Messages in this thread Patch in this message |  | | Date | Sun, 11 Jun 2006 04:54:21 -0700 | | From | "Robin H. Johnson" <> | | Subject | [PATCH] tmpfs time granularity fix for [acm]time going backwards. Also VFS time granularity bug on creat(). (Repost, more content) |
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[Please CC me on replies].
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.
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.
Puesdo-code of my testcase: int fd = creat(name,0644); fstat(fd,&st); printf("...[acm]time...",...) futime(fd,NULL); fstat(fd,&st); printf("...[acm]time...",...) close(fd);
Tested against: linus 2.6.13, linus 2.6.17-rc6.
Test output from a filesystem not supporting sub-second timestamps (ext3, reiserfs): creat: m=1149891410.0 c=1149891410.0 a=1149891407.0 futimes: m=1149891410.0 c=1149891410.0 a=1149891410.0
Test output from a filesystem supporting sub-second timestamps (jfs,xfs,ramfs): creat: m=1149891452.928796249 c=1149891452.928796249 a=1149891452.928796249 futimes: m=1149891452.928796249 c=1149891452.928796249 a=1149891452.928796249
Test output from the tmpfs filesystem before the fix: creat: m=1149892052.562029884 c=1149892052.562029884 a=1149892052.562029884 futimes: m=1149892052.0 c=1149892052.0 a=1149892052.0
Test output from the tmpfs filesystem with the patch below: creat: m=1149892086.382150894 c=1149892086.382150894 a=1149892075.473249075 futimes: m=1149892086.383150885 c=1149892086.383150885 a=1149892086.383150885
The output above of jfs/xfs/ramfs having identical ctime/mtime in the utime/creat calls is just co-incidence, my box is reasonably fast, on a slower machine, they do diverge more. My box is just fast enough that they happened in the same tick (I have HZ=1000).
After some digging, I found that this was being caused by tmpfs not having a time granularity set, thus inheriting the default 1s granularity.
NOTE: The further bug: I believe this also indicates there is another bug at the VFS layer, where the initial timestamp from the creat() operation did not have the granularity applied to it. I haven't traced this problem down yet, others that are more familiar with the VFS might have a bit better luck. Unless this is fixed, similar issues may crop up with other filesystems. It just needs a bit of code like this: inode->i_[acm]time = current_fs_time(inode->i_sb);
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
--- mm/shmem.c +++ mm/shmem.c @@ -2102,6 +2102,7 @@ #endif 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) -- Robin Hugh Johnson E-Mail : robbat2@gentoo.org GnuPG FP : 11AC BA4F 4778 E3F6 E4ED F38E B27B 944E 3488 4E85 [unhandled content-type:application/pgp-signature] |  |