Re: netmem series needs some love and Acks from MM folks

[David Hildenbrand <david@redhat.com>]

On 17.06.25 04:31, Harry Yoo wrote:
> On Fri, Jun 13, 2025 at 07:19:07PM -0700, Mina Almasry wrote:
>> On Thu, Jun 12, 2025 at 6:13 PM Byungchul Park <byungchul@sk.com> wrote:
>>>
>>> On Wed, Jun 11, 2025 at 06:55:42PM -0700, Jakub Kicinski wrote:
>>>> On Tue, 10 Jun 2025 10:30:01 +0900 Byungchul Park wrote:
>>>>>> What's the intended relation between the types?
>>>>>
>>>>> One thing I'm trying to achieve is to remove pp fields from struct page,
>>>>> and make network code use struct netmem_desc { pp fields; } instead of
>>>>> sturc page for that purpose.
>>>>>
>>>>> The reason why I union'ed it with the existing pp fields in struct
>>>>> net_iov *temporarily* for now is, to fade out the existing pp fields
>>>>> from struct net_iov so as to make the final form like:
>>>>
>>>> I see, I may have mixed up the complaints there. I thought the effort
>>>> was also about removing the need for the ref count. And Rx is
>>>> relatively light on use of ref counting.
>>>>
>>>>>> netmem_ref exists to clearly indicate that memory may not be readable.
>>>>>> Majority of memory we expect to allocate from page pool must be
>>>>>> kernel-readable. What's the plan for reading the "single pointer"
>>>>>> memory within the kernel?
>>>>>>
>>>>>> I think you're approaching this problem from the easiest and least
>>>>>
>>>>> No, I've never looked for the easiest way.  My bad if there are a better
>>>>> way to achieve it.  What would you recommend?
>>>>
>>>> Sorry, I don't mean that the approach you took is the easiest way out.
>>>> I meant that between Rx and Tx handling Rx is the easier part because
>>>> we already have the suitable abstraction. It's true that we use more
>>>> fields in page struct on Rx, but I thought Tx is also more urgent
>>>> as there are open reports for networking taking references on slab
>>>> pages.
>>>>
>>>> In any case, please make sure you maintain clear separation between
>>>> readable and unreadable memory in the code you produce.
>>>
>>> Do you mean the current patches do not?  If yes, please point out one
>>> as example, which would be helpful to extract action items.
>>>
>>
>> I think one thing we could do to improve separation between readable
>> (pages/netmem_desc) and unreadable (net_iov) is to remove the struct
>> netmem_desc field inside the net_iov, and instead just duplicate the
>> pp/pp_ref_count/etc fields. The current code gives off the impression
>> that net_iov may be a container of netmem_desc which is not really
>> accurate.
>>
>> But I don't think that's a major blocker. I think maybe the real issue
>> is that there are no reviews from any mm maintainers?
> 
> Let's try changing the subject to draw some attention from MM people :)

Hi, it worked! :P

I hope Willy will find his way to this thread as well.

> 
>> So I'm not 100%
>> sure this is in line with their memdesc plans. I think probably
>> patches 2->8 are generic netmem-ifications that are good to merge
>> anyway, but I would say patch 1 and 9 need a reviewed by from someone
>> on the mm side. Just my 2 cents.
> 
> As someone who worked on the zpdesc series, I think it is pretty much
> in line with the memdesc plans.
> 
> I mean, it does differ a bit from the initial idea of generalizing it as
> "bump" allocator, but overall, it's still aligned with the memdesc
> plans, and looks like a starting point, IMHO.

Just to summarize (not that there is any misunderstanding), the first 
step of the memdesc plan is simple:

1) have a dedicated data-structure we will allocate alter dynamically.

2) Make it overlay "struct page" for now in a way that doesn't break things

3) Convert all users of "struct page" to the new data-structure

Later, the memdesc data-structure will then actually come be allocated 
dynamically, so "struct page" content will not apply anymore, and we can 
shrink "struct page".


What I see in this patch is exactly 1) and 2).

I am not 100% sure about existing "struct net_iov" and how that 
interacts with "struct page" overlay. I suspects it's just a dynamically 
allocated structure?

Because this patch changes the layout of "struct net_iov", which is a 
bit confusing at first sight?

-- 
Cheers,

David / dhildenb