Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for custom page providers

[Mina Almasry]

On Mon, Jun 3, 2024 at 7:52 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
>
> On 6/3/24 15:17, Mina Almasry wrote:
> > On Fri, May 31, 2024 at 10:35 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> >>
> >> On Thu, May 30, 2024 at 08:16:01PM +0000, Mina Almasry wrote:
> >>> I'm unsure if the discussion has been resolved yet. Sending the series
> >>> anyway to get reviews/feedback on the (unrelated) rest of the series.
> >>
> >> As far as I'm concerned it is not.  I've not seen any convincing
> >> argument for more than page/folio allocator including larger order /
> >> huge page and dmabuf.
> >>
> >
> > Thanks Christoph, this particular patch series adds dmabuf, so I
> > assume no objection there. I assume the objection is that you want the
> > generic, extensible hooks removed.
> >
> > To be honest, I don't think the hooks are an integral part of the
> > design, and at this point I think we've argued for them enough. I
> > think we can easily achieve the same thing with just raw if statements
> > in a couple of places. We can always add the hooks if and only if we
> > actually justify many memory providers.
> >
> > Any objections to me removing the hooks and directing to memory
> > allocations via simple if statements? Something like (very rough
> > draft, doesn't compile):
>
> The question for Christoph is what exactly is the objection here? Why we
> would not be using well defined ops when we know there will be more
> users? Repeating what I said in the last thread, for io_uring it's used
> to implement the flow of buffers from userspace to the kernel, the ABI,
> which is orthogonal to the issue of what memory type it is and how it
> came there. And even if you mandate unnecessary dmabuf condoms for user
> memory in one form or another IMHO for no clear reason, the callbacks
> (or yet another if-else) would still be needed.
>
> Sure, Mina can drop and hard code devmem path to easy the pain for
> him and delay the discussion, but then shortly after I will be
> re-sending same shit.

You don't need to re-send the same ops again, right? You can add io
uring support without ops. Something like:

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 92be1aaf18ccc..2cc986455bce6 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -557,8 +557,8 @@ netmem_ref page_pool_alloc_netmem(struct page_pool
*pool, gfp_t gfp)
                return netmem;

        /* Slow-path: cache empty, do real allocation */
-       if (static_branch_unlikely(&page_pool_mem_providers) && pool->mp_ops)
-               netmem = pool->mp_ops->alloc_pages(pool, gfp);
+       if (unlikely(page_pool_is_dmabuf(pool)))
+               netmem = mp_dmabuf_devmem_alloc_pages():
+       else if (unlikely(page_pool_is_iouring(pool)))
+               netmem = mp_io_uring_alloc_pages():
       else
                netmem = __page_pool_alloc_pages_slow(pool, gfp);
        return netmem;

So IMO, the ops themselves, which Christoph is repeatedly nacking, are
not that important.

I humbly think the energy should be spent convincing maintainers of
the use case of io uring memory, not the ops. The ops are a cosmetic
change to the code, and can be added later. Christoph is nacking the
ops because it gives people too much rope [1].

But if you disagree and think the ops themselves are important for a
reason I missed, I'm happy waiting until agreement is reached here.
Sorry, just voicing my 2 cents.

[1] https://lore.kernel.org/netdev/ZjjHUh1eINPg1wkn@xxxxxxxxxxxxx/

-- 
Thanks,
Mina