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? 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. Thanks. > > Thanks >
