btrfs: auto enable discard=async when possible

There's a request to automatically enable async discard for capable
devices. We can do that, the async mode is designed to wait for larger
freed extents and is not intrusive, with limits to iops, kbps or latency.

The status and tunables will be exported in /sys/fs/btrfs/FSID/discard .

The automatic selection is done if there's at least one discard capable
device in the filesystem (not capable devices are skipped). Mounting
with any other discard option will honor that option, notably mounting
with nodiscard will keep it disabled.

Link: https://lore.kernel.org/linux-btrfs/CAEg-Je_b1YtdsCR0zS5XZ_SbvJgN70ezwvRwLiCZgDGLbeMB=w@mail.gmail.com/Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -1396,6 +1396,7 @@ enum {
 	BTRFS_MOUNT_DISCARD_ASYNC		= (1UL << 28),
 	BTRFS_MOUNT_IGNOREBADROOTS		= (1UL << 29),
 	BTRFS_MOUNT_IGNOREDATACSUMS		= (1UL << 30),
+	BTRFS_MOUNT_NODISCARD			= (1UL << 31),
 };
 
 #define BTRFS_DEFAULT_COMMIT_INTERVAL	(30)

fs/btrfs/disk-io.c

@@ -3746,6 +3746,20 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 		btrfs_set_and_info(fs_info, SSD, "enabling ssd optimizations");
 	}
 
+	/*
+	 * For devices supporting discard turn on discard=async automatically,
+	 * unless it's already set or disabled. This could be turned off by
+	 * nodiscard for the same mount.
+	 */
+	if (!(btrfs_test_opt(fs_info, DISCARD_SYNC) ||
+	      btrfs_test_opt(fs_info, DISCARD_ASYNC) ||
+	      btrfs_test_opt(fs_info, NODISCARD)) &&
+	    fs_info->fs_devices->discardable) {
+		btrfs_set_and_info(fs_info, DISCARD_ASYNC,
+				   "auto enabling async discard");
+		btrfs_clear_opt(fs_info->mount_opt, NODISCARD);
+	}
+
 	/*
 	 * Mount does not set all options immediately, we can do it now and do
 	 * not have to wait for transaction commit

fs/btrfs/super.c

@@ -918,12 +918,14 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
 				ret = -EINVAL;
 				goto out;
 			}
+			btrfs_clear_opt(info->mount_opt, NODISCARD);
 			break;
 		case Opt_nodiscard:
 			btrfs_clear_and_info(info, DISCARD_SYNC,
 					     "turning off discard");
 			btrfs_clear_and_info(info, DISCARD_ASYNC,
 					     "turning off async discard");
+			btrfs_set_opt(info->mount_opt, NODISCARD);
 			break;
 		case Opt_space_cache:
 		case Opt_space_cache_version:

fs/btrfs/volumes.c

@@ -641,6 +641,9 @@ static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
 	if (!bdev_nonrot(bdev))
 		fs_devices->rotating = true;
 
+	if (bdev_max_discard_sectors(bdev))
+		fs_devices->discardable = true;
+
 	device->bdev = bdev;
 	clear_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state);
 	device->mode = flags;

fs/btrfs/volumes.h

@@ -354,6 +354,8 @@ struct btrfs_fs_devices {
 	 * nonrot flag set
 	 */
 	bool rotating;
+	/* Devices support TRIM/discard commands */
+	bool discardable;
 
 	struct btrfs_fs_info *fs_info;
 	/* sysfs kobjects */