[PATCH 1/7] iomap: move pos+len BUG_ON() to after folio lookup

[Brian Foster <bfoster@redhat.com>]

The bug checks at the top of iomap_write_begin() assume the pos/len
reflect exactly the next range to process. This may no longer be the
case once the get folio path is able to process a folio batch from
the filesystem. Move the check a bit further down after the folio
lookup and range trim to verify everything lines up with the current
iomap.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/iomap/buffered-io.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 3729391a18f3..16499655e7b0 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -805,15 +805,12 @@ static int iomap_write_begin(struct iomap_iter *iter, struct folio **foliop,
 {
 	const struct iomap_folio_ops *folio_ops = iter->iomap.folio_ops;
 	const struct iomap *srcmap = iomap_iter_srcmap(iter);
-	loff_t pos = iter->pos;
+	loff_t pos;
 	u64 len = min_t(u64, SIZE_MAX, iomap_length(iter));
 	struct folio *folio;
 	int status = 0;
 
 	len = min_not_zero(len, *plen);
-	BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
-	if (srcmap != &iter->iomap)
-		BUG_ON(pos + len > srcmap->offset + srcmap->length);
 
 	if (fatal_signal_pending(current))
 		return -EINTR;
@@ -843,6 +840,9 @@ static int iomap_write_begin(struct iomap_iter *iter, struct folio **foliop,
 	}
 
 	pos = iomap_trim_folio_range(iter, folio, poffset, &len);
+	BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
+	if (srcmap != &iter->iomap)
+		BUG_ON(pos + len > srcmap->offset + srcmap->length);
 
 	if (srcmap->type == IOMAP_INLINE)
 		status = iomap_write_begin_inline(iter, folio);
-- 
2.49.0