iomap: add a IOMAP_F_ANON_WRITE flag

Add a IOMAP_F_ANON_WRITE flag that indicates that the write I/O does not
have a target block assigned to it yet at iomap time and the file system
will do that in the bio submission handler, splitting the I/O as needed.

This is used to implement Zone Append based I/O for zoned XFS, where
splitting writes to the hardware limits and assigning a zone to them
happens just before sending the I/O off to the block layer, but could
also be useful for other things like compressed I/O.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250206064035.2323428-4-hch@lst.de
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Christoph Hellwig

Documentation/filesystems/iomap/design.rst

@@ -246,6 +246,10 @@ The fields are as follows:
    * **IOMAP_F_PRIVATE**: Starting with this value, the upper bits can
      be set by the filesystem for its own purposes.
 
+   * **IOMAP_F_ANON_WRITE**: Indicates that (write) I/O does not have a target
+     block assigned to it yet and the file system will do that in the bio
+     submission handler, splitting the I/O as needed.
+
    These flags can be set by iomap itself during file operations.
    The filesystem should supply an ``->iomap_end`` function if it needs
    to observe these flags:

fs/iomap/buffered-io.c

@@ -1691,10 +1691,14 @@ static int iomap_submit_ioend(struct iomap_writepage_ctx *wpc, int error)
 	 * failure happened so that the file system end I/O handler gets called
 	 * to clean up.
 	 */
-	if (wpc->ops->submit_ioend)
+	if (wpc->ops->submit_ioend) {
 		error = wpc->ops->submit_ioend(wpc, error);
-	else if (!error)
-		submit_bio(&wpc->ioend->io_bio);
+	} else {
+		if (WARN_ON_ONCE(wpc->iomap.flags & IOMAP_F_ANON_WRITE))
+			error = -EIO;
+		if (!error)
+			submit_bio(&wpc->ioend->io_bio);
+	}
 
 	if (error) {
 		wpc->ioend->io_bio.bi_status = errno_to_blk_status(error);
@@ -1744,7 +1748,8 @@ static bool iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t pos,
 		return false;
 	if (pos != wpc->ioend->io_offset + wpc->ioend->io_size)
 		return false;
-	if (iomap_sector(&wpc->iomap, pos) !=
+	if (!(wpc->iomap.flags & IOMAP_F_ANON_WRITE) &&
+	    iomap_sector(&wpc->iomap, pos) !=
 	    bio_end_sector(&wpc->ioend->io_bio))
 		return false;
 	/*

fs/iomap/direct-io.c

@@ -81,10 +81,12 @@ static void iomap_dio_submit_bio(const struct iomap_iter *iter,
 		WRITE_ONCE(iocb->private, bio);
 	}
 
-	if (dio->dops && dio->dops->submit_io)
+	if (dio->dops && dio->dops->submit_io) {
 		dio->dops->submit_io(iter, bio, pos);
-	else
+	} else {
+		WARN_ON_ONCE(iter->iomap.flags & IOMAP_F_ANON_WRITE);
 		submit_bio(bio);
+	}
 }
 
 ssize_t iomap_dio_complete(struct iomap_dio *dio)

include/linux/iomap.h

@@ -56,6 +56,10 @@ struct vm_fault;
  *
  * IOMAP_F_BOUNDARY indicates that I/O and I/O completions for this iomap must
  * never be merged with the mapping before it.
+ *
+ * IOMAP_F_ANON_WRITE indicates that (write) I/O does not have a target block
+ * assigned to it yet and the file system will do that in the bio submission
+ * handler, splitting the I/O as needed.
  */
 #define IOMAP_F_NEW		(1U << 0)
 #define IOMAP_F_DIRTY		(1U << 1)
@@ -68,6 +72,7 @@ struct vm_fault;
 #endif /* CONFIG_BUFFER_HEAD */
 #define IOMAP_F_XATTR		(1U << 5)
 #define IOMAP_F_BOUNDARY	(1U << 6)
+#define IOMAP_F_ANON_WRITE	(1U << 7)
 
 /*
  * Flags set by the core iomap code during operations:
@@ -111,6 +116,8 @@ struct iomap {
 
 static inline sector_t iomap_sector(const struct iomap *iomap, loff_t pos)
 {
+	if (iomap->flags & IOMAP_F_ANON_WRITE)
+		return U64_MAX; /* invalid */
 	return (iomap->addr + pos - iomap->offset) >> SECTOR_SHIFT;
 }