Commit 96ed320d authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge tag 'vfs-5.8-merge-1' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux 

Pull DAX updates part one from Darrick Wong:
 "After many years of LKML-wrangling about how to enable programs to
  query and influence the file data access mode (DAX) when a filesystem
  resides on storage devices such as persistent memory, Ira Weiny has
  emerged with a proposed set of standard behaviors that has not been
  shot down by anyone! We're more or less standardizing on the current
  XFS behavior and adapting ext4 to do the same.

  This is the first of a handful pull requests that will make ext4 and
  XFS present a consistent interface for user programs that care about
  DAX. We add a statx attribute that programs can check to see if DAX is
  enabled on a particular file. Then, we update the DAX documentation to
  spell out the user-visible behaviors that filesystems will guarantee
  (until the next storage industry shakeup). The on-disk inode flag has
  been in XFS for a few years now.

  Summary:

   - Clean up io_is_direct.

   - Add a new statx flag to indicate when file data access is being
     done via DAX (as opposed to the page cache).

   - Update the documentation for how system administrators and
     application programmers can take advantage of the (still
     experimental DAX) feature"

Link: https://lore.kernel.org/lkml/20200505002016.1085071-1-ira.weiny@intel.com/

* tag 'vfs-5.8-merge-1' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux:
  Documentation/dax: Update Usage section
  fs/stat: Define DAX statx attribute
  fs: Remove unneeded IS_DAX() check in io_is_direct()
parents 16d91548 83d90886

Documentation/filesystems/dax.txt

+139 −3
Original line number Diff line number Diff line
@@ -20,8 +20,144 @@ Usage 
If you have a block device which supports DAX, you can make a filesystem

on it as usual.  The DAX code currently only supports files with a block

size equal to your kernel's PAGE_SIZE, so you may need to specify a block

size when creating the filesystem.  When mounting it, use the "-o dax"

option on the command line or add 'dax' to the options in /etc/fstab.

size when creating the filesystem.



Currently 3 filesystems support DAX: ext2, ext4 and xfs.  Enabling DAX on them

is different.



Enabling DAX on ext4 and ext2

-----------------------------



When mounting the filesystem, use the "-o dax" option on the command line or

add 'dax' to the options in /etc/fstab.  This works to enable DAX on all files

within the filesystem.  It is equivalent to the '-o dax=always' behavior below.





Enabling DAX on xfs

-------------------



Summary

-------



 1. There exists an in-kernel file access mode flag S_DAX that corresponds to

    the statx flag STATX_ATTR_DAX.  See the manpage for statx(2) for details

    about this access mode.



 2. There exists a persistent flag FS_XFLAG_DAX that can be applied to regular

    files and directories. This advisory flag can be set or cleared at any

    time, but doing so does not immediately affect the S_DAX state.



 3. If the persistent FS_XFLAG_DAX flag is set on a directory, this flag will

    be inherited by all regular files and subdirectories that are subsequently

    created in this directory. Files and subdirectories that exist at the time

    this flag is set or cleared on the parent directory are not modified by

    this modification of the parent directory.



 4. There exist dax mount options which can override FS_XFLAG_DAX in the

    setting of the S_DAX flag.  Given underlying storage which supports DAX the

    following hold:



    "-o dax=inode"  means "follow FS_XFLAG_DAX" and is the default.



    "-o dax=never"  means "never set S_DAX, ignore FS_XFLAG_DAX."



    "-o dax=always" means "always set S_DAX ignore FS_XFLAG_DAX."



    "-o dax"        is a legacy option which is an alias for "dax=always".

		    This may be removed in the future so "-o dax=always" is

		    the preferred method for specifying this behavior.



    NOTE: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain

    the same even when the filesystem is mounted with a dax option.  However,

    in-core inode state (S_DAX) will be overridden until the filesystem is

    remounted with dax=inode and the inode is evicted from kernel memory.



 5. The S_DAX policy can be changed via:



    a) Setting the parent directory FS_XFLAG_DAX as needed before files are

       created



    b) Setting the appropriate dax="foo" mount option



    c) Changing the FS_XFLAG_DAX flag on existing regular files and

       directories.  This has runtime constraints and limitations that are

       described in 6) below.



