Re: [PATCH net-next v3 2/5] mm: add a signature in struct page

[Shakeel Butt]

On Sun, Apr 18, 2021 at 10:12 PM Ilias Apalodimas
<ilias.apalodimas@xxxxxxxxxx> wrote:
>
> On Wed, Apr 14, 2021 at 01:09:47PM -0700, Shakeel Butt wrote:
> > On Wed, Apr 14, 2021 at 12:42 PM Jesper Dangaard Brouer
> > <brouer@xxxxxxxxxx> wrote:
> > >
> > [...]
> > > > >
> > > > > Can this page_pool be used for TCP RX zerocopy? If yes then PageType
> > > > > can not be used.
> > > >
> > > > Yes it can, since it's going to be used as your default allocator for
> > > > payloads, which might end up on an SKB.
> > >
> > > I'm not sure we want or should "allow" page_pool be used for TCP RX
> > > zerocopy.
> > > For several reasons.
> > >
> > > (1) This implies mapping these pages page to userspace, which AFAIK
> > > means using page->mapping and page->index members (right?).
> > >
> >
> > No, only page->_mapcount is used.
> >
>
> I am not sure I like leaving out TCP RX zerocopy. Since we want driver to
> adopt the recycling mechanism we should try preserving the current
> functionality of the network stack.
>
> The question is how does it work with the current drivers that already have an
> internal page recycling mechanism.
>

I think the current drivers check page_ref_count(page) to decide to
reuse (or not) the already allocated pages.

Some examples from the drivers:
drivers/net/ethernet/intel/ixgbe/ixgbe_main.c:ixgbe_can_reuse_rx_page()
drivers/net/ethernet/intel/igb/igb_main.c:igb_can_reuse_rx_page()
drivers/net/ethernet/mellanox/mlx5/core/en_rx.c:mlx5e_rx_cache_get()

> > > (2) It feels wrong (security wise) to keep the DMA-mapping (for the
> > > device) and also map this page into userspace.
> > >
> >
> > I think this is already the case i.e pages still DMA-mapped and also
> > mapped into userspace.
> >
> > > (3) The page_pool is optimized for refcnt==1 case, and AFAIK TCP-RX
> > > zerocopy will bump the refcnt, which means the page_pool will not
> > > recycle the page when it see the elevated refcnt (it will instead
> > > release its DMA-mapping).
> >
> > Yes this is right but the userspace might have already consumed and
> > unmapped the page before the driver considers to recycle the page.
>
> Same question here. I'll have a closer look in a few days and make sure we are
> not breaking anything wrt zerocopy.
>

Pages mapped into the userspace have their refcnt elevated, so the
page_ref_count() check by the drivers indicates to not reuse such
pages.

> >
> > >
> > > (4) I remember vaguely that this code path for (TCP RX zerocopy) uses
> > > page->private for tricks.  And our patch [3/5] use page->private for
> > > storing xdp_mem_info.
> > >
> > > IMHO when the SKB travel into this TCP RX zerocopy code path, we should
> > > call page_pool_release_page() to release its DMA-mapping.
> > >
> >
> > I will let TCP RX zerocopy experts respond to this but from my high
> > level code inspection, I didn't see page->private usage.
>
> Shakeel are you aware of any 'easy' way I can have rx zerocopy running?
>

I would recommend tools/testing/selftests/net/tcp_mmap.c.