On Wed, 24 Apr 2024 10:45:49 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote: > On Wed, Apr 24, 2024 at 10:42 AM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote: > > > > On Wed, 24 Apr 2024 10:34:56 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > On Wed, Apr 24, 2024 at 9:10 AM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > On Wed, 24 Apr 2024 08:43:21 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > > > On Tue, Apr 23, 2024 at 8:38 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > On Tue, 23 Apr 2024 12:36:42 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > > > > > On Mon, Apr 22, 2024 at 3:24 PM Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx/ > > > > > > > > > > > > > > > > Signed-off-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> > > > > > > > > --- > > > > > > > > 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. > > See this: > > https://lore.kernel.org/netdev/20210411114307.5087f958@carbon/ I think that is because that the page pool will share the page with the skbs. I'm not entirely sure. In our case, virtio-net fully owns the page. After the page is referenced by skb, virtio-net no longer references the page. I don't think there is any problem here. The key is that who owns the page, who can use the page private space (20/40 bytes). Is that? Thanks. > > Thanks >
