Re: [PATCH] mm: print a warning once the vm dirtiness settings is illogical

[Yafang Shao <laoar.shao@gmail.com>]

+cc fcicq

2017-11-26 18:38 GMT+08:00 Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>:
> Yafang Shao wrote:
>> 2017-11-26 16:03 GMT+08:00 Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>:
>> > Yafang Shao wrote:
>> >> >> I have also verified your test code on my machine, but can not find
>> >> >> this message.
>> >> >>
>> >> >
>> >> > Not always printed. It is timing dependent.
>> >> >
>> >>
>> >> I will try and analysis why this happen.
>> >>
>> > I see.
>> >
>> > Here is dump of variables. Always mostly 0 when this happens.
>> >
>> > --- a/mm/page-writeback.c
>> > +++ b/mm/page-writeback.c
>> > @@ -434,7 +434,8 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
>> >                 bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
>> >
>> >         if (unlikely(bg_thresh >= thresh)) {
>> > -               pr_warn("vm direct limit must be set greater than background limit.\n");
>> > +               pr_warn("vm direct limit must be set greater than background limit. bg_thresh=%lu thresh=%lu bg_bytes=%lu bytes=%lu bg_ratio=%lu ratio=%lu gdtc=%p gdtc->vail=%lu vm_dirty_bytes=%lu dirty_background_bytes=%lu\n",
>> > +                       bg_thresh, thresh, bg_bytes, bytes, bg_ratio, ratio, gdtc, gdtc ? gdtc->avail : 0UL, vm_dirty_bytes, dirty_background_bytes);
>> >                 bg_thresh = thresh / 2;
>> >         }
>>
>> You could print dtc->avail as well.
>> Seems bg_thresh and thresh are not so acurate as they are interger
>> other than float.
>
> Indeed, dtc->avail < 4 when this message is printed.
>
> [  314.730541] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=2
> [  315.864111] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=1
> [  315.864126] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=1
> [  315.993866] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=2
> [  355.807392] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=2
> [  406.819939] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=1
> [  407.782790] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=1
> [  416.939906] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=1
> [  417.090872] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093164] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093176] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093183] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093191] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093198] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093206] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093213] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093223] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093232] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3
> [  417.093240] vm direct limit must be set greater than background limit. bg_thresh=0 thresh=0 bg_bytes=0 bytes=0 bg_ratio=409 ratio=1228 gdtc=          (null) gdtc->avail=0 vm_dirty_bytes=0 dirty_background_bytes=0 dtc->avail=3


What about bellow change ?

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 8a15511..6c5c018 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -377,7 +377,16 @@ static unsigned long global_dirtyable_memory(void)
    if (!vm_highmem_is_dirtyable)
        x -= highmem_dirtyable_memory(x);

-   return x + 1;   /* Ensure that we never return 0 */
+   /*
+    * - Why 100 ?
+    * - Because the return value will be used by dirty ratio and
+    *   dirty background ratio to calculate dirty thresh and bg thresh,
+    *   so if the return value is two small, the thresh value maybe
+    *   calculated to 0.
+    *   As the max value of ratio is 100, so the return value is added
+    *   by 100 here.
+    */
+   return x + 100;



Thanks
Yafang

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