[PATCH][UBUNTU] sync before umount to reduce time taken by ext4 umount

[Stefan Bader]

It looks like this should at least give no badness. Only it should be made a
SAUCE patch.

Surbhi Palande wrote:
> This patch is a tempory fix for the bug 543617 on launchpad where users have
> reported huge amount of time for an ext4 unmount. When sync is called prior to
> umount, the time taken by umount is reduced to an acceptable value. The
> reason due to which ext4 umount takes a long time is:
> * a journal transaction is involved with every inode associated with a flush
> at umount time. A wait is performed till this journal transaction is flushed
> to the disk.
> 
> In the example mentioned by Kees Cook on LP, there are 65K inodes and their
> associated journal data blocks which are flushed to the disk and 65K barriers
> called with a single unmount call. Due to the large dirty data flush interval
> time, the journal flush is not happening before umount is called in this
> particular case.
> 
> The following patch puts the sync() related code before umount, so that users
> get an acceptable wait period whenever umount is called.
> 
> Do consider merging this for Lucid, till a real fix is available.
> 
> From bc7dddbc909a17c63b13ba4c62d420256bd11758 Mon Sep 17 00:00:00 2001
> From: Surbhi Palande <surbhi.palande at canonical.com>
> Date: Wed, 7 Apr 2010 15:03:11 +0300
> Subject: [PATCH] sync before umount to reduce time taken by ext4 umount
> 
> This is a temporary fix to reduce the time it takes an unmount, when no prior
> sync is called. The appropriate solution would be to fix the underlying ext4
> journal, sync and barrier calls.
> 
> BugLink: http://launchpad.net/bugs/543617
> 
> Signed-off-by: Surbhi Palande <surbhi.palande at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>
> ---
>  fs/namespace.c     |    7 +++++++
>  fs/sync.c          |    2 +-
>  include/linux/fs.h |    1 +
>  3 files changed, 9 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index a2cadcf..a648d7f 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -33,6 +33,7 @@
>  #include <asm/unistd.h>
>  #include "pnode.h"
>  #include "internal.h"
> +#include <linux/fs.h>
>  
>  #define HASH_SHIFT ilog2(PAGE_SIZE / sizeof(struct list_head))
>  #define HASH_SIZE (1UL << HASH_SHIFT)
> @@ -1134,6 +1135,12 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
>  	if (!capable(CAP_SYS_ADMIN))
>  		goto dput_and_out;
>  
> +	/* Temporary solution to fix long umount periods till
> +	 * we find the real fix
> +	 */
> +	sync_filesystems(0);
> +	sync_filesystems(1);
> +
>  	retval = do_umount(path.mnt, flags);
>  dput_and_out:
>  	/* we mustn't call path_put() as that would clear mnt_expiry_mark */
> diff --git a/fs/sync.c b/fs/sync.c
> index d104591..09e3734 100644
> --- a/fs/sync.c
> +++ b/fs/sync.c
> @@ -89,7 +89,7 @@ EXPORT_SYMBOL_GPL(sync_filesystem);
>   * flags again, which will cause process A to resync everything.  Fix that with
>   * a local mutex.
>   */
> -static void sync_filesystems(int wait)
> +void sync_filesystems(int wait)
>  {
>  	struct super_block *sb;
>  	static DEFINE_MUTEX(mutex);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 692a3ee..1a51c61 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1973,6 +1973,7 @@ static inline int thaw_bdev(struct block_device *bdev, struct super_block *sb)
>  	return 0;
>  }
>  #endif
> +extern void sync_filesystems(int wait);
>  extern int sync_filesystem(struct super_block *);
>  extern const struct file_operations def_blk_fops;
>  extern const struct file_operations def_chr_fops;