linux-kernel - Re: [PATCH 1/6] ext4: Add statx support for atomic writes

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Message-ID: <314835ec-98bf-472c-8be7-0b26e50cfc9b@oracle.com>
Date: Fri, 25 Oct 2024 10:41:39 +0100
From: John Garry <john.g.garry@...cle.com>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>, linux-ext4@...r.kernel.org
Cc: Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Christoph Hellwig
 <hch@...radead.org>,
        Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
        Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/6] ext4: Add statx support for atomic writes

On 25/10/2024 04:45, 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.
> 
> Later patches adds support for bigalloc as well so that ext4 can also
> support doing atomic writes for bs = ps systems.
> 
> Co-developed-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
> Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
> ---
>   fs/ext4/ext4.h  |  7 ++++++-
>   fs/ext4/inode.c | 14 ++++++++++++++
>   fs/ext4/super.c | 32 ++++++++++++++++++++++++++++++++
>   3 files changed, 52 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 44b0d418143c..a41e56c2c628 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 */
> +	unsigned int fs_awu_min;
> +	unsigned int fs_awu_max;
> +
>   	/* Ext4 fast commit sub transaction ID */
>   	atomic_t s_fc_subtid;
>   
> @@ -1820,7 +1824,8 @@ static inline int ext4_valid_inum(struct super_block *sb, unsigned long ino)
>    */
>   enum {
>   	EXT4_MF_MNTDIR_SAMPLED,
> -	EXT4_MF_FC_INELIGIBLE	/* Fast commit ineligible */
> +	EXT4_MF_FC_INELIGIBLE,	/* Fast commit ineligible */
> +	EXT4_MF_ATOMIC_WRITE	/* Supports atomic write */

Does this flag really buy us much?

>   };
>   
>   static inline void ext4_set_mount_flag(struct super_block *sb, int bit)
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 54bdd4884fe6..897c028d5bc9 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_test_mount_flag(inode->i_sb, EXT4_MF_ATOMIC_WRITE)) {

I'd use ext4_inode_can_atomicwrite() here, similar to what is done for xfs

> +			awu_min = sbi->fs_awu_min;
> +			awu_max = sbi->fs_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..f5c075aff060 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4425,6 +4425,37 @@ static int ext4_handle_clustersize(struct super_block *sb)
>   	return 0;
>   }
>   
> +/*