Re: [RFC PATCH 07/11] xfs: disable direct read path for fs-verity sealed files

[Dave Chinner <david@fromorbit.com>]

On Tue, Dec 13, 2022 at 06:29:31PM +0100, Andrey Albershteyn wrote:
> The direct path is not supported on verity files. Attempts to use direct
> I/O path on such files should fall back to buffered I/O path.
> 
> Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
> ---
>  fs/xfs/xfs_file.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 5eadd9a37c50e..fb4181e38a19d 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -245,7 +245,8 @@ xfs_file_dax_read(
>  	struct kiocb		*iocb,
>  	struct iov_iter		*to)
>  {
> -	struct xfs_inode	*ip = XFS_I(iocb->ki_filp->f_mapping->host);
> +	struct inode		*inode = iocb->ki_filp->f_mapping->host;
> +	struct xfs_inode	*ip = XFS_I(inode);
>  	ssize_t			ret = 0;
>  
>  	trace_xfs_file_dax_read(iocb, to);
> @@ -298,10 +299,17 @@ xfs_file_read_iter(
>  
>  	if (IS_DAX(inode))
>  		ret = xfs_file_dax_read(iocb, to);

fsverity is supported on DAX?

Eric, I was under the impression that the DAX io path does not
support fsverity, but I can't see anything that prevents ext4 from
using fsverity on dax enabled filesystems. Does this work (is it
tested regularly?), or is the lack of checking simply an oversight
in that nobody thought to check DAX status when fsverity is enabled?

> -	else if (iocb->ki_flags & IOCB_DIRECT)
> +	else if (iocb->ki_flags & IOCB_DIRECT && !fsverity_active(inode))
>  		ret = xfs_file_dio_read(iocb, to);
> -	else
> +	else {
> +		/*
> +		 * In case fs-verity is enabled, we also fallback to the
> +		 * buffered read from the direct read path. Therefore,
> +		 * IOCB_DIRECT is set and need to be cleared
> +		 */
> +		iocb->ki_flags &= ~IOCB_DIRECT;
>  		ret = xfs_file_buffered_read(iocb, to);
> +	}

Is this IOCB_DIRECT avoidance a limitation of the XFS
implementation, or a generic limitation of the fsverity
infrastructure?

If it's a limitation of the fsverity infrastructure, then we
shouldn't be working around this in every single filesystem that
supports fsverity.  If all the major filesystems are having to check
fsverity_active() and clear IOCB_DIRECT on every single IOCB_DIRECT
IO that is issued on a fsverity inode, then shouldn't we just elide
IOCB_DIRECT from file->f_iocb_flags in the first place?

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com