Re: [PATCH v3 3/3] shmem: stable directory offsets

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 6/6/23 15:11, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> The current cursor-based directory offset mechanism doesn't work
> when a tmpfs filesystem is exported via NFS. This is because NFS
> clients do not open directories. Each server-side READDIR operation
> has to open the directory, read it, then close it. The cursor state
> for that directory, being associated strictly with the opened
> struct file, is thus discarded after each NFS READDIR operation.
> 
> Directory offsets are cached not only by NFS clients, but also by
> user space libraries on those clients. Essentially there is no way
> to invalidate those caches when directory offsets have changed on
> an NFS server after the offset-to-dentry mapping changes. Thus the
> whole application stack depends on unchanging directory offsets.
> 
> The solution we've come up with is to make the directory offset for
> each file in a tmpfs filesystem stable for the life of the directory
> entry it represents.
> 
> shmem_readdir() and shmem_dir_llseek() now use an xarray to map each
> directory offset (an loff_t integer) to the memory address of a
> struct dentry.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>   mm/shmem.c |   39 +++++++++++++++++++++++++++++++++++----
>   1 file changed, 35 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 721f9fd064aa..fd9571056181 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2410,7 +2410,8 @@ static struct inode *shmem_get_inode(struct mnt_idmap *idmap, struct super_block
>   			/* Some things misbehave if size == 0 on a directory */
>   			inode->i_size = 2 * BOGO_DIRENT_SIZE;
>   			inode->i_op = &shmem_dir_inode_operations;
> -			inode->i_fop = &simple_dir_operations;
> +			inode->i_fop = &stable_dir_operations;
> +			stable_offset_init(inode);
>   			break;
>   		case S_IFLNK:
>   			/*
> @@ -2950,6 +2951,10 @@ shmem_mknod(struct mnt_idmap *idmap, struct inode *dir,
>   		if (error && error != -EOPNOTSUPP)
>   			goto out_iput;
>   
> +		error = stable_offset_add(dir, dentry);
> +		if (error)
> +			goto out_iput;
> +
>   		error = 0;

This line can be removed?

>   		dir->i_size += BOGO_DIRENT_SIZE;
>   		dir->i_ctime = dir->i_mtime = current_time(dir);
> @@ -3027,6 +3032,10 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
>   			goto out;
>   	}
>   
> +	ret = stable_offset_add(dir, dentry);
> +	if (ret)
> +		goto out;
> +

I think this should call shmem_free_inode() before goto out - reverse 
what shmem_reserve_inode() has done.

>   	dir->i_size += BOGO_DIRENT_SIZE;
>   	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
>   	inode_inc_iversion(dir);
> @@ -3045,6 +3054,8 @@ static int shmem_unlink(struct inode *dir, struct dentry *dentry)
>   	if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode))
>   		shmem_free_inode(inode->i_sb);
>   
> +	stable_offset_remove(dir, dentry);
> +
>   	dir->i_size -= BOGO_DIRENT_SIZE;
>   	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
>   	inode_inc_iversion(dir);
> @@ -3103,24 +3114,37 @@ static int shmem_rename2(struct mnt_idmap *idmap,
>   {
>   	struct inode *inode = d_inode(old_dentry);
>   	int they_are_dirs = S_ISDIR(inode->i_mode);
> +	int error;
>   
>   	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE | RENAME_WHITEOUT))
>   		return -EINVAL;
>   
> -	if (flags & RENAME_EXCHANGE)
> +	if (flags & RENAME_EXCHANGE) {
> +		stable_offset_remove(old_dir, old_dentry);
> +		stable_offset_remove(new_dir, new_dentry);
> +		error = stable_offset_add(new_dir, old_dentry);
> +		if (error)
> +			return error;
> +		error = stable_offset_add(old_dir, new_dentry);
> +		if (error)
> +			return error;
>   		return simple_rename_exchange(old_dir, old_dentry, new_dir, new_dentry);
> +	}

Hmm, error handling issues? Everything needs to be reversed when any of 
the operations fails?

>   
>   	if (!simple_empty(new_dentry))
>   		return -ENOTEMPTY;
>   
>   	if (flags & RENAME_WHITEOUT) {
> -		int error;
> -
>   		error = shmem_whiteout(idmap, old_dir, old_dentry);
>   		if (error)
>   			return error;
>   	}
>   
> +	stable_offset_remove(old_dir, old_dentry);
> +	error = stable_offset_add(new_dir, old_dentry);
> +	if (error)
> +		return error;
> +
>   	if (d_really_is_positive(new_dentry)) {
>   		(void) shmem_unlink(new_dir, new_dentry);
>   		if (they_are_dirs) {
> @@ -3185,6 +3209,11 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
>   		folio_unlock(folio);
>   		folio_put(folio);
>   	}
> +
> +	error = stable_offset_add(dir, dentry);
> +	if (error)
> +		goto out_iput;
> +

Error handling, there is a kmemdup() above which needs to be freed? I'm 
not sure about folio, automatically released with the inode?

>   	dir->i_size += BOGO_DIRENT_SIZE;
>   	dir->i_ctime = dir->i_mtime = current_time(dir);
>   	inode_inc_iversion(dir);
> @@ -3920,6 +3949,8 @@ static void shmem_destroy_inode(struct inode *inode)
>   {
>   	if (S_ISREG(inode->i_mode))
>   		mpol_free_shared_policy(&SHMEM_I(inode)->policy);
> +	if (S_ISDIR(inode->i_mode))
> +		stable_offset_destroy(inode);
>   }
>   
>   static void shmem_init_inode(void *foo)
> 
> 

Thanks,
Bernd