[PATCH v5 08/14] iomap: set accurate iter->pos when reading folio ranges

[Joanne Koong <joannelkoong@gmail.com>]

Advance iter to the correct position before calling an IO helper to read
in a folio range. This allows the helper to reliably use iter->pos to
determine the starting offset for reading.

This will simplify the interface for reading in folio ranges when iomap
read/readahead supports caller-provided callbacks.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
---
 fs/iomap/buffered-io.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 4e6258fdb915..82bdf7c5e03c 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -375,10 +375,11 @@ static void iomap_bio_submit_read(struct iomap_read_folio_ctx *ctx)
 }
 
 static void iomap_bio_read_folio_range(const struct iomap_iter *iter,
-		struct iomap_read_folio_ctx *ctx, loff_t pos, size_t plen)
+		struct iomap_read_folio_ctx *ctx, size_t plen)
 {
 	struct folio *folio = ctx->cur_folio;
 	const struct iomap *iomap = &iter->iomap;
+	loff_t pos = iter->pos;
 	size_t poff = offset_in_folio(folio, pos);
 	loff_t length = iomap_length(iter);
 	sector_t sector;
@@ -470,7 +471,7 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 	loff_t length = iomap_length(iter);
 	struct folio *folio = ctx->cur_folio;
 	size_t poff, plen;
-	loff_t count;
+	loff_t pos_diff;
 	int ret;
 
 	if (iomap->type == IOMAP_INLINE) {
@@ -488,12 +489,16 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 		iomap_adjust_read_range(iter->inode, folio, &pos, length, &poff,
 				&plen);
 
-		count = pos - iter->pos + plen;
-		if (WARN_ON_ONCE(count > length))
+		pos_diff = pos - iter->pos;
+		if (WARN_ON_ONCE(pos_diff + plen > length))
 			return -EIO;
 
+		ret = iomap_iter_advance(iter, pos_diff);
+		if (ret)
+			return ret;
+
 		if (plen == 0)
-			return iomap_iter_advance(iter, count);
+			return 0;
 
 		/* zero post-eof blocks as the page may be mapped */
 		if (iomap_block_needs_zeroing(iter, pos)) {
@@ -503,13 +508,13 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 			if (!*bytes_pending)
 				iomap_read_init(folio);
 			*bytes_pending += plen;
-			iomap_bio_read_folio_range(iter, ctx, pos, plen);
+			iomap_bio_read_folio_range(iter, ctx, plen);
 		}
 
-		ret = iomap_iter_advance(iter, count);
+		ret = iomap_iter_advance(iter, plen);
 		if (ret)
 			return ret;
-		length -= count;
+		length -= pos_diff + plen;
 		pos = iter->pos;
 	}
 	return 0;
-- 
2.47.3