iomap: share iomap_unshare_iter predicate code with fsdax

The predicate code that iomap_unshare_iter uses to decide if it's really
needs to unshare a file range mapping should be shared with the fsdax
version, because right now they're opencoded and inconsistent.

Note that we simplify the predicate logic a bit -- we no longer allow
unsharing of inline data mappings, but there aren't any filesystems that
allow shared inline data currently.

This is a fix in the sense that it should have been ported to fsdax.

Fixes: b53fdb2 ("iomap: improve shared block detection in iomap_unshare_iter")
Signed-off-by: Darrick J. Wong <[email protected]>
Link: https://lore.kernel.org/r/172796813294.1131942.15762084021076932620.stgit@frogsfrogsfrogs
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: Darrick J. Wong

fs/dax.c

@@ -1268,8 +1268,7 @@ static s64 dax_unshare_iter(struct iomap_iter *iter)
 	s64 ret = 0;
 	void *daddr = NULL, *saddr = NULL;
 
-	/* don't bother with blocks that are not shared to start with */
-	if (!(iomap->flags & IOMAP_F_SHARED))
+	if (!iomap_want_unshare_iter(iter))
 		return length;
 
 	id = dax_read_lock();

fs/iomap/buffered-io.c

@@ -1309,29 +1309,31 @@ void iomap_file_buffered_write_punch_delalloc(struct inode *inode,
 }
 EXPORT_SYMBOL_GPL(iomap_file_buffered_write_punch_delalloc);
 
-static loff_t iomap_unshare_iter(struct iomap_iter *iter)
+bool iomap_want_unshare_iter(const struct iomap_iter *iter)
 {
-	struct iomap *iomap = &iter->iomap;
-	loff_t pos = iter->pos;
-	loff_t length = iomap_length(iter);
-	loff_t written = 0;
-
-	/* Don't bother with blocks that are not shared to start with. */
-	if (!(iomap->flags & IOMAP_F_SHARED))
-		return length;
-
 	/*
-	 * Don't bother with delalloc reservations, holes or unwritten extents.
+	 * Don't bother with blocks that are not shared to start with; or
+	 * mappings that cannot be shared, such as inline data, delalloc
+	 * reservations, holes or unwritten extents.
 	 *
 	 * Note that we use srcmap directly instead of iomap_iter_srcmap as
 	 * unsharing requires providing a separate source map, and the presence
 	 * of one is a good indicator that unsharing is needed, unlike
 	 * IOMAP_F_SHARED which can be set for any data that goes into the COW
 	 * fork for XFS.
 	 */
-	if (iter->srcmap.type == IOMAP_HOLE ||
-	    iter->srcmap.type == IOMAP_DELALLOC ||
-	    iter->srcmap.type == IOMAP_UNWRITTEN)
+	return (iter->iomap.flags & IOMAP_F_SHARED) &&
+		iter->srcmap.type == IOMAP_MAPPED;
+}
+
+static loff_t iomap_unshare_iter(struct iomap_iter *iter)
+{
+	struct iomap *iomap = &iter->iomap;
+	loff_t pos = iter->pos;
+	loff_t length = iomap_length(iter);
+	loff_t written = 0;
+
+	if (!iomap_want_unshare_iter(iter))
 		return length;
 
 	do {

include/linux/iomap.h

@@ -267,6 +267,7 @@ void iomap_invalidate_folio(struct folio *folio, size_t offset, size_t len);
 bool iomap_dirty_folio(struct address_space *mapping, struct folio *folio);
 int iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
 		const struct iomap_ops *ops);
+bool iomap_want_unshare_iter(const struct iomap_iter *iter);
 int iomap_zero_range(struct inode *inode, loff_t pos, loff_t len,
 		bool *did_zero, const struct iomap_ops *ops);
 int iomap_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,