Re: [PATCH 8/8] xfs: add initial DAX support

Boaz Harrosh
2015-03-24T12:52:53Z

On 03/24/2015 12:51 PM, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Add initial DAX support to XFS. To do this we need a new mount
> option to turn DAX on filesystem, and we need to propagate thi into
> the inode flags whenever an inode is instantiated so that the
> per-inode checks throughout the code Do The Right Thing.
>
> There are still some things remaining to be done:
>
> - needs per-inode flags to mark inodes as DAX enabled, and
> an inheritance flag to enable automatic filesystem
> propagation of the property
> - fails occasionally with zero length writes instead of
> ENOSPC errors, so error propagation inside/from the DAX
> code need work
> - occasionally creates two extents rather than a single
> larger extent like non-dax filesystems.
> - much more testing
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_iops.c | 24 ++++++++++++------------
> fs/xfs/xfs_mount.h | 2 ++
> fs/xfs/xfs_super.c | 25 +++++++++++++++++++++++--
> 3 files changed, 37 insertions(+), 14 deletions(-)
>
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 9ca5352..695d857 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -1195,22 +1195,22 @@ xfs_diflags_to_iflags(
> struct inode *inode,
> struct xfs_inode *ip)
> {
> - if (ip->i_d.di_flags & XFS_DIFLAG_IMMUTABLE)
> + uint16_t flags = ip->i_d.di_flags;
> +
> + inode->i_flags &= ~(S_IMMUTABLE | S_APPEND | S_SYNC |
> + S_NOATIME | S_DAX);
> +
> + if (flags & XFS_DIFLAG_IMMUTABLE)
> inode->i_flags |= S_IMMUTABLE;
> - else
> - inode->i_flags &= ~S_IMMUTABLE;
> - if (ip->i_d.di_flags & XFS_DIFLAG_APPEND)
> + if (flags & XFS_DIFLAG_APPEND)
> inode->i_flags |= S_APPEND;
> - else
> - inode->i_flags &= ~S_APPEND;
> - if (ip->i_d.di_flags & XFS_DIFLAG_SYNC)
> + if (flags & XFS_DIFLAG_SYNC)
> inode->i_flags |= S_SYNC;
> - else
> - inode->i_flags &= ~S_SYNC;
> - if (ip->i_d.di_flags & XFS_DIFLAG_NOATIME)
> + if (flags & XFS_DIFLAG_NOATIME)
> inode->i_flags |= S_NOATIME;
> - else
> - inode->i_flags &= ~S_NOATIME;
> + /* XXX: Also needs an on-disk per inode flag! */
> + if (ip->i_mount->m_flags & XFS_MOUNT_DAX)
> + inode->i_flags |= S_DAX;
> }
>
> /*
> diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> index 8c995a2..cd44e88 100644
> --- a/fs/xfs/xfs_mount.h
> +++ b/fs/xfs/xfs_mount.h
> @@ -179,6 +179,8 @@ typedef struct xfs_mount {
> allocator */
> #define XFS_MOUNT_NOATTR2 (1ULL << 25) /* disable use of attr2 format */
>
> +#define XFS_MOUNT_DAX (1ULL << 62) /* TEST ONLY! */
> +
>
> /*
> * Default minimum read and write sizes.
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 3ad0b17..0f26d7a 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -112,6 +112,8 @@ static struct xfs_kobj xfs_dbg_kobj; /* global debug sysfs attrs */
> #define MNTOPT_DISCARD "discard" /* Discard unused blocks */
> #define MNTOPT_NODISCARD "nodiscard" /* Do not discard unused blocks */
>
> +#define MNTOPT_DAX "dax" /* Enable direct access to bdev pages */
> +
> /*
> * Table driven mount option parser.
> *
> @@ -363,6 +365,10 @@ xfs_parseargs(
> mp->m_flags |= XFS_MOUNT_DISCARD;
> } else if (!strcmp(this_char, MNTOPT_NODISCARD)) {
> mp->m_flags &= ~XFS_MOUNT_DISCARD;
> +#ifdef CONFIG_FS_DAX
> + } else if (!strcmp(this_char, MNTOPT_DAX)) {
> + mp->m_flags |= XFS_MOUNT_DAX;

Hi

So what I see, (I might be wrong), is that once this flag is set here the
fs (At above xfs_diflags_to_iflags() ) will start serving DAX inodes.

This is a problem because the bdev passed in might not support direct_access
at all.

I think we might want a dax_supported(sb) and call somewhere at mount time.

> +#endif
> } else {
> xfs_warn(mp, "unknown mount option [%s].", this_char);
> return -EINVAL;
> @@ -452,8 +458,8 @@ done:
> }
>
> struct proc_xfs_info {
> - int flag;
> - char *str;
> + uint64_t flag;
> + char *str;
> };
>
> STATIC int
> @@ -474,6 +480,7 @@ xfs_showargs(
> { XFS_MOUNT_GRPID, "," MNTOPT_GRPID },
> { XFS_MOUNT_DISCARD, "," MNTOPT_DISCARD },
> { XFS_MOUNT_SMALL_INUMS, "," MNTOPT_32BITINODE },
> + { XFS_MOUNT_DAX, "," MNTOPT_DAX },
> { 0, NULL }
> };
> static struct proc_xfs_info xfs_info_unset[] = {
> @@ -1501,6 +1508,20 @@ xfs_fs_fill_super(
> if (XFS_SB_VERSION_NUM(&mp->m_sb) == XFS_SB_VERSION_5)
> sb->s_flags |= MS_I_VERSION;
>
> + if (mp->m_flags & XFS_MOUNT_DAX) {
> + xfs_warn(mp,
> + "DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
> + if (sb->s_blocksize != PAGE_SIZE) {
> + xfs_alert(mp,
> + "Filesystem block size invalid for DAX Turning DAX off.");
> + mp->m_flags &= ~XFS_MOUNT_DAX;
> + } else if (!sb->s_bdev->bd_disk->fops->direct_access) {
> + xfs_alert(mp,
> + "Block device does not support DAX Turning DAX off.");
> + mp->m_flags &= ~XFS_MOUNT_DAX;
> + }
> + }
>

If we agree about the s_flags MS_MOUNT_DAX then we can define a
if (MNTOPT_DAX)
dax_enable_if_supported(sb);

This will try a call to bdev_direct_access(sb->s_bdev, ...) and set the
flag if everything is OK, else will leave it off.

(I can do this patch if you want)

> error = xfs_mountfs(mp);
> if (error)
> goto out_filestream_unmount;
>

Thanks
Boaz