iomap: add a merge boundary flag

File systems might have boundaries over which merges aren't possible.
In fact these are very common, although most of the time some kind of
header at the beginning of this region (e.g. XFS alloation groups, ext4
block groups) automatically create a merge barrier.  But if that is
not present, say for a device purely used for data we need to manually
communicate that to iomap.

Add a IOMAP_F_BOUNDARY flag to never merge I/O into a previous mapping.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

Author: Christoph Hellwig

fs/iomap/buffered-io.c

@@ -1601,6 +1601,8 @@ iomap_ioend_can_merge(struct iomap_ioend *ioend, struct iomap_ioend *next)
 {
 	if (ioend->io_bio.bi_status != next->io_bio.bi_status)
 		return false;
+	if (next->io_flags & IOMAP_F_BOUNDARY)
+		return false;
 	if ((ioend->io_flags & IOMAP_F_SHARED) ^
 	    (next->io_flags & IOMAP_F_SHARED))
 		return false;
@@ -1720,6 +1722,8 @@ static struct iomap_ioend *iomap_alloc_ioend(struct iomap_writepage_ctx *wpc,
 	INIT_LIST_HEAD(&ioend->io_list);
 	ioend->io_type = wpc->iomap.type;
 	ioend->io_flags = wpc->iomap.flags;
+	if (pos > wpc->iomap.offset)
+		wpc->iomap.flags &= ~IOMAP_F_BOUNDARY;
 	ioend->io_inode = inode;
 	ioend->io_size = 0;
 	ioend->io_offset = pos;
@@ -1731,6 +1735,8 @@ static struct iomap_ioend *iomap_alloc_ioend(struct iomap_writepage_ctx *wpc,
 
 static bool iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t pos)
 {
+	if (wpc->iomap.offset == pos && (wpc->iomap.flags & IOMAP_F_BOUNDARY))
+		return false;
 	if ((wpc->iomap.flags & IOMAP_F_SHARED) !=
 	    (wpc->ioend->io_flags & IOMAP_F_SHARED))
 		return false;

include/linux/iomap.h

@@ -53,6 +53,9 @@ struct vm_fault;
  *
  * IOMAP_F_XATTR indicates that the iomap is for an extended attribute extent
  * rather than a file data extent.
+ *
+ * IOMAP_F_BOUNDARY indicates that I/O and I/O completions for this iomap must
+ * never be merged with the mapping before it.
  */
 #define IOMAP_F_NEW		(1U << 0)
 #define IOMAP_F_DIRTY		(1U << 1)
@@ -64,6 +67,7 @@ struct vm_fault;
 #define IOMAP_F_BUFFER_HEAD	0
 #endif /* CONFIG_BUFFER_HEAD */
 #define IOMAP_F_XATTR		(1U << 5)
+#define IOMAP_F_BOUNDARY	(1U << 6)
 
 /*
  * Flags set by the core iomap code during operations: