Re: [PATCH vhost v2 4/7] virtio_net: big mode support premapped

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Wed, 24 Apr 2024 10:34:56 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Wed, Apr 24, 2024 at 9:10 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > On Wed, 24 Apr 2024 08:43:21 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > On Tue, Apr 23, 2024 at 8:38 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > >
> > > > On Tue, 23 Apr 2024 12:36:42 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > > On Mon, Apr 22, 2024 at 3:24 PM Xuan Zhuo <xuanzhuo@linux.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@linux.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.
> >
> >
> > In the big mode, we will commit many pages to the virtio core by
> > virtqueue_add_inbuf().
> >
> > By virtqueue_get_buf_ctx(), we got the data. That is the first page.
> > Other pages are chained by the "private".
> >
> > If we use the page pool, how can we chain the pages.
> > After virtqueue_add_inbuf(), we need to get the pages to fill the skb.
>
> Right, technically it could be solved by providing helpers in the
> virtio core, but considering it's an optimization for big mode which
> is not popular, it's not worth to bother.
>
> >
> >
> >
> > >
> > > >
> > > > 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.
> >
> > No. we only need to store the "chain next" and the dma as this patch set did.
> > The size of the private space inside the page is  20(32bit)/40(64bit) bytes.
> > That is enough for us.
> >
> > If you worry about the change of the pp structure, we can use the "private" as
> > origin and use the "struct list_head lru" to store the dma.
>
> This looks even worse, as it uses fields belonging to the different
> structures in the union.

I mean we do not use the elems from the pp structure inside the page,
if we worry the change of the pp structure.

I mean use the "private" and "lru", these in the same structure.

I think this is a good way.

Thanks.

>
> >
> > The min size of "struct list_head lru" is 8 bytes, that is enough for the
> > dma_addr_t.
> >
> > We can do this more simper:
> >
> > static void page_chain_set_dma(struct page *p, dma_addr_t dma)
> > {
> >         BUILD_BUG_ON(sizeof(p->lru)) < sizeof(dma));
> >
> >         dma_addr_t *addr;
> >
> >         addr = &page->lru;
> >
> >         *addr = dma;
> > }
>
> So we had this in the kernel code.
>
>        /*
>          * Five words (20/40 bytes) are available in this union.
>          * WARNING: bit 0 of the first word is used for PageTail(). That
>          * means the other users of this union MUST NOT use the bit to
>          * avoid collision and false-positive PageTail().
>          */
>
> And by looking at the discussion that introduces the pp_magic, reusing
> fields seems to be tricky as we may end up with side effects of
> aliasing fields in page structure. Technically, we can invent new
> structures in the union, but it might not be worth it to bother.
>
> So I think we can leave the fallback code and revisit this issue in the future.
>
> Thanks
>
> >
> > Thanks.
> >
> >
> >
> > >
> > > Thanks
> > >
> > > >
> > > > Thanks.
> > > >
> > > >
> > > > >
> > > > > Thanks
> > > > >
> > > >
> > >
> >
>