iomap: simplify ->read_folio_range() error handling for reads

Instead of requiring that the caller calls iomap_finish_folio_read()
even if the ->read_folio_range() callback returns an error, account for
this internally in iomap instead, which makes the interface simpler and
makes it match writeback's ->read_folio_range() error handling
expectations.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20251111193658.3495942-6-joannelkoong@gmail.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Documentation/filesystems/iomap/operations.rst

@@ -149,10 +149,9 @@ These ``struct kiocb`` flags are significant for buffered I/O with iomap:
 iomap calls these functions:
 
   - ``read_folio_range``: Called to read in the range. This must be provided
-    by the caller. The caller is responsible for calling
-    iomap_finish_folio_read() after reading in the folio range. This should be
-    done even if an error is encountered during the read. This returns 0 on
-    success or a negative error on failure.
+    by the caller. If this succeeds, iomap_finish_folio_read() must be called
+    after the range is read in, regardless of whether the read succeeded or
+    failed.
 
   - ``submit_read``: Submit any pending read requests. This function is
     optional.

fs/fuse/file.c

@@ -922,13 +922,6 @@ static int fuse_iomap_read_folio_range_async(const struct iomap_iter *iter,
 
 	if (ctx->rac) {
 		ret = fuse_handle_readahead(folio, ctx->rac, data, pos, len);
-		/*
-		 * If fuse_handle_readahead was successful, fuse_readpages_end
-		 * will do the iomap_finish_folio_read, else we need to call it
-		 * here
-		 */
-		if (ret)
-			iomap_finish_folio_read(folio, off, len, ret);
 	} else {
 		/*
 		 *  for non-readahead read requests, do reads synchronously
@@ -936,6 +929,7 @@ static int fuse_iomap_read_folio_range_async(const struct iomap_iter *iter,
 		 *  out-of-order reads
 		 */
 		ret = fuse_do_readfolio(file, folio, off, len);
+		if (!ret)
 			iomap_finish_folio_read(folio, off, len, ret);
 	}
 	return ret;

fs/iomap/buffered-io.c

@@ -398,7 +398,8 @@ static void iomap_read_init(struct folio *folio)
 		 * has already finished reading in the entire folio.
 		 */
 		spin_lock_irq(&ifs->state_lock);
-		ifs->read_bytes_pending += len + 1;
+		WARN_ON_ONCE(ifs->read_bytes_pending != 0);
+		ifs->read_bytes_pending = len + 1;
 		spin_unlock_irq(&ifs->state_lock);
 	}
 }
@@ -414,43 +415,47 @@ static void iomap_read_init(struct folio *folio)
  */
 static void iomap_read_end(struct folio *folio, size_t bytes_submitted)
 {
-	struct iomap_folio_state *ifs;
+	struct iomap_folio_state *ifs = folio->private;
 
-	/*
-	 * If there are no bytes submitted, this means we are responsible for
-	 * unlocking the folio here, since no IO helper has taken ownership of
-	 * it.
-	 */
-	if (!bytes_submitted) {
-		folio_unlock(folio);
-		return;
-	}
-
-	ifs = folio->private;
 	if (ifs) {
 		bool end_read, uptodate;
+
+		spin_lock_irq(&ifs->state_lock);
+		if (!ifs->read_bytes_pending) {
+			WARN_ON_ONCE(bytes_submitted);
+			end_read = true;
+		} else {
 			/*
 			 * Subtract any bytes that were initially accounted to
-		 * read_bytes_pending but skipped for IO.
-		 * The +1 accounts for the bias we added in iomap_read_init().
+			 * read_bytes_pending but skipped for IO. The +1
+			 * accounts for the bias we added in iomap_read_init().
 			 */
 			size_t bytes_not_submitted = folio_size(folio) + 1 -
 					bytes_submitted;
-
-		spin_lock_irq(&ifs->state_lock);
 			ifs->read_bytes_pending -= bytes_not_submitted;
 			/*
-		 * If !ifs->read_bytes_pending, this means all pending reads
-		 * by the IO helper have already completed, which means we need
-		 * to end the folio read here. If ifs->read_bytes_pending != 0,
-		 * the IO helper will end the folio read.
+			 * If !ifs->read_bytes_pending, this means all pending
+			 * reads by the IO helper have already completed, which
+			 * means we need to end the folio read here. If
+			 * ifs->read_bytes_pending != 0, the IO helper will end
+			 * the folio read.
 			 */
 			end_read = !ifs->read_bytes_pending;
+		}
 		if (end_read)
 			uptodate = ifs_is_fully_uptodate(folio, ifs);
 		spin_unlock_irq(&ifs->state_lock);
 		if (end_read)
 			folio_end_read(folio, uptodate);
+	} else if (!bytes_submitted) {
+		/*
+		 * If there were no bytes submitted, this means we are
+		 * responsible for unlocking the folio here, since no IO helper
+		 * has taken ownership of it. If there were bytes submitted,
+		 * then the IO helper will end the read via
+		 * iomap_finish_folio_read().
+		 */
+		folio_unlock(folio);
 	}
 }
 
@@ -498,10 +503,10 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 		} else {
 			if (!*bytes_submitted)
 				iomap_read_init(folio);
-			*bytes_submitted += plen;
 			ret = ctx->ops->read_folio_range(iter, ctx, plen);
 			if (ret)
 				return ret;
+			*bytes_submitted += plen;
 		}
 
 		ret = iomap_iter_advance(iter, plen);

include/linux/iomap.h

@@ -495,9 +495,8 @@ struct iomap_read_ops {
 	/*
 	 * Read in a folio range.
 	 *
-	 * The caller is responsible for calling iomap_finish_folio_read() after
-	 * reading in the folio range. This should be done even if an error is
-	 * encountered during the read.
+	 * If this succeeds, iomap_finish_folio_read() must be called after the
+	 * range is read in, regardless of whether the read succeeded or failed.
 	 *
 	 * Returns 0 on success or a negative error on failure.
 	 */