreover, some
file-systems do not even need this end they do not register the
'->write_super()' method at all (e.g., btrfs).

So 'sync_supers()' most often just generates useless wake-ups and wastes
power.  I am trying to make all file-systems independent of
'->write_super()' and plan to remove 'sync_supers()' and '->write_super'
completely once there are no more users.

The '->write_supers()' method is mostly used by baroque file-systems like
hfs, udf, etc.  Modern file-systems like btrfs and xfs do not use it.
This justifies removing this stuff from VFS completely and make every FS
self-manage own superblock.

Tested with xfstests.

This patch:

Preparation for further changes.  It introduces a special inode
('fsinfo_inode') in FAT file-system which we'll later use for managing the
FSINFO block.  Note, this there is already one special inode ('fat_inode')
which is used for managing the FAT tables.

Introduce new 'MSDOS_FSINFO_INO' constant for this special inode.  It is
safe to do because FAT file-system does not store inode numbers on the
media but generates them run-time.

I've also cleaned up the comment to existing 'MSDOS_ROOT_INO' constant,
while on it.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index b3d290c1b513..84c602b4bb25 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -490,6 +490,7 @@ static void fat_put_super(struct super_block *sb)
         if (sb->s_dirt)
                 fat_write_super(sb);
 
+        iput(sbi->fsinfo_inode);
         iput(sbi->fat_inode);
 
         unload_nls(sbi->nls_disk);
@@ -1244,6 +1245,7 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
                    void (*setup)(struct super_block *))
 {
         struct inode *root_inode = NULL, *fat_inode = NULL;
+        struct inode *fsinfo_inode = NULL;
         struct buffer_head *bh;
         struct fat_boot_sector *b;
         struct msdos_sb_info *sbi;
@@ -1490,6 +1492,14 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
                 goto out_fail;
         MSDOS_I(fat_inode)->i_pos = 0;
         sbi->fat_inode = fat_inode;
+
+        fsinfo_inode = new_inode(sb);
+        if (!fsinfo_inode)
+                goto out_fail;
+        fsinfo_inode->i_ino = MSDOS_FSINFO_INO;
+        sbi->fsinfo_inode = fsinfo_inode;
+        insert_inode_hash(fsinfo_inode);
+
         root_inode = new_inode(sb);
         if (!root_inode)
                 goto out_fail;
@@ -1516,6 +1526,8 @@ out_invalid:
                 fat_msg(sb, KERN_INFO, "Can't find a valid FAT filesystem");
 
 out_fail:
+        if (fsinfo_inode)
+                iput(fsinfo_inode);
         if (fat_inode)
                 iput(fat_inode);
         unload_nls(sbi->nls_io);