[PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits

[PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits

[Maxim Patlasov]

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

Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>
---
 mm/page-writeback.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 518e2c3..e0c9430 100644
--- 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;
 
 	/*
 	 * In order to avoid the stacked BDI deadlock we need

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Re: [PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits

[Rik van Riel]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 07/11/2014 04:18 AM, Maxim Patlasov wrote:
> Under memory pressure, it is possible for dirty_thresh, calculated
> by global_dirty_limits() in balance_dirty_pages(), to equal zero.
> 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.
> 
> Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>

Acked-by: Rik van Riel <riel@redhat.com>

- -- 
All rights reversed
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Re: [PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits

[Andrew Morton]

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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Re: [PATCH] mm/page-writeback.c: fix divide by zero in bdi_dirty_limits

[Maxim Patlasov]

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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