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Subject[PATCH[RFC] kill sysrq-u (emergency remount r/o)
Hi there,

in two recent discussions (file_list_lock scalability and remount r/o
on suspend) I stumbled over this emergency remount feature. It's not
actually useful because it tries a potentially dangerous remount
despite writers still beeing in progress, which we can't get rid.

I've attached one patch in this mail that simply kills the
functionality, and in a reply to this mail I'll send a second one
that keeps the sysrq functionality, but removes the force argument
from do_remount_sb that overrides the still busy check. This version
is currently not useful, but makes a lot of sense once Dave Hansens
per-mountpoint r/o patches get in, as we can check for a real write
in progress instead of simply a file opened with write permission.

Any ideas and comments?


Index: linux-2.6/drivers/char/sysrq.c
===================================================================
--- linux-2.6.orig/drivers/char/sysrq.c 2007-02-05 18:05:11.000000000 +0100
+++ linux-2.6/drivers/char/sysrq.c 2007-02-05 18:12:47.000000000 +0100
@@ -159,17 +159,6 @@
.enable_mask = SYSRQ_ENABLE_SYNC,
};

-static void sysrq_handle_mountro(int key, struct tty_struct *tty)
-{
- emergency_remount();
-}
-static struct sysrq_key_op sysrq_mountro_op = {
- .handler = sysrq_handle_mountro,
- .help_msg = "Unmount",
- .action_msg = "Emergency Remount R/O",
- .enable_mask = SYSRQ_ENABLE_REMOUNT,
-};
-
#ifdef CONFIG_LOCKDEP
static void sysrq_handle_showlocks(int key, struct tty_struct *tty)
{
@@ -340,7 +329,7 @@
&sysrq_unraw_op, /* r */
&sysrq_sync_op, /* s */
&sysrq_showstate_op, /* t */
- &sysrq_mountro_op, /* u */
+ NULL, /* u */
/* v: May be registered at init time by SMP VOYAGER */
NULL, /* v */
&sysrq_showstate_blocked_op, /* w */
Index: linux-2.6/fs/namespace.c
===================================================================
--- linux-2.6.orig/fs/namespace.c 2007-02-05 18:03:23.000000000 +0100
+++ linux-2.6/fs/namespace.c 2007-02-05 18:04:24.000000000 +0100
@@ -597,7 +597,7 @@
if (!(sb->s_flags & MS_RDONLY)) {
lock_kernel();
DQUOT_OFF(sb);
- retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
+ retval = do_remount_sb(sb, MS_RDONLY, NULL);
unlock_kernel();
}
up_write(&sb->s_umount);
@@ -964,7 +964,7 @@
return -EINVAL;

down_write(&sb->s_umount);
- err = do_remount_sb(sb, flags, data, 0);
+ err = do_remount_sb(sb, flags, data);
if (!err)
nd->mnt->mnt_flags = mnt_flags;
up_write(&sb->s_umount);
Index: linux-2.6/fs/super.c
===================================================================
--- linux-2.6.orig/fs/super.c 2007-02-05 18:03:23.000000000 +0100
+++ linux-2.6/fs/super.c 2007-02-05 18:04:54.000000000 +0100
@@ -557,35 +557,14 @@
}

/**
- * mark_files_ro
- * @sb: superblock in question
- *
- * All files are marked read/only. We don't care about pending
- * delete files so this should be used in 'force' mode only
- */
-
-static void mark_files_ro(struct super_block *sb)
-{
- struct file *f;
-
- file_list_lock();
- list_for_each_entry(f, &sb->s_files, f_u.fu_list) {
- if (S_ISREG(f->f_path.dentry->d_inode->i_mode) && file_count(f))
- f->f_mode &= ~FMODE_WRITE;
- }
- file_list_unlock();
-}
-
-/**
* do_remount_sb - asks filesystem to change mount options.
* @sb: superblock in question
* @flags: numeric part of options
* @data: the rest of options
- * @force: whether or not to force the change
*
* Alters the mount options of a mounted file system.
*/
-int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
+int do_remount_sb(struct super_block *sb, int flags, void *data)
{
int retval;

@@ -601,9 +580,7 @@
/* If we are remounting RDONLY and current sb is read/write,
make sure there are no rw files opened */
if ((flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY)) {
- if (force)
- mark_files_ro(sb);
- else if (!fs_may_remount_ro(sb))
+ if (!fs_may_remount_ro(sb))
return -EBUSY;
}

@@ -618,37 +595,6 @@
return 0;
}

-static void do_emergency_remount(unsigned long foo)
-{
- struct super_block *sb;
-
- spin_lock(&sb_lock);
- list_for_each_entry(sb, &super_blocks, s_list) {
- sb->s_count++;
- spin_unlock(&sb_lock);
- down_read(&sb->s_umount);
- if (sb->s_root && sb->s_bdev && !(sb->s_flags & MS_RDONLY)) {
- /*
- * ->remount_fs needs lock_kernel().
- *
- * What lock protects sb->s_flags??
- */
- lock_kernel();
- do_remount_sb(sb, MS_RDONLY, NULL, 1);
- unlock_kernel();
- }
- drop_super(sb);
- spin_lock(&sb_lock);
- }
- spin_unlock(&sb_lock);
- printk("Emergency Remount complete\n");
-}
-
-void emergency_remount(void)
-{
- pdflush_operation(do_emergency_remount, 0);
-}
-
/*
* Unnamed block devices are dummy devices used by virtual
* filesystems which don't use real block-devices. -- jrs
@@ -861,7 +807,7 @@
}
s->s_flags |= MS_ACTIVE;
}
- do_remount_sb(s, flags, data, 0);
+ do_remount_sb(s, flags, data);
return simple_set_mnt(mnt, s);
}

Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h 2007-02-05 18:03:23.000000000 +0100
+++ linux-2.6/include/linux/fs.h 2007-02-05 18:05:16.000000000 +0100
@@ -1599,9 +1599,7 @@
extern void sync_filesystems(int wait);
extern void __fsync_super(struct super_block *sb);
extern void emergency_sync(void);
-extern void emergency_remount(void);
-extern int do_remount_sb(struct super_block *sb, int flags,
- void *data, int force);
+extern int do_remount_sb(struct super_block *sb, int flags, void *data);
#ifdef CONFIG_BLOCK
extern sector_t bmap(struct inode *, sector_t);
#endif
-
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