[Please CC me on replies].
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
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: This also indicates there is another bug at the VFS layer, where the
initial timestamp did not have the granularity applied to it.
Signed-off-by: Robin H. Johnson <[email protected]>
--- a/mm/shmem.c
+++ b/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 : [email protected]
GnuPG FP : 11AC BA4F 4778 E3F6 E4ED F38E B27B 944E 3488 4E85
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