drbd: bitmap: don't count unused bits (fix non-terminating resync)

We trusted the on-disk bitmap to have unused bits cleared.
In case that is not true for whatever reason,
and we take a code path where the unused bits don't get cleared
elsewhere (bm_clear_surplus is not called), we may miscount the bits,
and get confused during resync, waiting for bits to get cleared that we
don't even use: the resync process would not terminate.

Fix this by masking out unused bits in __bm_count_bits.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

1 files changed, 22 insertions, 3 deletions

diff --git a/drivers/block/drbd/drbd_bitmap.c b/drivers/block/drbd/drbd_bitmap.c
index 9390e9526786..c5361487cf47 100644
--- a/drivers/block/drbd/drbd_bitmap.c
+++ b/drivers/block/drbd/drbd_bitmap.c
@@ -376,9 +376,16 @@ static unsigned long __bm_count_bits(struct drbd_bitmap *b, const int swap_endia
         unsigned long *p_addr, *bm, offset = 0;
         unsigned long bits = 0;
         unsigned long i, do_now;
+        unsigned long words;
 
-        while (offset < b->bm_words) {
-                i = do_now = min_t(size_t, b->bm_words-offset, LWPP);
+        /* due to 64bit alignment, the last long on a 32bit arch
+         * may be not used at all. The last used long will likely
+         * be only partially used, always. Don't count those bits,
+         * but mask them out. */
+        words = (b->bm_bits + BITS_PER_LONG - 1) >> LN2_BPL;
+
+        while (offset < words) {
+                i = do_now = min_t(size_t, words-offset, LWPP);
                 p_addr = __bm_map_paddr(b, offset, KM_USER0);
                 bm = p_addr + MLPP(offset);
                 while (i--) {
@@ -388,8 +395,20 @@ static unsigned long __bm_count_bits(struct drbd_bitmap *b, const int swap_endia
 #endif
                         bits += hweight_long(*bm++);
                 }
-                __bm_unmap(p_addr, KM_USER0);
                 offset += do_now;
+                if (offset == words) {
+                        /* last word may only be partially used,
+                         * see also bm_clear_surplus. */
+                        i = (1UL << (b->bm_bits & (BITS_PER_LONG-1))) -1;
+                        if (i) {
+                                bits -= hweight_long(p_addr[do_now-1] & ~i);
+                                p_addr[do_now-1] &= i;
+                        }
+                        /* 32bit arch, may have an unused padding long */
+                        if (words != b->bm_words)
+                                p_addr[do_now] = 0;
+                }
+                __bm_unmap(p_addr, KM_USER0);
                 cond_resched();
         }