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Message-ID: <CACGkMEs=6Xfc1hELudF=+xvoJN+npQw11BqP0jjCxmUy2jaikg@mail.gmail.com>
Date: Wed, 24 Apr 2024 08:43:21 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux.dev, "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>, netdev@...r.kernel.org
Subject: Re: [PATCH vhost v2 4/7] virtio_net: big mode support premapped
On Tue, Apr 23, 2024 at 8:38 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Tue, 23 Apr 2024 12:36:42 +0800, Jason Wang <jasowang@...hat.com> wrote:
> > On Mon, Apr 22, 2024 at 3:24 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
> > >
> > > In big mode, pre-mapping DMA is beneficial because if the pages are not
> > > used, we can reuse them without needing to unmap and remap.
> > >
> > > We require space to store the DMA address. I use the page.dma_addr to
> > > store the DMA address from the pp structure inside the page.
> > >
> > > Every page retrieved from get_a_page() is mapped, and its DMA address is
> > > stored in page.dma_addr. When a page is returned to the chain, we check
> > > the DMA status; if it is not mapped (potentially having been unmapped),
> > > we remap it before returning it to the chain.
> > >
> > > Based on the following points, we do not use page pool to manage these
> > > pages:
> > >
> > > 1. virtio-net uses the DMA APIs wrapped by virtio core. Therefore,
> > > we can only prevent the page pool from performing DMA operations, and
> > > let the driver perform DMA operations on the allocated pages.
> > > 2. But when the page pool releases the page, we have no chance to
> > > execute dma unmap.
> > > 3. A solution to #2 is to execute dma unmap every time before putting
> > > the page back to the page pool. (This is actually a waste, we don't
> > > execute unmap so frequently.)
> > > 4. But there is another problem, we still need to use page.dma_addr to
> > > save the dma address. Using page.dma_addr while using page pool is
> > > unsafe behavior.
> > >
> > > More:
> > > https://lore.kernel.org/all/CACGkMEu=Aok9z2imB_c5qVuujSh=vjj1kx12fy9N7hqyi+M5Ow@mail.gmail.com/
> > >
> > > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > > ---
> > > drivers/net/virtio_net.c | 123 ++++++++++++++++++++++++++++++++++-----
> > > 1 file changed, 108 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 2c7a67ad4789..d4f5e65b247e 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -439,6 +439,81 @@ skb_vnet_common_hdr(struct sk_buff *skb)
> > > return (struct virtio_net_common_hdr *)skb->cb;
> > > }
> > >
> > > +static void sg_fill_dma(struct scatterlist *sg, dma_addr_t addr, u32 len)
> > > +{
> > > + sg->dma_address = addr;
> > > + sg->length = len;
> > > +}
> > > +
> > > +/* For pages submitted to the ring, we need to record its dma for unmap.
> > > + * Here, we use the page.dma_addr and page.pp_magic to store the dma
> > > + * address.
> > > + */
> > > +static void page_chain_set_dma(struct page *p, dma_addr_t addr)
> > > +{
> > > + if (sizeof(dma_addr_t) > sizeof(unsigned long)) {
> >
> > Need a macro like PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA.
> >
> > > + p->dma_addr = lower_32_bits(addr);
> > > + p->pp_magic = upper_32_bits(addr);
> >
> > And this uses three fields on page_pool which I'm not sure the other
> > maintainers are happy with. For example, re-using pp_maing might be
> > dangerous. See c07aea3ef4d40 ("mm: add a signature in struct page").
> >
> > I think a more safe way is to reuse page pool, for example introducing
> > a new flag with dma callbacks?
>
> If we use page pool, how can we chain the pages allocated for a packet?
I'm not sure I get this, it is chained via the descriptor flag.
>
> Yon know the "private" can not be used.
>
>
> If the pp struct inside the page is not safe, how about:
>
> struct { /* Page cache and anonymous pages */
> /**
> * @lru: Pageout list, eg. active_list protected by
> * lruvec->lru_lock. Sometimes used as a generic list
> * by the page owner.
> */
> union {
> struct list_head lru;
>
> /* Or, for the Unevictable "LRU list" slot */
> struct {
> /* Always even, to negate PageTail */
> void *__filler;
> /* Count page's or folio's mlocks */
> unsigned int mlock_count;
> };
>
> /* Or, free page */
> struct list_head buddy_list;
> struct list_head pcp_list;
> };
> /* See page-flags.h for PAGE_MAPPING_FLAGS */
> struct address_space *mapping;
> union {
> pgoff_t index; /* Our offset within mapping. */
> unsigned long share; /* share count for fsdax */
> };
> /**
> * @private: Mapping-private opaque data.
> * Usually used for buffer_heads if PagePrivate.
> * Used for swp_entry_t if PageSwapCache.
> * Indicates order in the buddy system if PageBuddy.
> */
> unsigned long private;
> };
>
> Or, we can map the private space of the page as a new structure.
It could be a way. But such allocation might be huge if we are using
indirect descriptors or I may miss something.
Thanks
>
> Thanks.
>
>
> >
> > Thanks
> >
>
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