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

[Alex Shi]

Ops, 

Sorry for missing Yosry and Nhat. How stupid I am!
Here is the first patch on lore: https://lore.kernel.org/all/20240628031138.429622-1-alexs@xxxxxxxxxx/

Sorry for this!
Alex

On 7/1/24 11:03 AM, Alex Shi wrote:
> 
> 
> On 6/30/24 9:45 PM, Hyeonggon Yoo wrote:
>> On Fri, Jun 28, 2024 at 12:06 PM <alexs@xxxxxxxxxx> wrote:
>>>
>>> From: Alex Shi <alexs@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxx>
>>> Signed-off-by: Alex Shi <alexs@xxxxxxxxxx>
>>> ---
>>>  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@xxxxxxxxxx>
>>> + *           Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
>>> + */
>>> +#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@xxxxxxxxxxxxxx/
> 
> 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