[PATCH 1/2] Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again"

[Jan Kara <jack@suse.cz>]

This reverts commit 9319b647902cbd5cc884ac08a8a6d54ce111fc78.

The commit is broken in several ways. Firstly, the removed (u64) cast
from the multiplication will introduce a multiplication overflow on
32-bit archs if wb_thresh * bg_thresh >= 1<<32 (which is actually common
- the default settings with 4GB of RAM will trigger this). Secondly, the
  div64_u64() is unnecessarily expensive on 32-bit archs. We have
div64_ul() in case we want to be safe & cheap. Thirdly, if dirty
thresholds are larger than 1<<32 pages, then dirty balancing is
going to blow up in many other spectacular ways anyway so trying to fix
one possible overflow is just moot.

CC: stable@vger.kernel.org
Fixes: 9319b647902c ("mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again")
Signed-off-by: Jan Kara <jack@suse.cz>
---
 mm/page-writeback.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 12c9297ed4a7..2573e2d504af 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1660,7 +1660,7 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
 	 */
 	dtc->wb_thresh = __wb_calc_thresh(dtc, dtc->thresh);
 	dtc->wb_bg_thresh = dtc->thresh ?
-		div64_u64(dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
+		div_u64((u64)dtc->wb_thresh * dtc->bg_thresh, dtc->thresh) : 0;
 
 	/*
 	 * In order to avoid the stacked BDI deadlock we need
-- 
2.35.3