btrfs: use named constant for reserved device space

There's a reserved space on each device of size 1MiB that can be used by
bootloaders or to avoid accidental overwrite. Use a symbolic constant
with the explaining comment instead of hard coding the value and
multiple comments.

Note: since btrfs-progs v4.1, mkfs.btrfs will reserve the first 1MiB for
the primary super block (at offset 64KiB), until then the range could
have been used by mistake. Kernel has been always respecting the 1MiB
range for writes.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index e5f19b49efa804fe02752b6c8f676eafe3853dd0..6d9f81d4e99c09f7373f963120bd0f79e98cb9cf 100644 (file)
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -229,6 +229,13 @@ struct btrfs_root_backup {
 #define BTRFS_SUPER_INFO_OFFSET                        SZ_64K
 #define BTRFS_SUPER_INFO_SIZE                  4096
 
+/*
+ * The reserved space at the beginning of each device.
+ * It covers the primary super block and leaves space for potential use by other
+ * tools like bootloaders or to lower potential damage of accidental overwrite.
+ */
+#define BTRFS_DEVICE_RANGE_RESERVED                    (SZ_1M)
+
 /*
  * the super block basically lists the main trees of the FS
  * it currently lacks any block count etc etc

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 91d2c20c7c8a6f20bfcf436941e6efacf19d80f6..a1696e3ffb1e38277318fa3f3e867d2f1e00084d 100644 (file)
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -5976,7 +5976,7 @@ int btrfs_error_unpin_extent_range(struct btrfs_fs_info *fs_info,
  */
 static int btrfs_trim_free_extents(struct btrfs_device *device, u64 *trimmed)
 {
-       u64 start = SZ_1M, len = 0, end = 0;
+       u64 start = BTRFS_DEVICE_RANGE_RESERVED, len = 0, end = 0;
        int ret;
 
        *trimmed = 0;
@@ -6020,8 +6020,8 @@ static int btrfs_trim_free_extents(struct btrfs_device *device, u64 *trimmed)
                        break;
                }
 
-               /* Ensure we skip the reserved area in the first 1M */
-               start = max_t(u64, start, SZ_1M);
+               /* Ensure we skip the reserved space on each device. */
+               start = max_t(u64, start, BTRFS_DEVICE_RANGE_RESERVED);
 
                /*
                 * If find_first_clear_extent_bit find a range that spans the

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 719dda57dc7a0a1c2a575ab0eb544c8657948874..41652dcd16f436eeb2142cb1fc89ee17c37c8634 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2272,17 +2272,13 @@ static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
                avail_space = rounddown(avail_space, BTRFS_STRIPE_LEN);
 
                /*
-                * In order to avoid overwriting the superblock on the drive,
-                * btrfs starts at an offset of at least 1MB when doing chunk
-                * allocation.
-                *
-                * This ensures we have at least min_stripe_size free space
-                * after excluding 1MB.
+                * Ensure we have at least min_stripe_size on top of the
+                * reserved space on the device.
                 */
-               if (avail_space <= SZ_1M + min_stripe_size)
+               if (avail_space <= BTRFS_DEVICE_RANGE_RESERVED + min_stripe_size)
                        continue;
 
-               avail_space -= SZ_1M;
+               avail_space -= BTRFS_DEVICE_RANGE_RESERVED;
 
                devices_info[i].dev = device;
                devices_info[i].max_avail = avail_space;

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 80636fbf28b7a3f46d4d134e49580532bbf5853b..e6af960a69614fea94cf070e28453fcb7718a3f3 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1403,12 +1403,7 @@ static u64 dev_extent_search_start(struct btrfs_device *device, u64 start)
 {
        switch (device->fs_devices->chunk_alloc_policy) {
        case BTRFS_CHUNK_ALLOC_REGULAR:
-               /*
-                * We don't want to overwrite the superblock on the drive nor
-                * any area used by the boot loader (grub for example), so we
-                * make sure to start at an offset of at least 1MB.
-                */
-               return max_t(u64, start, SZ_1M);
+               return max_t(u64, start, BTRFS_DEVICE_RANGE_RESERVED);
        case BTRFS_CHUNK_ALLOC_ZONED:
                /*
                 * We don't care about the starting region like regular