Hi Andrew,
On 07/12/2014 02:27 AM, Andrew Morton wrote:
On Fri, 11 Jul 2014 12:18:27 +0400 Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx> 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
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>