 6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag,

    the change in behaviour for existing regular files may not occur

    immediately.  If the change must take effect immediately, the administrator

    needs to:



    a) stop the application so there are no active references to the data set

       the policy change will affect



    b) evict the data set from kernel caches so it will be re-instantiated when

       the application is restarted. This can be achieved by:



       i. drop-caches

       ii. a filesystem unmount and mount cycle

       iii. a system reboot





Details

-------



There are 2 per-file dax flags.  One is a persistent inode setting (FS_XFLAG_DAX)

and the other is a volatile flag indicating the active state of the feature

(S_DAX).



FS_XFLAG_DAX is preserved within the filesystem.  This persistent config

setting can be set, cleared and/or queried using the FS_IOC_FS[GS]ETXATTR ioctl

(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'.



New files and directories automatically inherit FS_XFLAG_DAX from

their parent directory _when_ _created_.  Therefore, setting FS_XFLAG_DAX at

directory creation time can be used to set a default behavior for an entire

sub-tree.



To clarify inheritance, here are 3 examples:



Example A:



mkdir -p a/b/c

xfs_io -c 'chattr +x' a

mkdir a/b/c/d

mkdir a/e



	dax: a,e

	no dax: b,c,d



Example B:



mkdir a

xfs_io -c 'chattr +x' a

mkdir -p a/b/c/d



	dax: a,b,c,d

	no dax:



Example C:



mkdir -p a/b/c

xfs_io -c 'chattr +x' c

mkdir a/b/c/d



	dax: c,d

	no dax: a,b





The current enabled state (S_DAX) is set when a file inode is instantiated in

memory by the kernel.  It is set based on the underlying media support, the

value of FS_XFLAG_DAX and the filesystem's dax mount option.



statx can be used to query S_DAX.  NOTE that only regular files will ever have

S_DAX set and therefore statx will never indicate that S_DAX is set on

directories.



Setting the FS_XFLAG_DAX flag (specifically or through inheritance) occurs even

if the underlying media does not support dax and/or the filesystem is

overridden with a mount option.







Implementation Tips for Block Driver Writers

drivers/block/loop.c

+3 −3
Original line number Diff line number Diff line
@@ -644,7 +644,7 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq) 


static inline void loop_update_dio(struct loop_device *lo)

{

	__loop_update_dio(lo, io_is_direct(lo->lo_backing_file) |

	__loop_update_dio(lo, (lo->lo_backing_file->f_flags & O_DIRECT) |

				lo->use_dio);

}
@@ -1149,7 +1149,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode, 


	if (config->block_size)

		bsize = config->block_size;

	else if (io_is_direct(lo->lo_backing_file) && inode->i_sb->s_bdev)

	else if ((lo->lo_backing_file->f_flags & O_DIRECT) && inode->i_sb->s_bdev)

		/* In case of direct I/O, match underlying block size */

		bsize = bdev_logical_block_size(inode->i_sb->s_bdev);

	else

fs/stat.c

+3 −0
Original line number Diff line number Diff line
@@ -80,6 +80,9 @@ 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);

include/linux/fs.h

+1 −6
Original line number Diff line number Diff line
@@ -3413,11 +3413,6 @@ extern void setattr_copy(struct inode *inode, const struct iattr *attr); 


extern int file_update_time(struct file *file);



static inline bool io_is_direct(struct file *filp)

{

	return (filp->f_flags & O_DIRECT) || IS_DAX(filp->f_mapping->host);

}



static inline bool vma_is_dax(const struct vm_area_struct *vma)

{

	return vma->vm_file && IS_DAX(vma->vm_file->f_mapping->host);
@@ -3442,7 +3437,7 @@ static inline int iocb_flags(struct file *file) 
	int res = 0;

	if (file->f_flags & O_APPEND)

		res |= IOCB_APPEND;

	if (io_is_direct(file))

	if (file->f_flags & O_DIRECT)

		res |= IOCB_DIRECT;

	if ((file->f_flags & O_DSYNC) || IS_SYNC(file->f_mapping->host))

		res |= IOCB_DSYNC;

include/uapi/linux/stat.h

+1 −0
Original line number Diff line number Diff line
@@ -183,6 +183,7 @@ struct statx { 
#define STATX_ATTR_AUTOMOUNT		0x00001000 /* Dir: Automount trigger */

#define STATX_ATTR_MOUNT_ROOT		0x00002000 /* Root of a mount */

#define STATX_ATTR_VERITY		0x00100000 /* [I] Verity protected file */

#define STATX_ATTR_DAX			0x00002000 /* [I] File is DAX */





#endif /* _UAPI_LINUX_STAT_H */

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