On Tue, Apr 21, 2020 at 12:17:45PM -0700, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
>
> In order for users to determine if a file is currently operating in DAX
> state (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
>
> The file is in the DAX (cpu direct access) state. DAX state
> attempts to minimize software cache effects for both I/O and
> memory mappings of this file. It requires a file system which
> has been configured to support DAX.
>
> DAX generally assumes all accesses are via cpu load / store
> instructions which can minimize overhead for small accesses, but
> may adversely affect cpu utilization for large transfers.
>
> File I/O is done directly to/from user-space buffers and memory
> mapped I/O may be performed with direct memory mappings that
> bypass kernel page cache.
>
> While the DAX property tends to result in data being transferred
> synchronously, it does not give the same guarantees of O_SYNC
> where data and the necessary metadata are transferred together.
>
> A DAX file may support being mapped with the MAP_SYNC flag,
> which enables a program to use CPU cache flush instructions to
> persist CPU store operations without an explicit fsync(2). See
> mmap(2) for more information.
One thing I hadn't noticed before -- this is a change to userspace API,
so please cc this series to linux-api@xxxxxxxxxxxxxxx when you send V10.
Also, I've started to think about commit order sequencing for actually
landing this series. Usually I try to put vfs and documentation things
before xfs stuff, which means I came up with:
vfs xfs I_DONTCACHE
2 3 11 1 4 5 6 7 8 9 10
Note that I separated the DONTCACHE part because it touches VFS
internals, which implies a higher standard of review (aka Al) and I do
not wish to hold up the 2-3-11-1-4-5-6-7 patches if the dontcache part
becomes contentious.
What do you think of that ordering?
(Heck, maybe I'll just put patch 1 in the queue for 5.8 right now...)
--D
> Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
>
> ---
> Changes from V2:
> Update man page text with comments from Darrick, Jan, Dan, and
> Dave.
> ---
> 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 030008796479..894699c74dde 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -79,6 +79,9 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
> if (IS_AUTOMOUNT(inode))
> stat->attributes |= STATX_ATTR_AUTOMOUNT;
>
> + if (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 ad80a5c885d5..e5f9d5517f6b 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_VERITY 0x00100000 /* [I] Verity protected file */
> +#define STATX_ATTR_DAX 0x00002000 /* [I] File is DAX */
>
>
> #endif /* _UAPI_LINUX_STAT_H */
> --
> 2.25.1
>
