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

[Christian Brauner <brauner@kernel.org>]

On Thu, Mar 28, 2024 at 12:18:06PM +1100, Dave Chinner wrote:
> 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...

Yeah, I know but since your current implementation raises them for both
I just did it 1:1:

STATIC int
xfs_file_open(
        struct inode    *inode,
        struct file     *file)
{
        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;
        return generic_file_open(inode, file);
}

STATIC int
xfs_dir_open(
        struct inode    *inode,
        struct file     *file)
{
        struct xfs_inode *ip = XFS_I(inode);
        unsigned int    mode;
        int             error;

        error = xfs_file_open(inode, file);
        if (error)
                return error;

	<snip>
}

> 
> ....
> 
> > @@ -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