Re: [patch] fs: add a remount,ro coherency point

[Wu Fengguang <fengguang.wu@intel.com>]

Hi Nick,

On Mon, Mar 30, 2009 at 01:33:54PM +0200, Nick Piggin wrote:
> 
> fs: invalidate sb->s_bdev on remount,ro
> 
> Fixes a problem reported by "Jorge Boncompte [DTI2]" <jorge@dti2.net>
> who is trying to snapshot a minix filesystem image.
> 
> Signed-off-by: Nick Piggin <npiggin@suse.de>
> 
> ---
>  fs/super.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> Index: linux-2.6/fs/super.c
> ===================================================================
> --- linux-2.6.orig/fs/super.c
> +++ linux-2.6/fs/super.c
> @@ -647,6 +647,14 @@ int do_remount_sb(struct super_block *sb
>  		acct_auto_close(sb);
>  	shrink_dcache_sb(sb);

Not a question to this patch: can we avoid shrink_dcache_sb() on
ro=>rw remounts? They will happen at system boot time, and eliminating
the shrink_dcache_sb() could help reduce the boot time.

>  	fsync_super(sb);
> +	 /* Some filesystems modify their metadata via some other path
> +	    than the bdev buffer cache (eg. use a private mapping, or
> +	    directories in pagecache, etc). Also file data modifications
> +	    go via their own mappings. So If we try to mount readonly
> +	    then copy the filesystem from bdev, we could get stale data,
> +	    so invalidate it to give a best effort at coherency. */
> +	if (flags & MS_RDONLY && sb->s_bdev)
> +		invalidate_bdev(sb->s_bdev);

Or move the above lines to...

>  	/* 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))
                        return -EBUSY;
                retval = vfs_dq_off(sb, 1);
                if (retval < 0 && retval != -ENOSYS)
                        return -EBUSY;
        }

...here?

Thanks,
Fengguang