FAILED: patch "[PATCH] Revert "mm/writeback: fix possible divide-by-zero in" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 8dfcffa37094fef2c8cf8b602316766a86956d07
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024073032-uniquely-herring-53cb@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

8dfcffa37094 ("Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again"") 9319b647902c ("mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8dfcffa37094fef2c8cf8b602316766a86956d07 Mon Sep 17 00:00:00 2001
From: Jan Kara <[email protected]>
Date: Fri, 21 Jun 2024 16:42:37 +0200
Subject: [PATCH] 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 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.

index 7168e25f88e5..c4aa6e84c20a 100644
@@ -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