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 <[email protected]>
---
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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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 <[email protected]>
Acked-by: Rik van Riel <[email protected]>
- --
All rights reversed
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On Fri, 11 Jul 2014 12:18:27 +0400 Maxim Patlasov <[email protected]> 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.
Hi Andrew,
On 07/12/2014 02:27 AM, Andrew Morton wrote:
> On Fri, 11 Jul 2014 12:18:27 +0400 Maxim Patlasov <[email protected]> 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