Re: [PATCH 01/20] mm/zsmalloc: add zpdesc memory descriptor for zswap.zpool

[Alex Shi <seakeel@gmail.com>]

On 7/1/24 9:42 PM, Yosry Ahmed wrote:
> [..]
>>>> +       union {
>>>> +               /* Next zpdescs in a zspage in zsmalloc zpool */
>>>> +               struct zpdesc *next;
>>>> +               /* For huge zspage in zsmalloc zpool */
>>>> +               unsigned long handle;
>>>> +       };
>>>> +       struct zspage *zspage;
>>>
>>> There was a discussion with Yosry on including memcg_data on zpdesc
>>> even if it's not used at the moment.
>>>
>>> Maybe you can look at:
>>> https://lore.kernel.org/linux-mm/CAB=+i9Quz9iP2-Lq=oQfKVVnzPDtOaKMm=hUPbnRg5hRxH+qaA@mail.gmail.com/
>>
>> Thanks for notice.
>> The memcg_data isn't used for zpdesc. And I have a bit confusion since Yosry said: "I think to drop memcg_data we need to enlighten the code that ...". So we actually don't need to have this unused member, is this right, Yosry?
> 
> I think we need to have it, at least for now. When the pages are
> freed, put_page() -> folio_put() -> __folio_put() will call
> mem_cgroup_uncharge(). The latter will call folio_memcg() (which reads
> folio->memcg_data) to figure out if uncharging needs to be done.
> 
> There are also other similar code paths that will check
> folio->memcg_data. It is currently expected to be present for all
> folios. So until we have custom code paths per-folio type for
> allocation/freeing/etc, we need to keep folio->memcg_data present and
> properly initialized.

Hi Yosry,

Many thanks for detailed comments and requirements! 
I'm going to add this member in next version, maybe tomorrow, unless there are more comments for this patchset. :)

Thank a lot!
Alex