Re: [PATCH net-next v7 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

[Jakub Kicinski]

On Thu, 17 Aug 2023 19:59:37 +0300 Ilias Apalodimas wrote:
> > Can we assume the DMA mapping of page pool is page aligned? We should
> > be, right?  
> 
> Yes
> 
> > That means we're storing 12 bits of 0 at the lower end.
> > So even with 32b of space we can easily store addresses for 32b+12b =>
> > 16TB of memory. "Ought to be enough" to paraphrase Bill G, and the
> > problem is only in our heads?  
> 
> Do you mean moving the pp_frag_count there? 

Right, IIUC we don't have enough space to fit dma_addr_t and the
refcount, but if we store the dma addr on a shifted u32 instead 
of using dma_addr_t explicitly - the refcount should fit?

> I was questioning the need to have PP_FLAG_PAGE_SPLIT_IN_DRIVER
> overall.  With Yunshengs patches such a platform would allocate a
> page, so why should we prevent it from splitting it internally?

Splitting it is fine, the problem is that the refcount AKA
page->pp_frag_count** counts outstanding PP-aware references
and page->refcount counts PP-unaware references.

If we want to use page->refcount directly we'd need to unmap 
the page whenever drivers calls page_pool_defrag_page().
But the driver may assume the page is still mapped afterwards.

We can change the API to make this behavior explicit. Although
IMHO that's putting the burden of rare platforms on non-rare
platforms which we should avoid.

** I said it before and I will keep saying this until someone gets 
   angry at me - I really think we should rename this field because
   the association with frags is a coincidence.