[PATCH] tmpfs: time granularity fix for [acm]time going backwards

[Hugh Dickins <hugh@veritas.com>]

From: Robin H. Johnson <robbat2@gentoo.org>

I noticed a strange behavior in a tmpfs file system the other day, while
building packages - occasionally, and seemingly at random, make decided to
rebuild a target. However, only on tmpfs.

A file would be created, and if checked, it had a sub-second timestamp.
However, after an utimes related call where sub-seconds should be set, they
were zeroed instead. In the case that a file was created, and utimes(...,NULL)
was used on it in the same second, the timestamp on the file moved backwards.

After some digging, I found that this was being caused by tmpfs not having a
time granularity set, thus inheriting the default 1 second granularity.

Hugh adds: yes, we missed tmpfs when the s_time_gran mods went into 2.6.11.
Unfortunately, the granularity of CURRENT_TIME, often used in filesystems,
does not match the default granularity set by alloc_super.  A few more such
discrepancies have been found, but this is the most important to fix now.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Acked-by: Andi Kleen <ak@suse.de>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
This patch should probably be included for 2.6.17, despite how long the
bug has been around. It's a one-liner, with no side-effects.

 mm/shmem.c |    1 +
 1 file changed, 1 insertion(+)

--- 2.6.17-rc6-git/mm/shmem.c
+++ linux/mm/shmem.c
@@ -2102,6 +2102,7 @@ static int shmem_fill_super(struct super
 	sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
 	sb->s_magic = TMPFS_MAGIC;
 	sb->s_op = &shmem_ops;
+	sb->s_time_gran = 1;
 
 	inode = shmem_get_inode(sb, S_IFDIR | mode, 0);
 	if (!inode)