On 04.01.25 02:20, Stefan Roesch wrote:
During testing it has been detected, that it is possible to get div by
zero error in bdi_set_min_bytes. The error is caused by the function
bdi_ratio_from_pages(). bdi_ratio_from_pages() calls
global_dirty_limits. If the dirty threshold is 0, the div by zero is
raised. This can happen if the root user is setting:
echo 0 > /proc/sys/vm/dirty_ration.
The following is a test case:
echo 0 > /proc/sys/vm/dirty_ratio
cd /sys/class/bdi/<device>
echo 1 > strict_limit
echo 8192 > min_bytes
==> error is raised.
The problem is addressed by returning -EINVAL if dirty_ratio or
dirty_bytes is set to 0.
Reported-by: cheung wall <zzqq0103.hey@xxxxxxxxx>
Closes: https://lore.kernel.org/linux-mm/87pll35yd0.fsf@xxxxxxxxxxxx/T/#t
Signed-off-by: Stefan Roesch <shr@xxxxxxxxxxxx>
---
mm/page-writeback.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d213ead95675..91aa7a5c0078 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -692,6 +692,8 @@ static unsigned long bdi_ratio_from_pages(unsigned long pages)
unsigned long ratio;
global_dirty_limits(&background_thresh, &dirty_thresh);
+ if (!dirty_thresh)
+ return -EINVAL;
ratio = div64_u64(pages * 100ULL * BDI_RATIO_SCALE, dirty_thresh);
return ratio;
bdi_set_min_bytes() calls bdi_ratio_from_pages() and passes the result
to __bdi_set_min_ratio().
__bdi_set_min_ratio() expects an "unsigned int min_ratio". I assume this
will work because "max_ratio > 100 * BDI_RATIO_SCALE", but it is rather
confusing ...
Maybe we want something like:
/* Use 101% to indicate "invalid" */
#define BDI_RATIO_INVALID (101 * BDI_RATIO_SCALE)
Or alternatively, just handle it in the callers of
bdi_ratio_from_pages(), checking for -EINVAL manually.
--
Cheers,
David / dhildenb
