Re: [PATCH 1/4] fs: replace FOP_DIO_PARALLEL_WRITE with a fmode bits

["Nirjhar Roy (IBM)" <nirjhar.roy.lists@gmail.com>]

On Wed, 2025-10-29 at 08:15 +0100, Christoph Hellwig wrote:
> To properly handle the direct to buffered I/O fallback for devices that
> require stable writes, we need to be able to set the DIO_PARALLEL_WRITE
> on a per-file basis and no statically for a given file_operations
> instance.
So, is the fallback configurable(like we can turn it on/off)? Looking at the code it seems like it
is not. Any reason for not making it configurable?
--NR
> 
> This effectively reverts a part of 210a03c9d51a ("fs: claw back a few
> FMODE_* bits").
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/ext4/file.c      | 2 +-
>  fs/xfs/xfs_file.c   | 4 ++--
>  include/linux/fs.h  | 7 ++-----
>  io_uring/io_uring.c | 2 +-
>  4 files changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 7a8b30932189..b484e98b9c78 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -924,6 +924,7 @@ static int ext4_file_open(struct inode *inode, struct file *filp)
>  		filp->f_mode |= FMODE_CAN_ATOMIC_WRITE;
>  
>  	filp->f_mode |= FMODE_NOWAIT | FMODE_CAN_ODIRECT;
> +	filp->f_mode |= FMODE_DIO_PARALLEL_WRITE;
>  	return dquot_file_open(inode, filp);
>  }
>  
> @@ -978,7 +979,6 @@ const struct file_operations ext4_file_operations = {
>  	.splice_write	= iter_file_splice_write,
>  	.fallocate	= ext4_fallocate,
>  	.fop_flags	= FOP_MMAP_SYNC | FOP_BUFFER_RASYNC |
> -			  FOP_DIO_PARALLEL_WRITE |
>  			  FOP_DONTCACHE,
>  };
>  
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 2702fef2c90c..5703b6681b1d 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1553,6 +1553,7 @@ xfs_file_open(
>  	if (xfs_is_shutdown(XFS_M(inode->i_sb)))
>  		return -EIO;
>  	file->f_mode |= FMODE_NOWAIT | FMODE_CAN_ODIRECT;
> +	file->f_mode |= FMODE_DIO_PARALLEL_WRITE;
>  	if (xfs_get_atomic_write_min(XFS_I(inode)) > 0)
>  		file->f_mode |= FMODE_CAN_ATOMIC_WRITE;
>  	return generic_file_open(inode, file);
> @@ -1951,8 +1952,7 @@ const struct file_operations xfs_file_operations = {
>  	.fadvise	= xfs_file_fadvise,
>  	.remap_file_range = xfs_file_remap_range,
>  	.fop_flags	= FOP_MMAP_SYNC | FOP_BUFFER_RASYNC |
> -			  FOP_BUFFER_WASYNC | FOP_DIO_PARALLEL_WRITE |
> -			  FOP_DONTCACHE,
> +			  FOP_BUFFER_WASYNC | FOP_DONTCACHE,
>  };
>  
>  const struct file_operations xfs_dir_file_operations = {
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index c895146c1444..09b47effc55e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -128,9 +128,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
>  #define FMODE_WRITE_RESTRICTED	((__force fmode_t)(1 << 6))
>  /* File supports atomic writes */
>  #define FMODE_CAN_ATOMIC_WRITE	((__force fmode_t)(1 << 7))
> -
> -/* FMODE_* bit 8 */
> -
> +/* Supports non-exclusive O_DIRECT writes from multiple threads */
> +#define FMODE_DIO_PARALLEL_WRITE ((__force fmode_t)(1 << 8))
>  /* 32bit hashes as llseek() offset (for directories) */
>  #define FMODE_32BITHASH         ((__force fmode_t)(1 << 9))
>  /* 64bit hashes as llseek() offset (for directories) */
> @@ -2317,8 +2316,6 @@ struct file_operations {
>  #define FOP_BUFFER_WASYNC	((__force fop_flags_t)(1 << 1))
>  /* Supports synchronous page faults for mappings */
>  #define FOP_MMAP_SYNC		((__force fop_flags_t)(1 << 2))
> -/* Supports non-exclusive O_DIRECT writes from multiple threads */
> -#define FOP_DIO_PARALLEL_WRITE	((__force fop_flags_t)(1 << 3))
>  /* Contains huge pages */
>  #define FOP_HUGE_PAGES		((__force fop_flags_t)(1 << 4))
>  /* Treat loff_t as unsigned (e.g., /dev/mem) */
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 296667ba712c..668937da27e8 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -469,7 +469,7 @@ static void io_prep_async_work(struct io_kiocb *req)
>  
>  		/* don't serialize this request if the fs doesn't need it */
>  		if (should_hash && (req->file->f_flags & O_DIRECT) &&
> -		    (req->file->f_op->fop_flags & FOP_DIO_PARALLEL_WRITE))
> +		    (req->file->f_mode & FMODE_DIO_PARALLEL_WRITE))
>  			should_hash = false;
>  		if (should_hash || (ctx->flags & IORING_SETUP_IOPOLL))
>  			io_wq_hash_work(&req->work, file_inode(req->file));