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

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

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;
>         }
>
> [  259.641324] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0
> [  317.798913] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0
> [  317.798935] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0
> [  317.976210] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0
> [  417.781194] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0
> [  466.322615] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0
> [  466.322618] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0
> [  466.497893] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0
> [  466.504687] 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->vail=0 vm_dirty_bytes=0 dirty_background_bytes=0

Hi Tetsuo,

Bellow is the code analysis,
     // global_dirtyable_memory() will return number of globally dirtyable page
    gdtc.avail = global_dirtyable_memory();
    domain_dirty_limits(&gdtc);
        unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;   // 1228
        unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE)
/ 100;  // 409
        available_memory = dtc->avail;
        thresh = (ratio * available_memory) / PAGE_SIZE;
        bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;


So if available_memory is less than 4 pages, thresh and bg_thresh will
be both 0,
then the message will be printed.

Thanks
Yafang

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>