xfs: refine atomic write size check in xfs_file_write_iter()

Currently the size of atomic write allowed is fixed at the blocksize.

To start to lift this restriction, partly refactor
xfs_report_atomic_write() to into helpers -
xfs_get_atomic_write_{min, max}() - and use those helpers to find the
per-inode atomic write limits and check according to that.

Also add xfs_get_atomic_write_max_opt() to return the optimal limit, and
just return 0 since large atomics aren't supported yet.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>

fs/xfs/xfs_file.c

@@ -1032,14 +1032,12 @@ xfs_file_write_iter(
 		return xfs_file_dax_write(iocb, from);
 
 	if (iocb->ki_flags & IOCB_ATOMIC) {
-		/*
+		if (ocount < xfs_get_atomic_write_min(ip))
-		 * Currently only atomic writing of a single FS block is
-		 * supported. It would be possible to atomic write smaller than
-		 * a FS block, but there is no requirement to support this.
-		 * Note that iomap also does not support this yet.
-		 */
-		if (ocount != ip->i_mount->m_sb.sb_blocksize)
 			return -EINVAL;
+
+		if (ocount > xfs_get_atomic_write_max(ip))
+			return -EINVAL;
+
 		ret = generic_atomic_write_valid(iocb, from);
 		if (ret)
 			return ret;

fs/xfs/xfs_iops.c

@@ -601,16 +601,42 @@ xfs_report_dioalign(
 		stat->dio_offset_align = stat->dio_read_offset_align;
 }
 
+unsigned int
+xfs_get_atomic_write_min(
+	struct xfs_inode	*ip)
+{
+	if (!xfs_inode_can_hw_atomic_write(ip))
+		return 0;
+
+	return ip->i_mount->m_sb.sb_blocksize;
+}
+
+unsigned int
+xfs_get_atomic_write_max(
+	struct xfs_inode	*ip)
+{
+	if (!xfs_inode_can_hw_atomic_write(ip))
+		return 0;
+
+	return ip->i_mount->m_sb.sb_blocksize;
+}
+
+unsigned int
+xfs_get_atomic_write_max_opt(
+	struct xfs_inode	*ip)
+{
+	return 0;
+}
+
 static void
 xfs_report_atomic_write(
 	struct xfs_inode	*ip,
 	struct kstat		*stat)
 {
-	unsigned int		unit_min = 0, unit_max = 0;
+	generic_fill_statx_atomic_writes(stat,
-
+			xfs_get_atomic_write_min(ip),
-	if (xfs_inode_can_hw_atomic_write(ip))
+			xfs_get_atomic_write_max(ip),
-		unit_min = unit_max = ip->i_mount->m_sb.sb_blocksize;
+			xfs_get_atomic_write_max_opt(ip));
-	generic_fill_statx_atomic_writes(stat, unit_min, unit_max, 0);
 }
 
 STATIC int

fs/xfs/xfs_iops.h

@@ -19,5 +19,8 @@ int xfs_inode_init_security(struct inode *inode, struct inode *dir,
 extern void xfs_setup_inode(struct xfs_inode *ip);
 extern void xfs_setup_iops(struct xfs_inode *ip);
 extern void xfs_diflags_to_iflags(struct xfs_inode *ip, bool init);
+unsigned int xfs_get_atomic_write_min(struct xfs_inode *ip);
+unsigned int xfs_get_atomic_write_max(struct xfs_inode *ip);
+unsigned int xfs_get_atomic_write_max_opt(struct xfs_inode *ip);
 
 #endif /* __XFS_IOPS_H__ */