dm: split discards further if target sets max_discard_granularity

The block core (bio_split_discard) will already split discards based
on the 'discard_granularity' and 'max_discard_sectors' queue_limits.
But the DM thin target also needs to ensure that it doesn't receive a
discard that spans a 'max_discard_sectors' boundary.

Introduce a dm_target 'max_discard_granularity' flag that if set will
cause DM core to split discard bios relative to 'max_discard_sectors'.
This treats 'discard_granularity' as a "min_discard_granularity" and
'max_discard_sectors' as a "max_discard_granularity".

Requested-by: Joe Thornber <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm.c

···
 	return ti->len - target_offset;
 }
 
-static sector_t max_io_len(struct dm_target *ti, sector_t sector)
+static sector_t __max_io_len(struct dm_target *ti, sector_t sector,
+			     unsigned int max_granularity)
 {
 	sector_t target_offset = dm_target_offset(ti, sector);
 	sector_t len = max_io_len_target_boundary(ti, target_offset);
···
 	 *   explains why stacked chunk_sectors based splitting via
 	 *   bio_split_to_limits() isn't possible here.
 	 */
-	if (!ti->max_io_len)
+	if (!max_granularity)
 		return len;
 	return min_t(sector_t, len,
 		min(queue_max_sectors(ti->table->md->queue),
-		    blk_chunk_sectors_left(target_offset, ti->max_io_len)));
+		    blk_chunk_sectors_left(target_offset, max_granularity)));
+}
+
+static inline sector_t max_io_len(struct dm_target *ti, sector_t sector)
+{
+	return __max_io_len(ti, sector, ti->max_io_len);
 }
 
 int dm_set_target_max_io_len(struct dm_target *ti, sector_t len)
···
 }
 
 static void __send_changing_extent_only(struct clone_info *ci, struct dm_target *ti,
-					unsigned int num_bios)
+					unsigned int num_bios,
+					unsigned int max_granularity)
 {
 	unsigned int len, bios;
 
 	len = min_t(sector_t, ci->sector_count,
-		    max_io_len_target_boundary(ti, dm_target_offset(ti, ci->sector)));
+		    __max_io_len(ti, ci->sector, max_granularity));
 
 	atomic_add(num_bios, &ci->io->io_count);
 	bios = __send_duplicate_bios(ci, ti, num_bios, &len);
···
 					  struct dm_target *ti)
 {
 	unsigned int num_bios = 0;
+	unsigned int max_granularity = 0;
 
 	switch (bio_op(ci->bio)) {
 	case REQ_OP_DISCARD:
 		num_bios = ti->num_discard_bios;
+		if (ti->max_discard_granularity) {
+			struct queue_limits *limits =
+				dm_get_queue_limits(ti->table->md);
+			max_granularity = limits->max_discard_sectors;
+		}
 		break;
 	case REQ_OP_SECURE_ERASE:
 		num_bios = ti->num_secure_erase_bios;
···
 	if (unlikely(!num_bios))
 		return BLK_STS_NOTSUPP;
 
-	__send_changing_extent_only(ci, ti, num_bios);
+	__send_changing_extent_only(ci, ti, num_bios, max_granularity);
 	return BLK_STS_OK;
 }
 

include/linux/device-mapper.h

···
 	bool discards_supported:1;
 
 	/*
+	 * Set if this target requires that discards be split on both
+	 * 'discard_granularity' and 'max_discard_sectors' boundaries.
+	 */
+	bool max_discard_granularity:1;
+
+	/*
 	 * Set if we need to limit the number of in-flight bios when swapping.
 	 */
 	bool limit_swap_bios:1;

include/uapi/linux/dm-ioctl.h

···
 #define DM_DEV_SET_GEOMETRY	_IOWR(DM_IOCTL, DM_DEV_SET_GEOMETRY_CMD, struct dm_ioctl)
 
 #define DM_VERSION_MAJOR	4
-#define DM_VERSION_MINOR	47
+#define DM_VERSION_MINOR	48
 #define DM_VERSION_PATCHLEVEL	0
-#define DM_VERSION_EXTRA	"-ioctl (2022-07-28)"
+#define DM_VERSION_EXTRA	"-ioctl (2023-03-01)"
 
 /* Status bits */
 #define DM_READONLY_FLAG	(1 << 0) /* In/Out */