netdev - Re: [PATCH v2] mm, page_pool: introduce a new page type for page pool in page type

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Message-ID: <20250729010410.GC56089@system.software.com>
Date: Tue, 29 Jul 2025 10:04:10 +0900
From: Byungchul Park <byungchul@...com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Mina Almasry <almasrymina@...gle.com>, linux-mm@...ck.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v2] mm, page_pool: introduce a new page type for page
 pool in page type

On Mon, Jul 28, 2025 at 07:49:30PM +0100, Pavel Begunkov wrote:
> On 7/28/25 19:39, Mina Almasry wrote:
> > On Mon, Jul 28, 2025 at 11:35 AM Pavel Begunkov <asml.silence@...il.com> wrote:
> > > 
> > > On 7/28/25 06:27, Byungchul Park wrote:
> > > > Changes from v1:
> > > >        1. Rebase on linux-next.
> > > 
> > > net-next is closed, looks like until August 11.
> > > 
> > > >        2. Initialize net_iov->pp = NULL when allocating net_iov in
> > > >           net_devmem_bind_dmabuf() and io_zcrx_create_area().
> > > >        3. Use ->pp for net_iov to identify if it's pp rather than
> > > >           always consider net_iov as pp.
> > > >        4. Add Suggested-by: David Hildenbrand <david@...hat.com>.
> > > 
> > > Oops, looks you killed my suggested-by tag now. Since it's still
> > > pretty much my diff spliced with David's suggestions, maybe
> > > Co-developed-by sounds more appropriate. Even more so goes for
> > > the second patch getting rid of __netmem_clear_lsb().
> > > 
> > > Looks fine, just one comment below.
> > > 
> > > ...> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
> > > > index 100b75ab1e64..34634552cf74 100644
> > > > --- a/io_uring/zcrx.c
> > > > +++ b/io_uring/zcrx.c
> > > > @@ -444,6 +444,7 @@ static int io_zcrx_create_area(struct io_zcrx_ifq *ifq,
> > > >                area->freelist[i] = i;
> > > >                atomic_set(&area->user_refs[i], 0);
> > > >                niov->type = NET_IOV_IOURING;
> > > > +             niov->pp = NULL;
> > > 
> > > It's zero initialised, you don't need it.
> > > 
> > 
> > This may be my bad since I said we should check if it's 0 initialized.
> > 
> > It looks like on the devmem side as well we kvmalloc_array the niovs,
> > and if I'm checking through the helpers right, kvmalloc_array does
> > 0-initialize indeed.
> 
> I wouldn't rely on that, it's just for zcrx I do:
> 
> kvmalloc_array(...,  GFP_KERNEL | __GFP_ZERO);

For net_devmem_bind_dmabuf(), __GFP_ZERO will add bigger overhead than
just assignment, 'niov->pp = NULL'.

I'd like to ask you if you are still good with __GFP_ZERO overhead
before going ahead.

	Byungchul

> 
> --
> Pavel Begunkov