[v4,1/5] vfs: Add file timestamp range support

Add fields to the superblock to track the min and max
timestamps supported by filesystems.

Initially, when a superblock is allocated, initialize
it to the max and min values the fields can hold.
Individual filesystems override these to match their
actual limits.

Pseudo filesystems are assumed to always support the
min and max allowable values for the fields.

Note that the time ranges are saved in type time64_t
rather than time_t.
This is required because if we save ranges in time_t
then we would not be able to save timestamp ranges
for files that support timestamps beyond y2038.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
---
 fs/super.c             | 2 ++
 include/linux/fs.h     | 3 +++
 include/linux/time64.h | 2 ++
 3 files changed, 7 insertions(+)

On Fri, Feb 24, 2017 at 05:40:59PM -0800, Deepa Dinamani wrote:
> Add fields to the superblock to track the min and max
> timestamps supported by filesystems.
> 
> Initially, when a superblock is allocated, initialize
> it to the max and min values the fields can hold.
> Individual filesystems override these to match their
> actual limits.
> 
> Pseudo filesystems are assumed to always support the
> min and max allowable values for the fields.
> 
> Note that the time ranges are saved in type time64_t
> rather than time_t.
> This is required because if we save ranges in time_t
> then we would not be able to save timestamp ranges
> for files that support timestamps beyond y2038.
> 
> Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
> ---
>  fs/super.c             | 2 ++
>  include/linux/fs.h     | 3 +++
>  include/linux/time64.h | 2 ++
>  3 files changed, 7 insertions(+)
> 
> diff --git a/fs/super.c b/fs/super.c
> index b8b6a08..f9c2241 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -247,6 +247,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
>  	s->s_maxbytes = MAX_NON_LFS;
>  	s->s_op = &default_op;
>  	s->s_time_gran = 1000000000;
> +	s->s_time_min = TIME64_MIN;
> +	s->s_time_max = TIME64_MAX;

Just out of curiosity, does this enable 64-bit timestamps for everything
by default?  I see that ext4 later sets its own values in fill_super;
what about things like XFS that really only have 32-bit seconds fields?

--D

>  	s->cleancache_poolid = CLEANCACHE_NO_POOL;
>  
>  	s->s_shrink.seeks = DEFAULT_SEEKS;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index de8ed0b..ef55dfb 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1337,6 +1337,9 @@ struct super_block {
>  	/* Granularity of c/m/atime in ns.
>  	   Cannot be worse than a second */
>  	u32		   s_time_gran;
> +	/* Time limits for c/m/atime in seconds. */
> +	time64_t           s_time_min;
> +	time64_t           s_time_max;
>  
>  	/*
>  	 * The next field is for VFS *only*. No filesystems have any business
> diff --git a/include/linux/time64.h b/include/linux/time64.h
> index 980c71b..25433b18 100644
> --- a/include/linux/time64.h
> +++ b/include/linux/time64.h
> @@ -38,6 +38,8 @@ struct itimerspec64 {
>  
>  /* Located here for timespec[64]_valid_strict */
>  #define TIME64_MAX			((s64)~((u64)1 << 63))
> +#define TIME64_MIN			(-TIME64_MAX - 1)
> +
>  #define KTIME_MAX			((s64)~((u64)1 << 63))
>  #define KTIME_SEC_MAX			(KTIME_MAX / NSEC_PER_SEC)
>  
> -- 
> 2.7.4
>

> Just out of curiosity, does this enable 64-bit timestamps for everything
> by default?  I see that ext4 later sets its own values in fill_super;
> what about things like XFS that really only have 32-bit seconds fields?

This is just initializing limits for each filesystem.
Filesystems then override these limits with the actual limits they support.
This is what ext4 patch in the series is doing.
As the cover letter says, I will be submitting patches for all
filesystems to update limits.

This is no different than what is happening right now.
Right now, on 64 bit systems, VFS assumes timestamps to be 64 bits
long for all filesystems.

On 32 bit systems, the series only effects handling of dates beyond
the year 2038.
This is until vfs is transitioned to support 64 bit times. At which
point, the behavior on these will also be similar to 64 bit machines.

-Deepa