Re: [PATCH 01/18] btrfs: create a mount option for dax

["Darrick J. Wong" <darrick.wong@oracle.com>]

[add Ted to the thread]

On Mon, Apr 29, 2019 at 12:26:32PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> This sets S_DAX in inode->i_flags, which can be used with
> IS_DAX().
> 
> The dax option is restricted to non multi-device mounts.
> dax interacts with the device directly instead of using bio, so
> all bio-hooks which we use for multi-device cannot be performed
> here. While regular read/writes could be manipulated with
> RAID0/1, mmap() is still an issue.
> 
> Auto-setting free space tree, because dealing with free space
> inode (specifically readpages) is a nightmare.
> Auto-setting nodatasum because we don't get callback for writing
> checksums after mmap()s.
> Deny compression because it does not go with direct I/O.
> 
> Store the dax_device in fs_info which will be used in iomap code.
> 
> I am aware of the push to directory-based flags for dax. Until, that
> code is in the kernel, we will work with mount flags.

Hmm.  This patchset was sent before LSFMM, and I've heard[1] that the
discussion there yielded some progress on how to move forward with the
user interface.  I've gotten the impression that means no new dax mount
options; a persistent flag that can be inherited by new files; and some
other means for userspace to check if writethrough worked.

However, the LWN article says Ted planned to summarize for fsdevel so
let's table this part until he does that.  Ted? :)

