[RFC]mm/zsmalloc,: trigger BUG_ON in function zs_map_object.

[RFC]mm/zsmalloc,: trigger BUG_ON in function zs_map_object.

[Yisheng Xie]

Hi, all,

We had backport the no-lru migration to linux-4.1, meanwhile change the
ZS_MAX_ZSPAGE_ORDER to 3. Then we met a BUG_ON(!page[1]).

It rarely happen, and presently, what I get is:
[6823.316528s]obj=a160701f, obj_idx=15, class{size:2176,objs_per_zspage:15,pages_per_zspage:8}
[...]
[6823.316619s]BUG: failure at /home/ethan/kernel/linux-4.1/mm/zsmalloc.c:1458/zs_map_object()! ----> BUG_ON(!page[1])

It seems that we have allocated an object from a ZS_FULL group?
(Actuallyi 1/4 ? I do not get the inuse number of this zspage, which I am trying to.)
And presently, I can not find why it happened. Any idea about it?

Any comment is more than welcome!

Thanks
Yisheng Xie



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Re: [RFC]mm/zsmalloc,: trigger BUG_ON in function zs_map_object.

[Minchan Kim]

Hello,

On Tue, Mar 28, 2017 at 03:20:22PM +0800, Yisheng Xie wrote:
> Hi, all,
> 
> We had backport the no-lru migration to linux-4.1, meanwhile change the
> ZS_MAX_ZSPAGE_ORDER to 3. Then we met a BUG_ON(!page[1]).

Hmm, I don't know how you backported.

There isn't any problem with default ZS_MAX_ZSPAGE_ORDER. Right?
So, it happens only if you changed it to 3?

Could you tell me what is your base kernel? and what zram/zsmalloc
version(ie, from what kernel version) you backported to your
base kernel?

> 
> It rarely happen, and presently, what I get is:
> [6823.316528s]obj=a160701f, obj_idx=15, class{size:2176,objs_per_zspage:15,pages_per_zspage:8}
> [...]
> [6823.316619s]BUG: failure at /home/ethan/kernel/linux-4.1/mm/zsmalloc.c:1458/zs_map_object()! ----> BUG_ON(!page[1])
> 
> It seems that we have allocated an object from a ZS_FULL group?
> (Actually， I do not get the inuse number of this zspage, which I am trying to.)
> And presently, I can not find why it happened. Any idea about it?

Although it happens rarely, always above same symptom once it happens?

> 
> Any comment is more than welcome!
> 
> Thanks
> Yisheng Xie
> 
> 
> 
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Re: [RFC]mm/zsmalloc,: trigger BUG_ON in function zs_map_object.

[Sergey Senozhatsky]

On (03/29/17 09:20), Minchan Kim wrote:
> Hello,
> 
> On Tue, Mar 28, 2017 at 03:20:22PM +0800, Yisheng Xie wrote:
> > Hi, all,
> > 
> > We had backport the no-lru migration to linux-4.1, meanwhile change the
> > ZS_MAX_ZSPAGE_ORDER to 3. Then we met a BUG_ON(!page[1]).
> 
> Hmm, I don't know how you backported.
> 
> There isn't any problem with default ZS_MAX_ZSPAGE_ORDER. Right?
> So, it happens only if you changed it to 3?

I agree with Minchan. too much things could have gone wrong during the backport.

> Could you tell me what is your base kernel? and what zram/zsmalloc
> version(ie, from what kernel version) you backported to your
> base kernel?

agree again.



Yisheng, do you have this commit applied?

commit c102f07ca0b04f2cb49cfc161c83f6239d17f491
Author: Junil Lee <junil0814.lee@lge.com>
Date:   Wed Jan 20 14:58:18 2016 -0800

    zsmalloc: fix migrate_zspage-zs_free race condition


	-ss

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Re: [RFC]mm/zsmalloc,: trigger BUG_ON in function zs_map_object.

[Yisheng Xie]

Hi Sergey,

Thanks for your comment!
On 2017/3/29 14:42, Sergey Senozhatsky wrote:
> On (03/29/17 09:20), Minchan Kim wrote:
>> Hello,
>>
>> On Tue, Mar 28, 2017 at 03:20:22PM +0800, Yisheng Xie wrote:
>>> Hi, all,
>>>
>>> We had backport the no-lru migration to linux-4.1, meanwhile change the
>>> ZS_MAX_ZSPAGE_ORDER to 3. Then we met a BUG_ON(!page[1]).
>>
>> Hmm, I don't know how you backported.
>>
>> There isn't any problem with default ZS_MAX_ZSPAGE_ORDER. Right?
>> So, it happens only if you changed it to 3?
> 
> I agree with Minchan. too much things could have gone wrong during the backport.
> 
>> Could you tell me what is your base kernel? and what zram/zsmalloc
>> version(ie, from what kernel version) you backported to your
>> base kernel?
> 
> agree again.
> 
> 
> 
> Yisheng, do you have this commit applied?
No, we missed this patch, I will try it. Really thanks for that.

Thanks
Yisheng Xie

> 
> commit c102f07ca0b04f2cb49cfc161c83f6239d17f491
> Author: Junil Lee <junil0814.lee@lge.com>
> Date:   Wed Jan 20 14:58:18 2016 -0800
> 
>     zsmalloc: fix migrate_zspage-zs_free race condition
> 
> 
> 	-ss
> 
> .
> 

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Re: [RFC]mm/zsmalloc,: trigger BUG_ON in function zs_map_object.

[Yisheng Xie]

Hi Minchan,

Thanks for your comment!
On 2017/3/29 8:20, Minchan Kim wrote:
> Hello,
> 
> On Tue, Mar 28, 2017 at 03:20:22PM +0800, Yisheng Xie wrote:
>> Hi, all,
>>
>> We had backport the no-lru migration to linux-4.1, meanwhile change the
>> ZS_MAX_ZSPAGE_ORDER to 3. Then we met a BUG_ON(!page[1]).
> 
> Hmm, I don't know how you backported.
Yes, maybe caused by our unsuitable backport.

> 
> There isn't any problem with default ZS_MAX_ZSPAGE_ORDER. Right?
> So, it happens only if you changed it to 3?
I will check whether it will default ZS_MAX_ZSPAGE_ORDER.

> 
> Could you tell me what is your base kernel? and what zram/zsmalloc
> version(ie, from what kernel version) you backported to your
> base kernel?
> 
We backport from kernel v4.8-rc8 to kernel v4.1.

>>
>> It rarely happen, and presently, what I get is:
>> [6823.316528s]obj=a160701f, obj_idx=15, class{size:2176,objs_per_zspage:15,pages_per_zspage:8}
>> [...]
>> [6823.316619s]BUG: failure at /home/ethan/kernel/linux-4.1/mm/zsmalloc.c:1458/zs_map_object()! ----> BUG_ON(!page[1])
>>
>> It seems that we have allocated an object from a ZS_FULL group?
>> (Actuallyi 1/4 ? I do not get the inuse number of this zspage, which I am trying to.)
>> And presently, I can not find why it happened. Any idea about it?
> 
> Although it happens rarely, always above same symptom once it happens?
Yes , though the class size is not the same, which means not from the same class.
however, the (obj_idx == objs_per_zspage) is always true.

Thanks
Yisheng Xie.

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