Re: [PATCH v7 06/15] net: page pool: add helper creating area from pages

[Mina Almasry <almasrymina@google.com>]

On Fri, Nov 1, 2024 at 11:16 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 11/1/24 17:33, Mina Almasry wrote:
> > On Tue, Oct 29, 2024 at 4:06 PM David Wei <dw@davidwei.uk> wrote:
> >>
> >> From: Pavel Begunkov <asml.silence@gmail.com>
> >>
> >> Add a helper that takes an array of pages and initialises passed in
> >> memory provider's area with them, where each net_iov takes one page.
> >> It's also responsible for setting up dma mappings.
> >>
> >> We keep it in page_pool.c not to leak netmem details to outside
> >> providers like io_uring, which don't have access to netmem_priv.h
> >> and other private helpers.
> >>
> >
> > I honestly prefer leaking netmem_priv.h details into the io_uring
> > rather than having io_uring provider specific code in page_pool.c.
>
> Even though Jakub didn't comment on this patch, but he definitely
> wasn't fond of giving all those headers to non net/ users. I guess
> I can't please everyone. One middle option is to make the page
> pool helper more granular, i.e. taking care of one netmem at
> a time, and moving the loop to io_uring, but I don't think it
> changes anything.
>

My issue is that these:

+int page_pool_mp_init_paged_area(struct page_pool *pool,
+                               struct net_iov_area *area,
+                               struct page **pages);
+void page_pool_mp_release_area(struct page_pool *pool,

Are masquerading as generic functions to be used by many mp but
they're really io_uring specific. dmabuf and the hugepage provider
would not use them AFAICT. Would have liked not to have code specific
to one mp in page_pool.c, and I was asked to move the dmabuf specific
functions to another file too IIRC.

These helpers depend on:

page_pool_set_pp_info: in netmem_priv.h
net_iov_to_netmem(niov): in netmem.h
page_pool_dma_map_page: can be put in page_pool/helpers.h?
page_pool_release_page_dma(pool, netmem):  can be put in page_pool/helpers.h?

I would prefer moving page_pool_set_pp_info (and the stuff it calls
into) to netmem.h and removing io_uring mp specific code from
page_pool.c.

> ...
> >>   #include <linux/dma-direction.h>
> >> @@ -459,7 +460,8 @@ page_pool_dma_sync_for_device(const struct page_pool *pool,
> >>                  __page_pool_dma_sync_for_device(pool, netmem, dma_sync_size);
> >>   }
> >>
> >> -static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem)
> >> +static bool page_pool_dma_map_page(struct page_pool *pool, netmem_ref netmem,
> >> +                                  struct page *page)
> >
> > I have to say this is confusing for me. Passing in both the netmem and
> > the page is weird. The page is the one being mapped and the
> > netmem->dma_addr is the one being filled with the mapping.
>
> the page argument provides a mapping, the netmem gives the object
> where the mapping is set. netmem could be the same as the page
> argument, but I don't think it's inherently wrong, and it's an
> internal helper anyway. I can entirely copy paste the function, I
> don't think it's anyhow an improvement.
>
> > Netmem is meant to be an abstraction over page. Passing both makes
> > little sense to me. The reason you're doing this is because the
> > io_uring memory provider is in a bit of a weird design IMO where the
> > memory comes in pages but it doesn't want to use paged-backed-netmem.
>
> Mina, as explained it before, I view it rather as an abstraction
> that helps with finer grained control over memory and extending
> it this way, I don't think it's such a stretch, and it doesn't
> change much for the networking stack overall. Not fitting into
> devmem TCP category doesn't make it weird.
>
> > Instead it uses net_iov-backed-netmem and there is an out of band page
> > to be managed.
> >
> > I think it would make sense to use paged-backed-netmem for your use
> > case, or at least I don't see why it wouldn't work. Memory providers
>
> It's a user page, we can't make assumptions about it, we can't
> reuse space in struct page like for pp refcounting (unlike when
> it's allocated by the kernel), we can't use the normal page
> refcounting.
>

You actually can't reuse space in struct net_iov either for your own
refcounting, because niov->pp_ref_count is a mirror to
page->pp_ref_count and strictly follows the patterns of that. But
that's the issue to be discussed on the other patch...

> If that's the direction people prefer, we can roll back to v1 from
> a couple years ago, fill skbs fill user pages, attach ubuf_info to
> every skb, and whack-a-mole'ing all places where the page could be
> put down or such, pretty similarly what net_iov does. Honestly, I
> thought that reusing common infra so that the net stack doesn't
> need a different path per interface was a good idea.
>

The common infra does support page-backed-netmem actually.

> > were designed to handle the hugepage usecase where the memory
> > allocated by the provider is pages. Is there a reason that doesn't
> > work for you as well?
> >
> > If you really need to use net_iov-backed-netmem, can we put this
> > weirdness in the provider? I don't know that we want a generic-looking
> > dma_map function which is a bit confusing to take a netmem and a page.>
> ...
> >> +
> >> +static void page_pool_release_page_dma(struct page_pool *pool,
> >> +                                      netmem_ref netmem)
> >> +{
> >> +       __page_pool_release_page_dma(pool, netmem);
> >> +}
> >> +
> >
> > Is this wrapper necessary? Do you wanna rename the original function
> > to remove the __ instead of a adding a wrapper?
>
> I only added it here to cast away __always_inline since it's used in
> a slow / setup path. It shouldn't change the binary, but I'm not a huge
> fan of leaving the hint for the code where it's not needed.
>

Right, it probably makes sense to make the modifications you want on
the original function rather than create a no-op wrapper to remove the
__always_inline.


--
Thanks,
Mina