Subject: Re: [net-next PATCH 1/2] mm: add dma_addr_t to struct page

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 12 Feb 2019 11:06:20 +0100

On Mon, 11 Feb 2019 08:55:51 -0800
Matthew Wilcox <[email-masked]> wrote:

> On Mon, Feb 11, 2019 at 05:06:46PM +0100, Jesper Dangaard Brouer wrote:
> > The page_pool API is using page->private to store DMA addresses.
> > As pointed out by David Miller we can't use that on 32-bit architectures
> > with 64-bit DMA
> > 
> > This patch adds a new dma_addr_t struct to allow storing DMA addresses
> > 
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > Signed-off-by: Ilias Apalodimas <[email-masked]>  
> 
> Reviewed-by: Matthew Wilcox <[email-masked]>
> 
> > +		struct {	/* page_pool used by netstack */
> > +			/**
> > +			 * @dma_addr: Page_pool need to store DMA-addr, and  
> 
> s/need/needs/
> 
> > +			 * cannot use @private, as DMA-mappings can be 64-bit  
> 
> s/DMA-mappings/DMA addresses/
> 
> > +			 * even on 32-bit Architectures.  
> 
> s/A/a/

Yes, that comments needs improvement. I think I'll use AKPMs suggestion.

> > +			 */
> > +			dma_addr_t dma_addr; /* Shares area with @lru */  
> 
> It also shares with @slab_list, @next, @compound_head, @pgmap and
> @rcu_head.  I think it's pointless to try to document which other fields
> something shares space with; the places which do it are a legacy from
> before I rearranged struct page last year.  Anyone looking at this should
> now be able to see "Oh, this is a union, only use the fields which are
> in the union for the type of struct page I have here".

I agree, I'll strip that comment.

> Are the pages allocated from this API ever supposed to be mapped to
> userspace?

I would like to know what fields on struct-page we cannot touch if we
want to keep this a possibility?

That said, I hope we don't need to do this. But as I integrate this
further into the netstack code, we might have to support this, or
at-least release the page_pool "state" (currently only DMA-addr) before
the skb_zcopy code path.  First iteration will not do zero-copy stuff,
and later I'll coordinate with Willem how to add this, if needed.

My general opinion is that if an end-user want to have pages mapped to
userspace, then page_pool (MEM_TYPE_PAGE_POOL) is not the right choice,
but instead use MEM_TYPE_ZERO_COPY (see enum xdp_mem_type).  We are
generally working towards allowing NIC drivers to have a different
memory type per RX-ring.

> You also say in the documentation:
> 
>  * If no DMA mapping is done, then it can act as shim-layer that
>  * fall-through to alloc_page.  As no state is kept on the page, the
>  * regular put_page() call is sufficient.
> 
> I think this is probably a dangerous precedent to set.  Better to require
> exactly one call to page_pool_put_page() (with the understanding that the
> refcount may be elevated, so this may not be the final free of the page,
> but the page will no longer be usable for its page_pool purpose).

Yes, this actually how it is implemented today, and the comment should
be improved.  Today __page_pool_put_page() in case of refcount is
elevated do call __page_pool_clean_page() to release page page_pool
state, and is in principle no longer "usable" for page_pool purposes.
BUT I have considered removing this, as it might not fit how want to
use the API. In our current RFC we found a need for (and introduced) a
page_pool_unmap_page() call (that call __page_pool_clean_page()), when
driver hits cases where the code path doesn't have a call-back to
page_pool_put_page() but instead end-up calling put_page().

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer