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

To: Dave Chinner <david@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
Subject: Re: [PATCH 8/8] xfs: add initial DAX support
From: Boaz Harrosh <boaz@xxxxxxxxxxxxx>
Date: Tue, 24 Mar 2015 14:52:48 +0200
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, willy@xxxxxxxxxxxxxxx, jack@xxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1427194266-2885-9-git-send-email-david@xxxxxxxxxxxxx>
References: <1427194266-2885-1-git-send-email-david@xxxxxxxxxxxxx> <1427194266-2885-9-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0
On 03/24/2015 12:51 PM, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> ---
>  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

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