Re: [PATCH v5 20/24] xfs: disable direct read path for fs-verity files

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

On Tue, Mar 12, 2024 at 01:02:52PM +0100, Andrey Albershteyn wrote:
> On 2024-03-07 14:11:08, Darrick J. Wong wrote:
> > On Mon, Mar 04, 2024 at 08:10:43PM +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 | 15 ++++++++++++---
> > >  1 file changed, 12 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > > index 17404c2e7e31..af3201075066 100644
> > > --- a/fs/xfs/xfs_file.c
> > > +++ b/fs/xfs/xfs_file.c
> > > @@ -281,7 +281,8 @@ 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);
> > > @@ -334,10 +335,18 @@ xfs_file_read_iter(
> > >  
> > >  	if (IS_DAX(inode))
> > >  		ret = xfs_file_dax_read(iocb, to);
> > > -	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 {
> > 
> > I think the earlier cases need curly braces {} too.
> > 
> > > +		/*
> > > +		 * 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 (see
> > > +		 * generic_file_read_iter())
> > > +		 */
> > > +		iocb->ki_flags &= ~IOCB_DIRECT;
> > 
> > I'm curious that you added this flag here; how have we gotten along
> > this far without clearing it?
> > 
> > --D
> 
> Do you know any better place? Not sure if that should be somewhere
> before. I've made it same as ext4 does it.

It's not the placement that I'm wondering about, it's /only/ the
clearing of IOCB_DIRECT.

Notice how directio writes (xfs_file_write_iter) fall back to buffered
without clearing that flag:


	if (iocb->ki_flags & IOCB_DIRECT) {
		/*
		 * Allow a directio write to fall back to a buffered
		 * write *only* in the case that we're doing a reflink
		 * CoW.  In all other directio scenarios we do not
		 * allow an operation to fall back to buffered mode.
		 */
		ret = xfs_file_dio_write(iocb, from);
		if (ret != -ENOTBLK)
			return ret;
	}

	return xfs_file_buffered_write(iocb, from);

Or is the problem here that generic_file_read_iter will trip over it?
Oh, haha, yes it will.

Maybe we should call filemap_read directly then?  But I guess this
(clearing IOCB_DIRECT) is fine the way it is.

--D

> -- 
> - Andrey
> 
>