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Date:   Mon, 27 Jul 2020 09:08:31 -0700
From:   "Darrick J. Wong" <darrick.wong@oracle.com>
To:     Xiao Yang <yangx.jy@cn.fujitsu.com>
Cc:     ira.weiny@intel.com, tytso@mit.edu, linux-ext4@vger.kernel.org
Subject: Re: [PATCH] chattr/lsattr: Support dax attribute
Message-ID: <20200727160831.GO7625@magnolia>
References: <20200727092901.2728-1-yangx.jy@cn.fujitsu.com>
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On Mon, Jul 27, 2020 at 05:29:01PM +0800, Xiao Yang wrote:
> Use the letter 'x' to set/get dax attribute on a directory/file.
> 
> Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
> ---
>  lib/e2p/pf.c         |  1 +
>  lib/ext2fs/ext2_fs.h |  1 +
>  misc/chattr.1.in     | 10 ++++++++--
>  misc/chattr.c        |  3 ++-
>  4 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/e2p/pf.c b/lib/e2p/pf.c
> index 0c6998c4..e59cccff 100644
> --- a/lib/e2p/pf.c
> +++ b/lib/e2p/pf.c
> @@ -44,6 +44,7 @@ static struct flags_name flags_array[] = {
>  	{ EXT2_TOPDIR_FL, "T", "Top_of_Directory_Hierarchies" },
>  	{ EXT4_EXTENTS_FL, "e", "Extents" },
>  	{ FS_NOCOW_FL, "C", "No_COW" },
> +	{ FS_DAX_FL, "x", "Dax" },
>  	{ EXT4_CASEFOLD_FL, "F", "Casefold" },
>  	{ EXT4_INLINE_DATA_FL, "N", "Inline_Data" },
>  	{ EXT4_PROJINHERIT_FL, "P", "Project_Hierarchy" },
> diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h
> index 6c20ea77..b5e2e42a 100644
> --- a/lib/ext2fs/ext2_fs.h
> +++ b/lib/ext2fs/ext2_fs.h
> @@ -334,6 +334,7 @@ struct ext2_dx_tail {
>  #define EXT4_EA_INODE_FL	        0x00200000 /* Inode used for large EA */
>  /* EXT4_EOFBLOCKS_FL 0x00400000 was here */
>  #define FS_NOCOW_FL			0x00800000 /* Do not cow file */
> +#define FS_DAX_FL			0x02000000 /* Inode is DAX */
>  #define EXT4_SNAPFILE_FL		0x01000000  /* Inode is a snapshot */
>  #define EXT4_SNAPFILE_DELETED_FL	0x04000000  /* Snapshot is being deleted */
>  #define EXT4_SNAPFILE_SHRUNK_FL		0x08000000  /* Snapshot shrink has completed */
> diff --git a/misc/chattr.1.in b/misc/chattr.1.in
> index ff2fcf00..b27c8e1f 100644
> --- a/misc/chattr.1.in
> +++ b/misc/chattr.1.in
> @@ -23,7 +23,7 @@ chattr \- change file attributes on a Linux file system
>  .B chattr
>  changes the file attributes on a Linux file system.
>  .PP
> -The format of a symbolic mode is +-=[aAcCdDeFijPsStTu].
> +The format of a symbolic mode is +-=[aAcCdDeFijPsStTux].
>  .PP
>  The operator '+' causes the selected attributes to be added to the
>  existing attributes of the files; '-' causes them to be removed; and '='
> @@ -45,7 +45,8 @@ secure deletion (s),
>  synchronous updates (S),
>  no tail-merging (t),
>  top of directory hierarchy (T),
> -and undeletable (u).
> +undeletable (u),
> +and direct access for files (x).
>  .PP
>  The following attributes are read-only, and may be listed by
>  .BR lsattr (1)
> @@ -210,6 +211,11 @@ saved.  This allows the user to ask for its undeletion.  Note: please
>  make sure to read the bugs and limitations section at the end of this
>  document.
>  .TP
> +.B x
> +A file with the 'x' attribute set is accessed directly on the memory-like
> +disk(e.g. /dev/pmem) by the kernel.  Kernel will skip page cache and do
> +reads/writes on the file directly.

There's much more to FS_DAX_FL than that.

See the "Enabling DAX on XFS and ext4" section of
Documentation/filesystems/dax.txt.

Note the part where you can set it on directories too; and also the part
where there's a separate state for whether or not you get the pagecache
bypass.

--D

> +.TP
>  .B V
>  A file with the 'V' attribute set has fs-verity enabled.  It cannot be
>  written to, and the filesystem will automatically verify all data read
> diff --git a/misc/chattr.c b/misc/chattr.c
> index a5d60170..c0337f86 100644
> --- a/misc/chattr.c
> +++ b/misc/chattr.c
> @@ -86,7 +86,7 @@ static unsigned long sf;
>  static void usage(void)
>  {
>  	fprintf(stderr,
> -		_("Usage: %s [-pRVf] [-+=aAcCdDeijPsStTuF] [-v version] files...\n"),
> +		_("Usage: %s [-pRVf] [-+=aAcCdDeijPsStTuFx] [-v version] files...\n"),
>  		program_name);
>  	exit(1);
>  }
> @@ -112,6 +112,7 @@ static const struct flags_char flags_array[] = {
>  	{ EXT2_NOTAIL_FL, 't' },
>  	{ EXT2_TOPDIR_FL, 'T' },
>  	{ FS_NOCOW_FL, 'C' },
> +	{ FS_DAX_FL, 'x' },
>  	{ EXT4_CASEFOLD_FL, 'F' },
>  	{ 0, 0 }
>  };
> -- 
> 2.21.0
> 
> 
>