Patch series "mm: Avoid possible overflows in dirty throttling". Dirty throttling logic assumes dirty limits in page units fit into 32-bits. This patch series makes sure this is true (see patch 2/2 for more details). This patch (of 2): 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. Link: https://lkml.kernel.org/r/20240621144017.30993-1-jack@xxxxxxx Link: https://lkml.kernel.org/r/20240621144246.11148-1-jack@xxxxxxx Fixes: 9319b647902c ("mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again") Signed-off-by: Jan Kara <jack@xxxxxxx> Reviewed-By: Zach O'Keefe <zokeefe@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 7168e25f88e5..c4aa6e84c20a 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -1683,7 +1683,7 @@ 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
