Re: [PATCH 1/5] fs/stat: Define DAX statx attribute

Boaz Harrosh
2019-10-22T11:32:12Z

On 20/10/2019 18:59, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
>
> In order for users to determine if a file is currently operating in DAX
> mode (effective DAX). Define a statx attribute value and set that
> attribute if the effective DAX flag is set.
>
> To go along with this we propose the following addition to the statx man
> page:
>
> STATX_ATTR_DAX
>
> DAX (cpu direct access) is a file mode that attempts to minimize
> software cache effects for both I/O and memory mappings of this
> file. It requires a capable device, a compatible filesystem
> block size, and filesystem opt-in. It generally assumes all
> accesses are via cpu load / store instructions which can
> minimize overhead for small accesses, but adversely affect cpu
> utilization for large transfers. File I/O is done directly
> to/from user-space buffers. While the DAX property tends to
> result in data being transferred synchronously it does not give
> the guarantees of synchronous I/O that data and necessary
> metadata are transferred. Memory mapped I/O may be performed
> with direct mappings that bypass system memory buffering. Again
> while memory-mapped I/O tends to result in data being
> transferred synchronously it does not guarantee synchronous
> metadata updates. A dax file may optionally support being mapped
> with the MAP_SYNC flag which does allow cpu store operations to
> be considered synchronous modulo cpu cache effects.
>
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> ---
> fs/stat.c | 3 +++
> include/uapi/linux/stat.h | 1 +
> 2 files changed, 4 insertions(+)
>
> diff --git a/fs/stat.c b/fs/stat.c
> index c38e4c2e1221..59ca360c1ffb 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -77,6 +77,9 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
> if (IS_AUTOMOUNT(inode))
> stat->attributes |= STATX_ATTR_AUTOMOUNT;
>
> + if (inode->i_flags & S_DAX)

Is there a reason not to use IS_DAX(inode) ?

> + stat->attributes |= STATX_ATTR_DAX;
> +
> if (inode->i_op->getattr)
> return inode->i_op->getattr(path, stat, request_mask,
> query_flags);
> diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
> index 7b35e98d3c58..5b0962121ef7 100644
> --- a/include/uapi/linux/stat.h
> +++ b/include/uapi/linux/stat.h
> @@ -169,6 +169,7 @@ struct statx {
> #define STATX_ATTR_ENCRYPTED 0x00000800 /* [I] File requires key to decrypt in fs */
>
> #define STATX_ATTR_AUTOMOUNT 0x00001000 /* Dir: Automount trigger */
> +#define STATX_ATTR_DAX 0x00002000 /* [I] File is DAX */
>
>
> #endif /* _UAPI_LINUX_STAT_H */
>