--D

[1] https://lwn.net/SubscriberLink/787973/ad85537bf8747e90/

> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/btrfs/ctree.h   |  2 ++
>  fs/btrfs/disk-io.c |  4 ++++
>  fs/btrfs/ioctl.c   |  5 ++++-
>  fs/btrfs/super.c   | 30 ++++++++++++++++++++++++++++++
>  4 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index b3642367a595..8ca1c0d120f4 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1067,6 +1067,7 @@ struct btrfs_fs_info {
>  	u32 metadata_ratio;
>  
>  	void *bdev_holder;
> +	struct dax_device *dax_dev;
>  
>  	/* private scrub information */
>  	struct mutex scrub_lock;
> @@ -1442,6 +1443,7 @@ static inline u32 BTRFS_MAX_XATTR_SIZE(const struct btrfs_fs_info *info)
>  #define BTRFS_MOUNT_FREE_SPACE_TREE	(1 << 26)
>  #define BTRFS_MOUNT_NOLOGREPLAY		(1 << 27)
>  #define BTRFS_MOUNT_REF_VERIFY		(1 << 28)
> +#define BTRFS_MOUNT_DAX			(1 << 29)
>  
>  #define BTRFS_DEFAULT_COMMIT_INTERVAL	(30)
>  #define BTRFS_DEFAULT_MAX_INLINE	(2048)
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 6fe9197f6ee4..2bbb63b2fcff 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -16,6 +16,7 @@
>  #include <linux/uuid.h>
>  #include <linux/semaphore.h>
>  #include <linux/error-injection.h>
> +#include <linux/dax.h>
>  #include <linux/crc32c.h>
>  #include <linux/sched/mm.h>
>  #include <asm/unaligned.h>
> @@ -2805,6 +2806,8 @@ int open_ctree(struct super_block *sb,
>  		goto fail_alloc;
>  	}
>  
> +	fs_info->dax_dev = fs_dax_get_by_bdev(fs_devices->latest_bdev);
> +
>  	/*
>  	 * We want to check superblock checksum, the type is stored inside.
>  	 * Pass the whole disk block of size BTRFS_SUPER_INFO_SIZE (4k).
> @@ -4043,6 +4046,7 @@ void close_ctree(struct btrfs_fs_info *fs_info)
>  #endif
>  
>  	btrfs_close_devices(fs_info->fs_devices);
> +	fs_put_dax(fs_info->dax_dev);
>  	btrfs_mapping_tree_free(&fs_info->mapping_tree);
>  
>  	percpu_counter_destroy(&fs_info->dirty_metadata_bytes);
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index cd4e693406a0..0138119cd9a3 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -149,8 +149,11 @@ void btrfs_sync_inode_flags_to_i_flags(struct inode *inode)
>  	if (binode->flags & BTRFS_INODE_DIRSYNC)
>  		new_fl |= S_DIRSYNC;
>  
> +	if ((btrfs_test_opt(btrfs_sb(inode->i_sb), DAX)) && S_ISREG(inode->i_mode))
> +		new_fl |= S_DAX;
> +
>  	set_mask_bits(&inode->i_flags,
> -		      S_SYNC | S_APPEND | S_IMMUTABLE | S_NOATIME | S_DIRSYNC,
> +		      S_SYNC | S_APPEND | S_IMMUTABLE | S_NOATIME | S_DIRSYNC | S_DAX,
>  		      new_fl);
>  }
>  
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 120e4340792a..3b85e61e5182 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -326,6 +326,7 @@ enum {
>  	Opt_treelog, Opt_notreelog,
>  	Opt_usebackuproot,
>  	Opt_user_subvol_rm_allowed,
> +	Opt_dax,
>  
>  	/* Deprecated options */
>  	Opt_alloc_start,
> @@ -393,6 +394,7 @@ static const match_table_t tokens = {
>  	{Opt_notreelog, "notreelog"},
>  	{Opt_usebackuproot, "usebackuproot"},
>  	{Opt_user_subvol_rm_allowed, "user_subvol_rm_allowed"},
> +	{Opt_dax, "dax"},
>  
>  	/* Deprecated options */
>  	{Opt_alloc_start, "alloc_start=%s"},
> @@ -745,6 +747,32 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
>  		case Opt_user_subvol_rm_allowed:
>  			btrfs_set_opt(info->mount_opt, USER_SUBVOL_RM_ALLOWED);
>  			break;
> +		case Opt_dax:
> +#ifdef CONFIG_FS_DAX
> +			if (btrfs_super_num_devices(info->super_copy) > 1) {
> +				btrfs_info(info,
> +					   "dax not supported for multi-device btrfs partition\n");
> +				ret = -EOPNOTSUPP;
> +				goto out;
> +			}
> +			btrfs_set_opt(info->mount_opt, DAX);
> +			btrfs_warn(info, "DAX enabled. Warning: EXPERIMENTAL, use at your own risk\n");
> +			btrfs_set_and_info(info, NODATASUM,
> +					   "auto-setting nodatasum (dax)");
> +			btrfs_clear_opt(info->mount_opt, SPACE_CACHE);
> +			btrfs_set_and_info(info, FREE_SPACE_TREE,
> +					"auto-setting free space tree (dax)");
> +			if (btrfs_test_opt(info, COMPRESS)) {
> +				btrfs_info(info, "disabling compress (dax)");
> +				btrfs_clear_opt(info->mount_opt, COMPRESS);
> +			}
> +			break;
> +#else
> +			btrfs_err(info,
> +				  "DAX option not supported\n");
> +			ret = -EINVAL;
> +			goto out;
> +#endif
>  		case Opt_enospc_debug:
>  			btrfs_set_opt(info->mount_opt, ENOSPC_DEBUG);
>  			break;
> @@ -1335,6 +1363,8 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
>  		seq_puts(seq, ",clear_cache");
>  	if (btrfs_test_opt(info, USER_SUBVOL_RM_ALLOWED))
>  		seq_puts(seq, ",user_subvol_rm_allowed");
> +	if (btrfs_test_opt(info, DAX))
> +		seq_puts(seq, ",dax");
>  	if (btrfs_test_opt(info, ENOSPC_DEBUG))
>  		seq_puts(seq, ",enospc_debug");
>  	if (btrfs_test_opt(info, AUTO_DEFRAG))
> -- 
> 2.16.4
>