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a question about slub in function __slab_free()

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  a question about slub in function __slab_free() Xishi Qiu <qiuxishi@huawei.com> - 2016-03-02 13:40 +0100
    Re: a question about slub in function __slab_free() Joonsoo Kim <js1304@gmail.com> - 2016-03-02 15:40 +0100
      Re: a question about slub in function __slab_free() Xishi Qiu <qiuxishi@huawei.com> - 2016-03-03 02:40 +0100
    Re: a question about slub in function __slab_free() Jianyu Zhan <nasa4836@gmail.com> - 2016-03-02 16:20 +0100

#1348002 — a question about slub in function __slab_free()

___slab_alloc()
	deactivate_slab()
		add_full(s, n, page);
The page will be added to full list and the frozen is 0, right?

__slab_free()
	prior = page->freelist;  // prior is NULL
	was_frozen = new.frozen;  // was_frozen is 0
	...
		/*
		 * Slab was on no list before and will be
		 * partially empty
		 * We can defer the list move and instead
		 * freeze it.
		 */
		new.frozen = 1;
	...

I don't understand why "Slab was on no list before"?

Thanks,
Xishi Qiu

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#1348090

2016-03-02 21:26 GMT+09:00 Xishi Qiu <qiuxishi@huawei.com>:
> ___slab_alloc()
>         deactivate_slab()
>                 add_full(s, n, page);
> The page will be added to full list and the frozen is 0, right?
>
> __slab_free()
>         prior = page->freelist;  // prior is NULL
>         was_frozen = new.frozen;  // was_frozen is 0
>         ...
>                 /*
>                  * Slab was on no list before and will be
>                  * partially empty
>                  * We can defer the list move and instead
>                  * freeze it.
>                  */
>                 new.frozen = 1;
>         ...
>
> I don't understand why "Slab was on no list before"?

add_full() is defined only for CONFIG_SLUB_DEBUG.
And, actual add happens if slub_debug=u is enabled.
In other cases, fully used slab isn't attached on any list.

Thanks.

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#1348670

On 2016/3/2 22:38, Joonsoo Kim wrote:

> 2016-03-02 21:26 GMT+09:00 Xishi Qiu <qiuxishi@huawei.com>:
>> ___slab_alloc()
>>         deactivate_slab()
>>                 add_full(s, n, page);
>> The page will be added to full list and the frozen is 0, right?
>>
>> __slab_free()
>>         prior = page->freelist;  // prior is NULL
>>         was_frozen = new.frozen;  // was_frozen is 0
>>         ...
>>                 /*
>>                  * Slab was on no list before and will be
>>                  * partially empty
>>                  * We can defer the list move and instead
>>                  * freeze it.
>>                  */
>>                 new.frozen = 1;
>>         ...
>>
>> I don't understand why "Slab was on no list before"?
> 
> add_full() is defined only for CONFIG_SLUB_DEBUG.
> And, actual add happens if slub_debug=u is enabled.
> In other cases, fully used slab isn't attached on any list.
> 
> Thanks.
> 

Hi Joonsoo,

You are right, thank you very much!

> 

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#1348142

On Wed, Mar 2, 2016 at 8:26 PM, Xishi Qiu <qiuxishi@huawei.com> wrote:
> __slab_free()
>         prior = page->freelist;  // prior is NULL
>         was_frozen = new.frozen;  // was_frozen is 0
>         ...
>                 /*
>                  * Slab was on no list before and will be
>                  * partially empty
>                  * We can defer the list move and instead
>                  * freeze it.
>                  */
>                 new.frozen = 1;
>         ...
>
> I don't understand why "Slab was on no list before"?

in this  __slab_free() code path, we are freeing an object to a remote CPU.

Consider the condition that leads to this branch, that is:

new.inuse  && !prior && !was_frozen

new.inuse means that slab page has free objects after we do this free operiton.

!prior && !was_frozen together means  that slab page has previously
depleted all objects
and forgotten(SLUB don't remember a slab page that has got all its
object allocated).

All these 3 conditions mean that,

A slab page on a remote CPU has got all tis objects allocated and it
was forgotten by SLUB,
so  "Slab was on no list before",

and then at the present, we (on local CPU) are freeing object back to
that CPU,  that "will make
the slab page partially empty".

But we don't bother to immediately add it back to the node partial
list( to avoid the list->lock contention),
so "we can defer the list move".

But how do we handle this?  Easy, just mark it frozen,  and latter
that CPU's  per-cpu freelist queue will
use it.


Regards,
Jianyu Zhan

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