When ->iomap_begin() sees this count being non-zero and determines that
the block map of the file needs to be modified to satisfy the I/O
request it will instead return an error. This is needed for MAP_DIRECT
where, due to locking constraints, we can't rely on xfs_break_layouts()
to protect against allocating write-faults either from the process that
setup the MAP_DIRECT mapping nor other processes that have the file
mapped.  xfs_break_layouts() requires XFS_IOLOCK which is problematic to
mix with the XFS_MMAPLOCK in the fault path.

Cc: Jan Kara <redacted>
Cc: Jeff Moyer <redacted>
Cc: Christoph Hellwig <redacted>
Cc: Dave Chinner <redacted>
Cc: "Darrick J. Wong" <redacted>
Cc: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
Cc: Jeff Layton <redacted>
Cc: "J. Bruce Fields" <redacted>
Signed-off-by: Dan Williams <redacted>
---
 fs/xfs/xfs_iomap.c |    9 +++++++++
 include/linux/fs.h |   31 +++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index a1909bc064e9..6816f8ebbdcf 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c

@@ -1053,6 +1053,15 @@ xfs_file_iomap_begin(
 			goto out_unlock;
 		}
 		/*
+		 * If a file has MAP_DIRECT mappings disable block map
+		 * updates. This should only effect mmap write faults as
+		 * other paths are protected by an FL_LAYOUT lease.
+		 */
+		if (i_mapdcount_read(inode)) {
+			error = -ETXTBSY;
+			goto out_unlock;
+		}

+		/*
 		 * We cap the maximum length we map here to MAX_WRITEBACK_PAGES
 		 * pages to keep the chunks of work done where somewhat symmetric
 		 * with the work writeback does. This is a completely arbitrary

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c2b9bf3dc4e9..f83871b188ff 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -642,6 +642,9 @@ struct inode {
 	atomic_t		i_count;
 	atomic_t		i_dio_count;
 	atomic_t		i_writecount;
+#ifdef CONFIG_FS_DAX
+	atomic_t		i_mapdcount;	/* count of MAP_DIRECT vmas */
+#endif