From: Xiaole He hexiaole1994@126.com
mainline inclusion from mainline-for-next commit fdbae121b4369fe49eb5f8efbd23604ab4c50116 category: doc
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The 'ctime', 'mtime', and 'atime' for inode is the type of 'xfs_timestamp_t', which is a 64-bit type:
/* fs/xfs/libxfs/xfs_format.h begin */ typedef __be64 xfs_timestamp_t; /* fs/xfs/libxfs/xfs_format.h end */
When the 'bigtime' feature is disabled, this 64-bit type is splitted into two parts of 32-bit, one part is encoded for seconds since 1970-01-01 00:00:00 UTC, the other part is encoded for nanoseconds above the seconds, this two parts are the type of 'xfs_legacy_timestamp' and the min and max time value of this type are defined as macros 'XFS_LEGACY_TIME_MIN' and 'XFS_LEGACY_TIME_MAX':
/* fs/xfs/libxfs/xfs_format.h begin */ struct xfs_legacy_timestamp { __be32 t_sec; /* timestamp seconds */ __be32 t_nsec; /* timestamp nanoseconds */ }; #define XFS_LEGACY_TIME_MIN ((int64_t)S32_MIN) #define XFS_LEGACY_TIME_MAX ((int64_t)S32_MAX) /* fs/xfs/libxfs/xfs_format.h end */ /* include/linux/limits.h begin */ #define U32_MAX ((u32)~0U) #define S32_MAX ((s32)(U32_MAX >> 1)) #define S32_MIN ((s32)(-S32_MAX - 1)) /* include/linux/limits.h end */
'XFS_LEGACY_TIME_MIN' is the min time value of the 'xfs_legacy_timestamp', that is -(2^31) seconds relative to the 1970-01-01 00:00:00 UTC, it can be converted to human-friendly time value by 'date' command:
/* command begin */ [root@~]# date --utc -d '@0' +'%Y-%m-%d %H:%M:%S' 1970-01-01 00:00:00 [root@~]# date --utc -d "@`echo '-(2^31)'|bc`" +'%Y-%m-%d %H:%M:%S' 1901-12-13 20:45:52 [root@~]# /* command end */
When 'bigtime' feature is enabled, this 64-bit type becomes a 64-bit nanoseconds counter, with the start time value is the min time value of 'xfs_legacy_timestamp'(start time means the value of 64-bit nanoseconds counter is 0). We have already caculated the min time value of 'xfs_legacy_timestamp', that is 1901-12-13 20:45:52 UTC, but the comment for the start time value of inode with 'bigtime' feature enabled writes the value is 1901-12-31 20:45:52 UTC:
/* fs/xfs/libxfs/xfs_format.h begin */ /* * XFS Timestamps * ============== * When the bigtime feature is enabled, ondisk inode timestamps become an * unsigned 64-bit nanoseconds counter. This means that the bigtime inode * timestamp epoch is the start of the classic timestamp range, which is * Dec 31 20:45:52 UTC 1901. ... ... */ /* fs/xfs/libxfs/xfs_format.h end */
That is a typo, and this patch corrects the typo, from 'Dec 31' to 'Dec 13'.
Suggested-by: Darrick J. Wong djwong@kernel.org Signed-off-by: Xiaole He hexiaole@kylinos.cn Reviewed-by: Darrick J. Wong djwong@kernel.org Signed-off-by: Darrick J. Wong djwong@kernel.org --- fs/xfs/libxfs/xfs_format.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h index dd764da08f6f..8faa2a6069e6 100644 --- a/fs/xfs/libxfs/xfs_format.h +++ b/fs/xfs/libxfs/xfs_format.h @@ -870,7 +870,7 @@ struct xfs_agfl { * When the bigtime feature is enabled, ondisk inode timestamps become an * unsigned 64-bit nanoseconds counter. This means that the bigtime inode * timestamp epoch is the start of the classic timestamp range, which is - * Dec 31 20:45:52 UTC 1901. Because the epochs are not the same, callers + * Dec 13 20:45:52 UTC 1901. Because the epochs are not the same, callers * /must/ use the bigtime conversion functions when encoding and decoding raw * timestamps. */