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Subject: Re: [PATCH 1/5] fs/stat: Define DAX statx attribute
To:     ira.weiny@intel.com, linux-kernel@vger.kernel.org
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Chinner <david@fromorbit.com>,
        Christoph Hellwig <hch@lst.de>,
        "Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
        linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
References: <20191020155935.12297-1-ira.weiny@intel.com>
 <20191020155935.12297-2-ira.weiny@intel.com>
From:   Boaz Harrosh <boaz@plexistor.com>
Message-ID: <119b57ed-2799-c499-00df-50da80d23612@plexistor.com>
Date:   Tue, 22 Oct 2019 14:32:04 +0300
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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 */
>