netdev - Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma

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Date: Wed, 16 Aug 2023 09:13:48 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux-foundation.org, 
	"Michael S. Tsirkin" <mst@...hat.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org, bpf@...r.kernel.org, 
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH vhost v13 05/12] virtio_ring: introduce virtqueue_dma_dev()

On Tue, Aug 15, 2023 at 5:40 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Tue, 15 Aug 2023 15:50:23 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Tue, Aug 15, 2023 at 2:32 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > >
> > >
> > > Hi, Jason
> > >
> > > Could you skip this patch?
> >
> > I'm fine with either merging or dropping this.
> >
> > >
> > > Let we review other patches firstly?
> >
> > I will be on vacation soon, and won't have time to do this until next week.
>
> Have a happly vacation.
>
> >
> > But I spot two possible "issues":
> >
> > 1) the DMA metadata were stored in the headroom of the page, this
> > breaks frags coalescing, we need to benchmark it's impact
>
> Not every page, just the first page of the COMP pages.
>
> So I think there is no impact.

Nope, see this:

        if (SKB_FRAG_PAGE_ORDER &&
            !static_branch_unlikely(&net_high_order_alloc_disable_key)) {
                /* Avoid direct reclaim but allow kswapd to wake */
                pfrag->page = alloc_pages((gfp & ~__GFP_DIRECT_RECLAIM) |
                                          __GFP_COMP | __GFP_NOWARN |
                                          __GFP_NORETRY,
                                          SKB_FRAG_PAGE_ORDER);
                if (likely(pfrag->page)) {
                        pfrag->size = PAGE_SIZE << SKB_FRAG_PAGE_ORDER;
                        return true;
                }
        }

The comp page might be disabled due to the SKB_FRAG_PAGE_ORDER and
net_high_order_alloc_disable_key.

>
>
> > 2) pre mapped DMA addresses were not reused in the case of XDP_TX/XDP_REDIRECT
>
> Because that the tx is not the premapped mode.

Yes, we can optimize this on top.

Thanks

>
> Thanks.
>
> >
> > I see Michael has merge this series so I'm fine to let it go first.
> >
> > Thanks
> >
> > >
> > > Thanks.
> > >
> >
>