Re: [PATCH 04/10] iomap: zeroing already holds invalidate_lock in iomap_file_buffered_write_punch_delalloc

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Aug 27, 2024 at 07:09:51AM +0200, Christoph Hellwig wrote:
> All callers of iomap_zero_range already hold invalidate_lock, so we can't
> take it again in iomap_file_buffered_write_punch_delalloc.

What about the xfs_zero_range call in xfs_file_write_checks?  AFAICT we
don't hold the invalidate lock there.  Did I misread that?

Also, would nested takings of the invalidate lock cause a livelock?  Or
is this actually quite broken now?

--D

> Use the passed in flags argument to detect if we're called from a zeroing
> operation and don't take the lock again in this case.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap/buffered-io.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 34de9f58794ad5..574ca413516443 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1198,8 +1198,8 @@ static int iomap_write_delalloc_scan(struct inode *inode,
>   * require sprinkling this code with magic "+ 1" and "- 1" arithmetic and expose
>   * the code to subtle off-by-one bugs....
>   */
> -static int iomap_write_delalloc_release(struct inode *inode,
> -		loff_t start_byte, loff_t end_byte, iomap_punch_t punch)
> +static int iomap_write_delalloc_release(struct inode *inode, loff_t start_byte,
> +		loff_t end_byte, unsigned flags, iomap_punch_t punch)
>  {
>  	loff_t punch_start_byte = start_byte;
>  	loff_t scan_end_byte = min(i_size_read(inode), end_byte);
> @@ -1210,8 +1210,13 @@ static int iomap_write_delalloc_release(struct inode *inode,
>  	 * folios and dirtying them via ->page_mkwrite whilst we walk the
>  	 * cache and perform delalloc extent removal. Failing to do this can
>  	 * leave dirty pages with no space reservation in the cache.
> +	 *
> +	 * For zeroing operations the callers already hold invalidate_lock.
>  	 */
> -	filemap_invalidate_lock(inode->i_mapping);
> +	if (flags & IOMAP_ZERO)
> +		rwsem_assert_held_write(&inode->i_mapping->invalidate_lock);
> +	else
> +		filemap_invalidate_lock(inode->i_mapping);
>  	while (start_byte < scan_end_byte) {
>  		loff_t		data_end;
>  
> @@ -1264,7 +1269,8 @@ static int iomap_write_delalloc_release(struct inode *inode,
>  		error = punch(inode, punch_start_byte,
>  				end_byte - punch_start_byte);
>  out_unlock:
> -	filemap_invalidate_unlock(inode->i_mapping);
> +	if (!(flags & IOMAP_ZERO))
> +		filemap_invalidate_unlock(inode->i_mapping);
>  	return error;
>  }
>  
> @@ -1328,7 +1334,7 @@ int iomap_file_buffered_write_punch_delalloc(struct inode *inode,
>  	if (start_byte >= end_byte)
>  		return 0;
>  
> -	return iomap_write_delalloc_release(inode, start_byte, end_byte,
> +	return iomap_write_delalloc_release(inode, start_byte, end_byte, flags,
>  					punch);
>  }
>  EXPORT_SYMBOL_GPL(iomap_file_buffered_write_punch_delalloc);
> -- 
> 2.43.0
> 
>