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

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Message-ID: <CACGkMEsC7AEi2SOmqNOo6KJDpx92raGWYwYzxZ_MVhmnco_LYQ@mail.gmail.com>
Date: Fri, 12 Apr 2024 12:47:55 +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>
> ---

Instead of doing a customized version of page pool, can we simply
switch to use page pool for big mode instead? Then we don't need to
bother the dma stuffs.

Thanks