Re: [PATCH] [RFC]: fs: claw back a few FMODE_* bits

[Dave Chinner <david@fromorbit.com>]

On Wed, Mar 27, 2024 at 05:45:09PM +0100, Christian Brauner wrote:
> There's a bunch of flags that are purely based on what the file
> operations support while also never being conditionally set or unset.
> IOW, they're not subject to change for individual file opens. Imho, such
> flags don't need to live in f_mode they might as well live in the fops
> structs itself. And the fops struct already has that lonely
> mmap_supported_flags member. We might as well turn that into a generic
> fops_flags member and move a few flags from FMODE_* space into FOP_*
> space. That gets us four FMODE_* bits back and the ability for new
> static flags that are about file ops to not have to live in FMODE_*
> space but in their own FOP_* space. It's not the most beautiful thing
> ever but it gets the job done. Yes, there'll be an additional pointer
> chase but hopefully that won't matter for these flags.
> 
> If this is palatable I suspect there's a few more we can move into there
> and that we can also redirect new flag suggestions that follow this
> pattern into the fops_flags field instead of f_mode. As of yet untested.
> 
> (Fwiw, FMODE_NOACCOUNT and FMODE_BACKING could live in fops_flags as
>  well because they're also completely static but they aren't really
>  about file operations so they're better suited for FMODE_* imho.)
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
.....
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 632653e00906..d13e21eb9a3c 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1230,8 +1230,7 @@ xfs_file_open(
>  {
>  	if (xfs_is_shutdown(XFS_M(inode->i_sb)))
>  		return -EIO;
> -	file->f_mode |= FMODE_NOWAIT | FMODE_BUF_RASYNC | FMODE_BUF_WASYNC |
> -			FMODE_DIO_PARALLEL_WRITE | FMODE_CAN_ODIRECT;
> +	file->f_mode |= FMODE_NOWAIT | FMODE_CAN_ODIRECT;
>  	return generic_file_open(inode, file);
>  }
>  
> @@ -1490,7 +1489,6 @@ const struct file_operations xfs_file_operations = {
>  	.compat_ioctl	= xfs_file_compat_ioctl,
>  #endif
>  	.mmap		= xfs_file_mmap,
> -	.mmap_supported_flags = MAP_SYNC,
>  	.open		= xfs_file_open,
>  	.release	= xfs_file_release,
>  	.fsync		= xfs_file_fsync,
> @@ -1498,6 +1496,8 @@ const struct file_operations xfs_file_operations = {
>  	.fallocate	= xfs_file_fallocate,
>  	.fadvise	= xfs_file_fadvise,
>  	.remap_file_range = xfs_file_remap_range,
> +	.fops_flags	= FOP_MMAP_SYNC | FOP_BUF_RASYNC | FOP_BUF_WASYNC |
> +			  FOP_DIO_PARALLEL_WRITE,
>  };
>  
>  const struct file_operations xfs_dir_file_operations = {
> @@ -1510,4 +1510,6 @@ const struct file_operations xfs_dir_file_operations = {
>  	.compat_ioctl	= xfs_file_compat_ioctl,
>  #endif
>  	.fsync		= xfs_dir_fsync,
> +	.fops_flags	= FOP_MMAP_SYNC | FOP_BUF_RASYNC | FOP_BUF_WASYNC |
> +			  FOP_DIO_PARALLEL_WRITE,
>  };

Why do we need to set any of these for directory operations now that
we have a clear choice? i.e. we can't mmap directories, and the rest
of these flags are for read() and write() operations which we also
can't do on directories...

....

> @@ -1024,7 +1024,7 @@ int io_write(struct io_kiocb *req, unsigned int issue_flags)
>  
>  		/* File path supports NOWAIT for non-direct_IO only for block devices. */
>  		if (!(kiocb->ki_flags & IOCB_DIRECT) &&
> -			!(kiocb->ki_filp->f_mode & FMODE_BUF_WASYNC) &&
> +			!fops_buf_wasync(kiocb->ki_filp) &&
>  			(req->flags & REQ_F_ISREG))
>  			goto copy_iov;

You should probably also fix that comment - WASYNC is set when the
filesystem supports NOWAIT for buffered writes.

-Dave.
-- 
Dave Chinner
david@fromorbit.com