btrfs: add allocator_hint to disable allocation completely

This is useful where you want to prevent new allocations of chunks to
a set of multiple disks which are going to be removed from the pool.
This acts as a multiple `btrfs dev remove` on steroids that can remove
multiple disks in parallel without moving data to disks which would be
removed in the next round. In such cases, it will avoid moving the
same data multiple times, and thus avoid placing it on potentially bad
disks.

Thanks to @Zygo for the explanation and suggestion.

Link: kdave/btrfs-progs#907 (comment)
Signed-off-by: Kai Krakow <kai@kaishome.de>

Author: kakra

fs/btrfs/volumes.c

@@ -191,6 +191,7 @@ enum btrfs_raid_types __attribute_const__ btrfs_bg_flags_to_raid_index(u64 flags
 		BTRFS_DEV_ALLOCATION_MASK_BIT_COUNT)
 
 static const char alloc_hint_map[BTRFS_DEV_ALLOCATION_MASK_COUNT] = {
+	[BTRFS_DEV_ALLOCATION_NONE_ONLY] = -99,
 	[BTRFS_DEV_ALLOCATION_DATA_ONLY] = -1,
 	[BTRFS_DEV_ALLOCATION_PREFERRED_DATA] = 0,
 	[BTRFS_DEV_ALLOCATION_PREFERRED_METADATA] = 1,
@@ -5344,6 +5345,11 @@ static int gather_device_info(struct btrfs_fs_devices *fs_devices,
 			 */
 			if (hint == BTRFS_DEV_ALLOCATION_METADATA_ONLY)
 				continue;
+			/*
+			 * skip BTRFS_DEV_NONE_ONLY disks
+			 */
+			if (hint == BTRFS_DEV_ALLOCATION_NONE_ONLY)
+				continue;
 			/*
 			 * if a data chunk must be allocated,
 			 * sort also by hint (data disk
@@ -5358,6 +5364,11 @@ static int gather_device_info(struct btrfs_fs_devices *fs_devices,
 			 */
 			if (hint == BTRFS_DEV_ALLOCATION_DATA_ONLY)
 				continue;
+			/*
+			 * skip BTRFS_DEV_NONE_ONLY disks
+			 */
+			if (hint == BTRFS_DEV_ALLOCATION_NONE_ONLY)
+				continue;
 			/*
 			 * if a data chunk must be allocated,
 			 * sort also by hint (metadata hint

include/uapi/linux/btrfs_tree.h

@@ -593,7 +593,9 @@ struct btrfs_node {
 #define BTRFS_DEV_ALLOCATION_DATA_ONLY		(3ULL)
 /* avoid chunk allocation if possible */
 #define BTRFS_DEV_ALLOCATION_PREFERRED_NONE	(4ULL)
-/* 5..7 are unused values */
+/* deny chunk allocation */
+#define BTRFS_DEV_ALLOCATION_NONE_ONLY	        (5ULL)
+/* 6..7 are unused values */
 
 #endif