linux-ext4 - Re: [RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute

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Date:   Wed, 15 Jan 2020 12:37:15 +0100
From:   Jan Kara <jack@...e.cz>
To:     ira.weiny@...el.com
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH V2 01/12] fs/stat: Define DAX statx attribute

On Fri 10-01-20 11:29:31, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.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@...el.com>

This looks good to me. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  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 */
> -- 
> 2.21.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR