linux-kernel - Re: [PATCH RFC 3/7] libfs: Add simple_offset

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Date: Thu, 15 Feb 2024 13:53:34 +0100
From: Jan Kara <jack@...e.cz>
To: Chuck Lever <cel@...nel.org>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	hughd@...gle.com, akpm@...ux-foundation.org,
	Liam.Howlett@...cle.com, oliver.sang@...el.com, feng.tang@...el.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	maple-tree@...ts.infradead.org, linux-mm@...ck.org, lkp@...el.com
Subject: Re: [PATCH RFC 3/7] libfs: Add simple_offset_empty()

On Tue 13-02-24 16:37:39, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@...cle.com>
> 
> For simple filesystems that use directory offset mapping, rely
> strictly on the directory offset map to tell when a directory has
> no children.
> 
> After this patch is applied, the emptiness test holds only the RCU
> read lock when the directory being tested has no children.
> 
> In addition, this adds another layer of confirmation that
> simple_offset_add/remove() are working as expected.
> 
> Signed-off-by: Chuck Lever <chuck.lever@...cle.com>

Makes sense. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/libfs.c         |   32 ++++++++++++++++++++++++++++++++
>  include/linux/fs.h |    1 +
>  mm/shmem.c         |    4 ++--
>  3 files changed, 35 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index a38af72f4719..3cf773950f93 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -313,6 +313,38 @@ void simple_offset_remove(struct offset_ctx *octx, struct dentry *dentry)
>  	offset_set(dentry, 0);
>  }
>  
> +/**
> + * simple_offset_empty - Check if a dentry can be unlinked
> + * @dentry: dentry to be tested
> + *
> + * Returns 0 if @dentry is a non-empty directory; otherwise returns 1.
> + */
> +int simple_offset_empty(struct dentry *dentry)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct offset_ctx *octx;
> +	struct dentry *child;
> +	unsigned long index;
> +	int ret = 1;
> +
> +	if (!inode || !S_ISDIR(inode->i_mode))
> +		return ret;
> +
> +	index = 2;
> +	octx = inode->i_op->get_offset_ctx(inode);
> +	xa_for_each(&octx->xa, index, child) {
> +		spin_lock(&child->d_lock);
> +		if (simple_positive(child)) {
> +			spin_unlock(&child->d_lock);
> +			ret = 0;
> +			break;
> +		}
> +		spin_unlock(&child->d_lock);
> +	}
> +
> +	return ret;
> +}
> +
>  /**
>   * simple_offset_rename_exchange - exchange rename with directory offsets
>   * @old_dir: parent of dentry being moved
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index ed5966a70495..03d141809a2c 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3267,6 +3267,7 @@ struct offset_ctx {
>  void simple_offset_init(struct offset_ctx *octx);
>  int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry);
>  void simple_offset_remove(struct offset_ctx *octx, struct dentry *dentry);
> +int simple_offset_empty(struct dentry *dentry);
>  int simple_offset_rename_exchange(struct inode *old_dir,
>  				  struct dentry *old_dentry,
>  				  struct inode *new_dir,
> diff --git a/mm/shmem.c b/mm/shmem.c
> index d7c84ff62186..6fed524343cb 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3374,7 +3374,7 @@ static int shmem_unlink(struct inode *dir, struct dentry *dentry)
>  
>  static int shmem_rmdir(struct inode *dir, struct dentry *dentry)
>  {
> -	if (!simple_empty(dentry))
> +	if (!simple_offset_empty(dentry))
>  		return -ENOTEMPTY;
>  
>  	drop_nlink(d_inode(dentry));
> @@ -3431,7 +3431,7 @@ static int shmem_rename2(struct mnt_idmap *idmap,
>  		return simple_offset_rename_exchange(old_dir, old_dentry,
>  						     new_dir, new_dentry);
>  
> -	if (!simple_empty(new_dentry))
> +	if (!simple_offset_empty(new_dentry))
>  		return -ENOTEMPTY;
>  
>  	if (flags & RENAME_WHITEOUT) {
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR