* [PATCH 1/2] Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again" [not found] <20240621144017.30993-1-jack@suse.cz> @ 2024-06-21 14:42 ` Jan Kara 2024-06-21 17:26 ` Zach O'Keefe 0 siblings, 1 reply; 2+ messages in thread From: Jan Kara @ 2024-06-21 14:42 UTC (permalink / raw) To: Andrew Morton; +Cc: linux-mm, linux-fsdevel, Jan Kara, stable 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 ^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH 1/2] Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again" 2024-06-21 14:42 ` [PATCH 1/2] Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again" Jan Kara @ 2024-06-21 17:26 ` Zach O'Keefe 0 siblings, 0 replies; 2+ messages in thread From: Zach O'Keefe @ 2024-06-21 17:26 UTC (permalink / raw) To: Jan Kara; +Cc: Andrew Morton, linux-mm, linux-fsdevel, stable 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> ^ permalink raw reply [flat|nested] 2+ messages in thread
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2024-06-21 14:42 ` [PATCH 1/2] Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again" Jan Kara
2024-06-21 17:26 ` Zach O'Keefe
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