Messages in this thread Patch in this message | | | Date | Mon, 14 Jul 2014 12:06:01 +0400 | From | Maxim Patlasov <> | Subject | Re: [PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits |
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Hi Andrew,
On 07/12/2014 02:27 AM, Andrew Morton wrote: > 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?
No, I used default settings:
vm_dirty_bytes = 0; dirty_background_bytes = 0; vm_dirty_ratio = 20; dirty_background_ratio = 10;
and a simple program like main() { while(1) { p = malloc(4096); mlock(p, 4096); } }. Of course, this triggers oom eventually, but immediately before oom, the system is under hard memory pressure.
> >> 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
No, the patch doesn't treat dirty_thresh==0 as infinity. In fact, in that case we have equation: x : 0 = 0 : 0, and the patch resolves it as x=0. Here is the reasoning:
1. A bdi counter is always a fraction of global one. Hence bdi_thresh is always not greater than dirty_thresh. So far as dirty_thresh is equal to zero, bdi_thresh is equal to zero too. 2. bdi_bg_thresh must be not greater than bdi_thresh because we want to start background process earlier than throttling it. So far as bdi_thresh is equal to zero, bdi_bg_thresh must be zero too.
> > 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. >
Firstly let me explain where available_memory equal to one came from. global_dirty_limits() calculates it by calling global_dirtyable_memory(). The latter takes into consideration three global counters and a global reserve. In my case corresponding values were:
NR_INACTIVE_FILE = 0 NR_ACTIVE_FILE = 0 NR_FREE_PAGES = 7006 dirty_balance_reserve = 7959.
Consequently, "x" in global_dirtyable_memory() was equal to zero, and the function returned one. Now global_dirty_limits() assigns available_memory to one and calculates "dirty" as a fraction of available_memory:
dirty = (vm_dirty_ratio * available_memory) / 100; So far as vm_drity_ratio is lesser than 100 (it is 20 by default), dirty is calculated as zero.
As for your question about conversion 0 to 1, I think that bdi_thresh = dirty_thresh = 0 makes natural sense: we are under strong memory pressure, please always start background writeback and throttle process (even if actual number of dirty pages is low). So other parts of balance_dirty_pages machinery must handle zero thresholds properly.
Thanks, Maxim
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