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

[Alex Shi <seakeel@gmail.com>]

On 6/30/24 9:45 PM, Hyeonggon Yoo wrote:
> On Fri, Jun 28, 2024 at 12:06 PM <alexs@kernel.org> wrote:
>>
>> From: Alex Shi <alexs@kernel.org>
>>
>> The 1st patch introduces new memory decriptor zpdesc and rename
>> zspage.first_page to zspage.first_zpdesc, no functional change.
>>
>> Originally-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
>> Signed-off-by: Alex Shi <alexs@kernel.org>
>> ---
>>  mm/zpdesc.h   | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>  mm/zsmalloc.c | 19 ++++++++---------
>>  2 files changed, 66 insertions(+), 9 deletions(-)
>>  create mode 100644 mm/zpdesc.h
> 
> Hi Alex, thanks for your effort in pushing this forward!
> 
>> diff --git a/mm/zpdesc.h b/mm/zpdesc.h
>> new file mode 100644
>> index 000000000000..a1ab5ebaa936
>> --- /dev/null
>> +++ b/mm/zpdesc.h
>> @@ -0,0 +1,56 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/* zpdesc.h: zswap.zpool memory descriptor
>> + *
>> + * Written by Alex Shi <alexs@kernel.org>
>> + *           Hyeonggon Yoo <42.hyeyoo@gmail.com>
>> + */
>> +#ifndef __MM_ZPDESC_H__
>> +#define __MM_ZPDESC_H__
>> +
>> +/*
>> + * struct zpdesc -     Memory descriptor for zpool memory, now is for zsmalloc
>> + * @flags:             Page flags, PG_private: identifies the first component page
>> + * @lru:               Indirected used by page migration
> 
> maybe Indirected -> Indirectly?

Hi Yoo,

Thanks for comments! Yes Indirectly is better. I will update it in next version.

> 
>> + * @next:              Next zpdesc in a zspage in zsmalloc zpool
>> + * @handle:            For huge zspage in zsmalloc zpool
>> + * @zspage:            Pointer to zspage in zsmalloc
>> + *
>> + * This struct overlays struct page for now. Do not modify without a good
>> + * understanding of the issues.
>> + */
>> +struct zpdesc {
>> +       unsigned long flags;
>> +       struct list_head lru;
>> +       unsigned long _zp_pad_1;
> 
> for understanding, I think it'd be better to replace _zp_pad_1 with movable ops,
> because mops reuses this 'mapping' field.

Right, 'mops' looks a bit more clear.

> 
>> +       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?

> 
>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>> index fec1a39e5bbe..67bb80b7413a 100644
>> --- a/mm/zsmalloc.c
>> +++ b/mm/zsmalloc.c
>> @@ -13,17 +13,17 @@
>>
>>  /*
>>   * Following is how we use various fields and flags of underlying
>> - * struct page(s) to form a zspage.
>> + * struct zpdesc(page) to form a zspage.
>>   *
>> - * Usage of struct page fields:
>> - *     page->private: points to zspage
>> - *     page->index: links together all component pages of a zspage
>> + * Usage of struct zpdesc fields:
>> + *     zpdesc->zspage: points to zspage
>> + *     zpdesc->next: links together all component pages of a zspage
>>   *             For the huge page, this is always 0, so we use this field
>>   *             to store handle.
>>   *     page->page_type: PG_zsmalloc, lower 16 bit locate the first object
>>   *             offset in a subpage of a zspage
>>   *
>> - * Usage of struct page flags:
>> + * Usage of struct zpdesc(page) flags:
>>   *     PG_private: identifies the first component page
>>   *     PG_owner_priv_1: identifies the huge component page
> 
> the comment for PG_owner_priv_1 can safely be removed as it's not used
> after commit a41ec880aa7b ("zsmalloc: move huge compressed obj from
> page to zspage")

Right, thanks for info!

> 
>> @@ -948,7 +949,7 @@ static void create_page_chain(struct size_class *class, struct zspage *zspage,
>>                 set_page_private(page, (unsigned long)zspage);
>>                 page->index = 0;
>>                 if (i == 0) {
>> -                       zspage->first_page = page;
>> +                       zspage->first_zpdesc = page_zpdesc(page);
>>                         SetPagePrivate(page);
>>                         if (unlikely(class->objs_per_zspage == 1 &&
>>                                         class->pages_per_zspage == 1))
>> @@ -1325,7 +1326,7 @@ static unsigned long obj_malloc(struct zs_pool *pool,
>>                 link->handle = handle;
>>         else
>>                 /* record handle to page->index */
>> -               zspage->first_page->index = handle;
>> +               zspage->first_zpdesc->handle = handle;
> 
> FYI this line seems to conflict with
> bcc6116e39f512 ("mm/zsmalloc: move record_obj() into obj_malloc()")
> on mm-unstable.

yes, a new commit made this conflict. will update this in next version.

Thanks
Alex
> 
> Best,
> Hyeonggon