drbd: cleanup ondisk meta data layout calculations and defines

Add a comment about our meta data layout variants,
and rename a few defines (e.g. MD_RESERVED_SECT -> MD_128MB_SECT)
to make it clear that they are short hand for fixed constants,
and not arbitrarily to be redefined as one may see fit.

Properly pad struct meta_data_on_disk to 4kB,
and initialize to zero not only the first 512 Byte,
but all of it in drbd_md_sync().

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 31 insertions, 11 deletions

diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index 2af26fc95280..581f6800cc30 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -696,12 +696,32 @@ out:
         return 0;
 }
 
-/* initializes the md.*_offset members, so we are able to find
- * the on disk meta data */
+/* Initializes the md.*_offset members, so we are able to find
+ * the on disk meta data.
+ *
+ * We currently have two possible layouts:
+ * external:
+ *   |----------- md_size_sect ------------------|
+ *   [ 4k superblock ][ activity log ][  Bitmap  ]
+ *   | al_offset == 8 |
+ *   | bm_offset = al_offset + X      |
+ *  ==> bitmap sectors = md_size_sect - bm_offset
+ *
+ * internal:
+ *            |----------- md_size_sect ------------------|
+ * [data.....][  Bitmap  ][ activity log ][ 4k superblock ]
+ *                        | al_offset < 0 |
+ *            | bm_offset = al_offset - Y |
+ *  ==> bitmap sectors = Y = al_offset - bm_offset
+ *
+ *  Activity log size used to be fixed 32kB,
+ *  but is about to become configurable.
+ */
 static void drbd_md_set_sector_offsets(struct drbd_conf *mdev,
                                        struct drbd_backing_dev *bdev)
 {
         sector_t md_size_sect = 0;
+        unsigned int al_size_sect = MD_32kB_SECT;
         int meta_dev_idx;
 
         rcu_read_lock();
@@ -710,23 +730,23 @@ static void drbd_md_set_sector_offsets(struct drbd_conf *mdev,
         switch (meta_dev_idx) {
         default:
                 /* v07 style fixed size indexed meta data */
-                bdev->md.md_size_sect = MD_RESERVED_SECT;
+                bdev->md.md_size_sect = MD_128MB_SECT;
                 bdev->md.md_offset = drbd_md_ss__(mdev, bdev);
-                bdev->md.al_offset = MD_AL_OFFSET;
-                bdev->md.bm_offset = MD_BM_OFFSET;
+                bdev->md.al_offset = MD_4kB_SECT;
+                bdev->md.bm_offset = MD_4kB_SECT + al_size_sect;
                 break;
         case DRBD_MD_INDEX_FLEX_EXT:
                 /* just occupy the full device; unit: sectors */
                 bdev->md.md_size_sect = drbd_get_capacity(bdev->md_bdev);
                 bdev->md.md_offset = 0;
-                bdev->md.al_offset = MD_AL_OFFSET;
-                bdev->md.bm_offset = MD_BM_OFFSET;
+                bdev->md.al_offset = MD_4kB_SECT;
+                bdev->md.bm_offset = MD_4kB_SECT + al_size_sect;
                 break;
         case DRBD_MD_INDEX_INTERNAL:
         case DRBD_MD_INDEX_FLEX_INT:
                 bdev->md.md_offset = drbd_md_ss__(mdev, bdev);
                 /* al size is still fixed */
-                bdev->md.al_offset = -MD_AL_SECTORS;
+                bdev->md.al_offset = -al_size_sect;
                 /* we need (slightly less than) ~ this much bitmap sectors: */
                 md_size_sect = drbd_get_capacity(bdev->backing_bdev);
                 md_size_sect = ALIGN(md_size_sect, BM_SECT_PER_EXT);
@@ -735,11 +755,11 @@ static void drbd_md_set_sector_offsets(struct drbd_conf *mdev,
 
                 /* plus the "drbd meta data super block",
                  * and the activity log; */
-                md_size_sect += MD_BM_OFFSET;
+                md_size_sect += MD_4kB_SECT + al_size_sect;
 
                 bdev->md.md_size_sect = md_size_sect;
                 /* bitmap offset is adjusted by 'super' block size */
-                bdev->md.bm_offset   = -md_size_sect + MD_AL_OFFSET;
+                bdev->md.bm_offset   = -md_size_sect + MD_4kB_SECT;
                 break;
         }
         rcu_read_unlock();
@@ -1416,7 +1436,7 @@ int drbd_adm_attach(struct sk_buff *skb, struct genl_info *info)
                 min_md_device_sectors = (2<<10);
         } else {
                 max_possible_sectors = DRBD_MAX_SECTORS;
-                min_md_device_sectors = MD_RESERVED_SECT * (new_disk_conf->meta_dev_idx + 1);
+                min_md_device_sectors = MD_128MB_SECT * (new_disk_conf->meta_dev_idx + 1);
         }
 
         if (drbd_get_capacity(nbc->md_bdev) < min_md_device_sectors) {