Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again"

JIRA: https://issues.redhat.com/browse/RHEL-50004

commit 30139c7
Author: Jan Kara <jack@suse.cz>
Date:   Fri Jun 21 16:42:37 2024 +0200

    Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again"

    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 9319b64.

    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@suse.cz
    Link: https://lkml.kernel.org/r/20240621144246.11148-1-jack@suse.cz
    Fixes: 9319b64 ("mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again")
    Signed-off-by: Jan Kara <jack@suse.cz>
    Reviewed-By: Zach O'Keefe <zokeefe@google.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Chris von Recklinghausen <crecklin@redhat.com>

Author: Chris von Recklinghausen

mm/page-writeback.c

@@ -1638,7 +1638,7 @@ static inline void wb_dirty_limits(struct dirty_throttle_control *dtc)
 	 */
 	dtc->wb_thresh = __wb_calc_thresh(dtc);
 	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