netdev - Re: [PATCH vhost 3/6] virtio_net: replace private by pp struct inside page

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Message-ID: <CACGkMEuWTpMSQVMvWgp2+wDPtRDuKiqFQdsSEnd9ors+E3iDWw@mail.gmail.com>
Date: Thu, 18 Apr 2024 14:11:10 +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 3/6] virtio_net: replace private by pp struct inside page

On Thu, Apr 11, 2024 at 10:51 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> Now, we chain the pages of big mode by the page's private variable.
> But a subsequent patch aims to make the big mode to support
> premapped mode. This requires additional space to store the dma addr.
>
> Within the sub-struct that contains the 'private', there is no suitable
> variable for storing the DMA addr.
>
>                 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;
>                 };
>
> But within the page pool struct, we have a variable called
> dma_addr that is appropriate for storing dma addr.
> And that struct is used by netstack. That works to our advantage.
>
>                 struct {        /* page_pool used by netstack */
>                         /**
>                          * @pp_magic: magic value to avoid recycling non
>                          * page_pool allocated pages.
>                          */
>                         unsigned long pp_magic;
>                         struct page_pool *pp;
>                         unsigned long _pp_mapping_pad;
>                         unsigned long dma_addr;
>                         atomic_long_t pp_ref_count;
>                 };
>
> On the other side, we should use variables from the same sub-struct.
> So this patch replaces the "private" with "pp".
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
>  drivers/net/virtio_net.c | 27 +++++++++++++++------------
>  1 file changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c22d1118a133..4446fb54de6d 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -48,6 +48,9 @@ module_param(napi_tx, bool, 0644);
>
>  #define VIRTIO_XDP_FLAG        BIT(0)
>
> +#define page_chain_next(p)     ((struct page *)((p)->pp))
> +#define page_chain_add(p, n)   ((p)->pp = (void *)n)
> +
>  /* RX packet size EWMA. The average packet size is used to determine the packet
>   * buffer size when refilling RX rings. As the entire RX ring may be refilled
>   * at once, the weight is chosen so that the EWMA will be insensitive to short-
> @@ -191,7 +194,7 @@ struct receive_queue {
>
>         struct virtnet_interrupt_coalesce intr_coal;
>
> -       /* Chain pages by the private ptr. */
> +       /* Chain pages by the page's pp struct. */
>         struct page *pages;
>
>         /* Average packet length for mergeable receive buffers. */
> @@ -432,16 +435,16 @@ skb_vnet_common_hdr(struct sk_buff *skb)
>  }
>
>  /*
> - * private is used to chain pages for big packets, put the whole
> - * most recent used list in the beginning for reuse
> + * put the whole most recent used list in the beginning for reuse
>   */

While at this, let's explain the pp is used to chain pages or we can
do it on the definition of page_chain_add().

Others look good.

Acked-by: Jason Wang <jasowang@...hat.com>

Thanks