Re: [PATCH v2 1/4] ext4: Add statx support for atomic writes

[John Garry <john.g.garry@oracle.com>]

On 27/10/2024 18:17, Ritesh Harjani (IBM) wrote:
> This patch adds base support for atomic writes via statx getattr.
> On bs < ps systems, we can create FS with say bs of 16k. That means
> both atomic write min and max unit can be set to 16k for supporting
> atomic writes.
> 
> Co-developed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
>   fs/ext4/ext4.h  |  9 +++++++++
>   fs/ext4/inode.c | 14 ++++++++++++++
>   fs/ext4/super.c | 31 +++++++++++++++++++++++++++++++
>   3 files changed, 54 insertions(+)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 44b0d418143c..6ee49aaacd2b 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1729,6 +1729,10 @@ struct ext4_sb_info {
>   	 */
>   	struct work_struct s_sb_upd_work;
>   
> +	/* Atomic write unit values in bytes */
> +	unsigned int s_awu_min;
> +	unsigned int s_awu_max;
> +
>   	/* Ext4 fast commit sub transaction ID */
>   	atomic_t s_fc_subtid;
>   
> @@ -3855,6 +3859,11 @@ static inline int ext4_buffer_uptodate(struct buffer_head *bh)
>   	return buffer_uptodate(bh);
>   }
>   
> +static inline bool ext4_can_atomic_write(struct super_block *sb)
> +{
> +	return EXT4_SB(sb)->s_awu_min > 0;
> +}
> +
>   extern int ext4_block_write_begin(handle_t *handle, struct folio *folio,
>   				  loff_t pos, unsigned len,
>   				  get_block_t *get_block);
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 54bdd4884fe6..fcdee27b9aa2 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5578,6 +5578,20 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
>   		}
>   	}
>   
> +	if (S_ISREG(inode->i_mode) && (request_mask & STATX_WRITE_ATOMIC)) {
> +		struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> +		unsigned int awu_min, awu_max;
> +
> +		if (ext4_can_atomic_write(inode->i_sb)) {
> +			awu_min = sbi->s_awu_min;
> +			awu_max = sbi->s_awu_max;
> +		} else {
> +			awu_min = awu_max = 0;
> +		}
> +
> +		generic_fill_statx_atomic_writes(stat, awu_min, awu_max);
> +	}
> +
>   	flags = ei->i_flags & EXT4_FL_USER_VISIBLE;
>   	if (flags & EXT4_APPEND_FL)
>   		stat->attributes |= STATX_ATTR_APPEND;
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 16a4ce704460..d6e3201a48be 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4425,6 +4425,36 @@ static int ext4_handle_clustersize(struct super_block *sb)
>   	return 0;
>   }
>   
> +/*
> + * ext4_atomic_write_init: Initializes filesystem min & max atomic write units.
> + * @sb: super block
> + * TODO: Later add support for bigalloc
> + */
> +static void ext4_atomic_write_init(struct super_block *sb)
> +{
> +	struct ext4_sb_info *sbi = EXT4_SB(sb);
> +	struct block_device *bdev = sb->s_bdev;
> +
> +	if (!bdev_can_atomic_write(bdev))

this check is duplicated, since bdev_atomic_write_unit_{min, 
max}_bytes() has this check

> +		return;
> +
> +	if (!ext4_has_feature_extents(sb))
> +		return;
> +
> +	sbi->s_awu_min = max(sb->s_blocksize,
> +			      bdev_atomic_write_unit_min_bytes(bdev));
> +	sbi->s_awu_max = min(sb->s_blocksize,
> +			      bdev_atomic_write_unit_max_bytes(bdev));
> +	if (sbi->s_awu_min && sbi->s_awu_max &&
> +	    sbi->s_awu_min <= sbi->s_awu_max) {

This looks a bit complicated. I would just follow the XFS example and 
ensure bdev_atomic_write_unit_min_bytes() <=  sb->s_blocksize <= 
bdev_atomic_write_unit_max_bytes() [which you are doing, but in a 
complicated way]

> +		ext4_msg(sb, KERN_NOTICE, "Supports atomic writes awu_min: %u, awu_max: %u",
> +			 sbi->s_awu_min, sbi->s_awu_max);
> +	} else {
> +		sbi->s_awu_min = 0;
> +		sbi->s_awu_max = 0;
> +	}
> +}
> +
>   static void ext4_fast_commit_init(struct super_block *sb)
>   {
>   	struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -5336,6 +5366,7 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
>   
>   	spin_lock_init(&sbi->s_bdev_wb_lock);
>   
> +	ext4_atomic_write_init(sb);
>   	ext4_fast_commit_init(sb);
>   
>   	sb->s_root = NULL;