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

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

On Fri, 12 Apr 2024 12:47:55 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Thu, Apr 11, 2024 at 10:51 AM Xuan Zhuo <xuanzhuo@linux.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@linux.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.


The page pool needs to do the dma by the DMA APIs.
So we can not use the page pool directly.

Thanks.


>
> Thanks
>