gfs2: Fix gfs2_testbit to use clone bitmaps

Function gfs2_testbit is called in three places. Two of those places,
gfs2_alloc_extent and gfs2_unaligned_extlen, should be using the clone
bitmaps, not the "real" bitmaps. Function gfs2_unaligned_extlen is used
by the block reservations scheme to determine the length of an extent of
free blocks. Before this patch, it wasn't using the clone bitmap, which
means recently-freed blocks were treated as free blocks for the purposes
of an allocation.

This patch adds a new parameter to gfs2_testbit to indicate whether or
not the clone bitmaps should be used (if available).

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/fs/gfs2/incore.h b/fs/gfs2/incore.h
index b50908211b69..b96d39c28e17 100644
--- a/fs/gfs2/incore.h
+++ b/fs/gfs2/incore.h
@@ -65,6 +65,27 @@ struct gfs2_log_operations {
 
 #define GBF_FULL 1
 
+/**
+ * Clone bitmaps (bi_clone):
+ *
+ * - When a block is freed, we remember the previous state of the block in the
+ *   clone bitmap, and only mark the block as free in the real bitmap.
+ *
+ * - When looking for a block to allocate, we check for a free block in the
+ *   clone bitmap, and if no clone bitmap exists, in the real bitmap.
+ *
+ * - For allocating a block, we mark it as allocated in the real bitmap, and if
+ *   a clone bitmap exists, also in the clone bitmap.
+ *
+ * - At the end of a log_flush, we copy the real bitmap into the clone bitmap
+ *   to make the clone bitmap reflect the current allocation state.
+ *   (Alternatively, we could remove the clone bitmap.)
+ *
+ * The clone bitmaps are in-core only, and is never written to disk.
+ *
+ * These steps ensure that blocks which have been freed in a transaction cannot
+ * be reallocated in that same transaction.
+ */
 struct gfs2_bitmap {
 	struct buffer_head *bi_bh;
 	char *bi_clone;