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

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

[Jan Kara]

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

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

[Zach O'Keefe]

On Fri, Jun 21, 2024 at 7:42 AM Jan Kara <jack@suse.cz> wrote:
>
> 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
>
>

Thanks Jan,

Reviewed-By: Zach O'Keefe <zokeefe@google.com>