iomap: improve argument passing to iomap_read_folio_sync

Pass the iomap_iter and derive the map inside iomap_read_folio_sync
instead of in the caller, and use the more descriptive srcmap name for
the source iomap.  Stop passing the offset into folio argument as it
can be derived from the folio and the file offset.  Rename the
variables for the offset into the file and the length to be more
descriptive and match the rest of the code.

Rename the function itself to iomap_read_folio_range to make the use
more clear.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: "Darrick J. Wong" <[email protected]>
Reviewed-by: Joanne Koong <[email protected]>

Author: Christoph Hellwig

fs/iomap/buffered-io.c

@@ -657,22 +657,22 @@ iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
 					 pos + len - 1);
 }
 
-static int iomap_read_folio_sync(loff_t block_start, struct folio *folio,
-		size_t poff, size_t plen, const struct iomap *iomap)
+static int iomap_read_folio_range(const struct iomap_iter *iter,
+		struct folio *folio, loff_t pos, size_t len)
 {
+	const struct iomap *srcmap = iomap_iter_srcmap(iter);
 	struct bio_vec bvec;
 	struct bio bio;
 
-	bio_init(&bio, iomap->bdev, &bvec, 1, REQ_OP_READ);
-	bio.bi_iter.bi_sector = iomap_sector(iomap, block_start);
-	bio_add_folio_nofail(&bio, folio, plen, poff);
+	bio_init(&bio, srcmap->bdev, &bvec, 1, REQ_OP_READ);
+	bio.bi_iter.bi_sector = iomap_sector(srcmap, pos);
+	bio_add_folio_nofail(&bio, folio, len, offset_in_folio(folio, pos));
 	return submit_bio_wait(&bio);
 }
 
 static int __iomap_write_begin(const struct iomap_iter *iter, size_t len,
 		struct folio *folio)
 {
-	const struct iomap *srcmap = iomap_iter_srcmap(iter);
 	struct iomap_folio_state *ifs;
 	loff_t pos = iter->pos;
 	loff_t block_size = i_blocksize(iter->inode);
@@ -721,8 +721,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, size_t len,
 			if (iter->flags & IOMAP_NOWAIT)
 				return -EAGAIN;
 
-			status = iomap_read_folio_sync(block_start, folio,
-					poff, plen, srcmap);
+			status = iomap_read_folio_range(iter, folio,
+					block_start, plen);
 			if (status)
 				return status;
 		}