[PATCH RFC 5/6] fs: xfs: iomap atomic write support

[John Garry <john.g.garry@xxxxxxxxxx>]

Ensure that when creating a mapping that we adhere to all the atomic
write rules.

We check that the mapping covers the complete range of the write to ensure
that we'll be just creating a single mapping.

Currently minimum granularity is the FS block size, but it should be
possibly to support lower in future.

Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
I am setting this as an RFC as I am not sure on the change in
xfs_iomap_write_direct() - it gives the desired result AFAICS.

 fs/xfs/xfs_iomap.c | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 18c8f168b153..758dc1c90a42 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -289,6 +289,9 @@ xfs_iomap_write_direct(
 		}
 	}
 
+	if (xfs_inode_atomicwrites(ip))
+		bmapi_flags = XFS_BMAPI_ZERO;
+
 	error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write, dblocks,
 			rblocks, force, &tp);
 	if (error)
@@ -812,6 +815,44 @@ xfs_direct_write_iomap_begin(
 	if (error)
 		goto out_unlock;
 
+	if (flags & IOMAP_ATOMIC) {
+		xfs_filblks_t unit_min_fsb, unit_max_fsb;
+		unsigned int unit_min, unit_max;
+
+		xfs_get_atomic_write_attr(ip, &unit_min, &unit_max);
+		unit_min_fsb = XFS_B_TO_FSBT(mp, unit_min);
+		unit_max_fsb = XFS_B_TO_FSBT(mp, unit_max);
+
+		if (!imap_spans_range(&imap, offset_fsb, end_fsb)) {
+			error = -EINVAL;
+			goto out_unlock;
+		}
+
+		if ((offset & mp->m_blockmask) ||
+		    (length & mp->m_blockmask)) {
+			error = -EINVAL;
+			goto out_unlock;
+		}
+
+		if (imap.br_blockcount == unit_min_fsb ||
+		    imap.br_blockcount == unit_max_fsb) {
+			/* ok if exactly min or max */
+		} else if (imap.br_blockcount < unit_min_fsb ||
+			   imap.br_blockcount > unit_max_fsb) {
+			error = -EINVAL;
+			goto out_unlock;
+		} else if (!is_power_of_2(imap.br_blockcount)) {
+			error = -EINVAL;
+			goto out_unlock;
+		}
+
+		if (imap.br_startoff &&
+		    imap.br_startoff & (imap.br_blockcount - 1)) {
+			error = -EINVAL;
+			goto out_unlock;
+		}
+	}
+
 	if (imap_needs_cow(ip, flags, &imap, nimaps)) {
 		error = -EAGAIN;
 		if (flags & IOMAP_NOWAIT)
-- 
2.31.1