iomap: improve shared block detection in iomap_unshare_iter

Currently iomap_unshare_iter relies on the IOMAP_F_SHARED flag to detect
blocks to unshare.  This is reasonable, but IOMAP_F_SHARED is also useful
for the file system to do internal book keeping for out of place writes.
XFS used to that, until it got removed in commit 72a048c1
("xfs: only set IOMAP_F_SHARED when providing a srcmap to a write")
because unshare for incorrectly unshare such blocks.

Add an extra safeguard by checking the explicitly provided srcmap instead
of the fallback to the iomap for valid data, as that catches the case
where we'd just copy from the same place we'd write to easily, allowing
to reinstate setting IOMAP_F_SHARED for all XFS writes that go to the
COW fork.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240910043949.3481298-3-hch@lst.deReviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/iomap/buffered-io.c

@@ -1338,16 +1338,25 @@ EXPORT_SYMBOL_GPL(iomap_file_buffered_write_punch_delalloc);
 static loff_t iomap_unshare_iter(struct iomap_iter *iter)
 {
 	struct iomap *iomap = &iter->iomap;
-	const struct iomap *srcmap = iomap_iter_srcmap(iter);
 	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 */
+	/* Don't bother with blocks that are not shared to start with. */
 	if (!(iomap->flags & IOMAP_F_SHARED))
 		return length;
-	/* don't bother with holes or unwritten extents */
-	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
+
+	/*
+	 * Don't bother with 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_UNWRITTEN)
 		return length;
 
 	do {