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SubjectRe: [PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits
On Fri, 11 Jul 2014 12:18:27 +0400 Maxim Patlasov <MPatlasov@parallels.com> wrote:

> Under memory pressure, it is possible for dirty_thresh, calculated by
> global_dirty_limits() in balance_dirty_pages(), to equal zero.

Under what circumstances? Really small values of vm_dirty_bytes?

> Then, if
> strictlimit is true, bdi_dirty_limits() tries to resolve the proportion:
>
> bdi_bg_thresh : bdi_thresh = background_thresh : dirty_thresh
>
> by dividing by zero.
>
> ...
>
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1306,9 +1306,9 @@ static inline void bdi_dirty_limits(struct backing_dev_info *bdi,
> *bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
>
> if (bdi_bg_thresh)
> - *bdi_bg_thresh = div_u64((u64)*bdi_thresh *
> - background_thresh,
> - dirty_thresh);
> + *bdi_bg_thresh = dirty_thresh ? div_u64((u64)*bdi_thresh *
> + background_thresh,
> + dirty_thresh) : 0;

This introduces a peculiar discontinuity:

if dirty_thresh==3, treat it as 3
if dirty_thresh==2, treat it as 2
if dirty_thresh==1, treat it as 1
if dirty_thresh==0, treat it as infinity

Would it not make more sense to change global_dirty_limits() to convert
0 to 1? With an appropriate comment, obviously.


Or maybe the fix lies elsewhere. Please do tell us how this zero comes
about.



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