Re: [PATCH v6 6/7] xfs: Validate atomic writes

[John Garry]

On 30/09/2024 17:41, Darrick J. Wong wrote:
> On Mon, Sep 30, 2024 at 12:54:37PM +0000, John Garry wrote:
> > Validate that an atomic write adheres to length/offset rules. Currently
> > we can only write a single FS block.
> >
> > For an IOCB with IOCB_ATOMIC set to get as far as xfs_file_dio_write(),
> > FMODE_CAN_ATOMIC_WRITE will need to be set for the file; for this,
> > ATOMICWRITES flags would also need to be set for the inode.
> >
> > Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> > ---
> >   fs/xfs/xfs_file.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 412b1d71b52b..fa6a44b88ecc 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -688,6 +688,13 @@ xfs_file_dio_write(
> >   	struct xfs_buftarg      *target = xfs_inode_buftarg(ip);
> >   	size_t			count = iov_iter_count(from);
> >   
> > +	if (iocb->ki_flags & IOCB_ATOMIC) {
> > +		if (count != ip->i_mount->m_sb.sb_blocksize)
> > +			return -EINVAL;
> > +		if (!generic_atomic_write_valid(iocb, from))
> > +			return -EINVAL;
> > +	}
>
> Does xfs_file_write_iter need a catch-all so that we don't fall back to
> buffered write for a directio write that returns ENOTBLK?
>
> 	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 || (iocb->ki_flags & IOCB_ATOMIC))
> 			return ret;
> 	}
>
> IIRC iomap_dio_rw can return ENOTBLK if pagecache invalidation fails for
> the region that we're trying to directio write.

I see where you are talking about. There is also a ENOTBLK from 
unaligned write for CoW, but we would not see that.

But I was thinking to use a common helper to catch this, like 
generic_write_checks_count() [which is called on the buffered IO path]:

----8<-----

diff --git a/fs/read_write.c b/fs/read_write.c
index 32b476bf9be0..222f25c6439c 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1774,6 +1774,10 @@ int generic_write_checks_count(struct kiocb 
*iocb, loff_t *count)
 	if (!*count)
 		return 0;

+	if (iocb->ki_flags & IOCB_ATOMIC &&
+	    !(iocb->ki_flags & IOCB_DIRECT))
+		return -EINVAL;
+
 	if (iocb->ki_flags & IOCB_APPEND)
 		iocb->ki_pos = i_size_read(inode);

---->8-----

But we keep the IOCB_DIRECT flag for the buffered IO fallback (so no good).

Another option would be:

----8<-----

--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -679,7 +679,12 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter 
*iter,
 			if (ret != -EAGAIN) {
 				trace_iomap_dio_invalidate_fail(inode, iomi.pos,
 								iomi.len);
-				ret = -ENOTBLK;
+				if (iocb->ki_flags & IOCB_ATOMIC) {
+					if (ret == -ENOTBLK)
+						ret = -EAGAIN;
+				}else {
+					ret = -ENOTBLK;
+				}
 			}
 			goto out_free_dio;
 		}
---->8-----

I suggest that, as other FSes (like ext4) handle -ENOTBLK and would need 
to be changed similar to XFS. But I am not sure if changing the error 
code from -ENOTBLK for IOCB_ATOMIC is ok.

Let me know what you think about possible alternative solutions.

Thanks,
John