From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-112.freemail.mail.aliyun.com (out30-112.freemail.mail.aliyun.com [115.124.30.112]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 130E183A19 for ; Wed, 24 Apr 2024 02:41:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.112 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713926512; cv=none; b=dDTNab+qQl6z1Ygn/ID6UWzOrpZf4h1znRSx+vC8llkvkgXNGkqVComGdj2uv9MIZKiZSRg4wuoOTkJA6nhSc6qIkzL/AfDddNBUdVAhB+z5OrvPUgLBfHuUgt/lD78YEhKMxDMVOY84A9utk5TDfJvx2vhgINb7ayky7U5VGOk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713926512; c=relaxed/simple; bh=buTiMV2ftYlqE79E0LoBGYdK2Bh3yp3XWW5qg4wrkiI=; h=Message-ID:Subject:Date:From:To:Cc:References:In-Reply-To: Content-Type; b=NRM4LtwvtTgaZY2+b9U3H/PRSJGWfOlpbZQR91Qdrs49HB4xNeFW9u2188McFzRNp93H+nbur0dwfWlGZGkE/w8GIEpJtSJ0lQK/HKUFAvZHuEwqciylfF1rs4DE5WxhTSClfxjZ1/VvLLAO9iQwZ3Sj9lR/229z+nPL/EDscR4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=hOKx1lZ/; arc=none smtp.client-ip=115.124.30.112 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="hOKx1lZ/" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1713926507; h=Message-ID:Subject:Date:From:To:Content-Type; bh=TEVqWVDSoS3QAyhMSLk4j1xzN2LSXFSWa8yzE48gNCE=; b=hOKx1lZ/7/SNKnw1og1ON68qz9JOmxfiqZ9NSY/dw45GXFL6fbjtNa7xhImkVVR9Wdj7i0VfQZyFN/CBzT1ag9VM07CxLD3QbTNQmPBZq2A6Uejl7KnLnQAGf75hZc9PyiISN+7lCzd+ivV/RoDbBPylF/+v92qn62rBz1oFlWE= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R661e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033037067113;MF=xuanzhuo@linux.alibaba.com;NM=1;PH=DS;RN=8;SR=0;TI=SMTPD_---0W5AbrlT_1713926505; Received: from localhost(mailfrom:xuanzhuo@linux.alibaba.com fp:SMTPD_---0W5AbrlT_1713926505) by smtp.aliyun-inc.com; Wed, 24 Apr 2024 10:41:46 +0800 Message-ID: <1713926353.64557-3-xuanzhuo@linux.alibaba.com> Subject: Re: [PATCH vhost v2 4/7] virtio_net: big mode support premapped Date: Wed, 24 Apr 2024 10:39:13 +0800 From: Xuan Zhuo To: Jason Wang Cc: virtualization@lists.linux.dev, "Michael S. Tsirkin" , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org References: <20240422072408.126821-1-xuanzhuo@linux.alibaba.com> <20240422072408.126821-5-xuanzhuo@linux.alibaba.com> <1713875473.8690095-1-xuanzhuo@linux.alibaba.com> <1713919985.3490202-1-xuanzhuo@linux.alibaba.com> In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: On Wed, 24 Apr 2024 10:34:56 +0800, Jason Wang wrote: > On Wed, Apr 24, 2024 at 9:10=E2=80=AFAM Xuan Zhuo wrote: > > > > On Wed, 24 Apr 2024 08:43:21 +0800, Jason Wang wr= ote: > > > On Tue, Apr 23, 2024 at 8:38=E2=80=AFPM Xuan Zhuo wrote: > > > > > > > > On Tue, 23 Apr 2024 12:36:42 +0800, Jason Wang wrote: > > > > > On Mon, Apr 22, 2024 at 3:24=E2=80=AFPM Xuan Zhuo 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_a= ddr to > > > > > > store the DMA address from the pp structure inside the page. > > > > > > > > > > > > Every page retrieved from get_a_page() is mapped, and its DMA a= ddress 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 un= mapped), > > > > > > we remap it before returning it to the chain. > > > > > > > > > > > > Based on the following points, we do not use page pool to manag= e these > > > > > > pages: > > > > > > > > > > > > 1. virtio-net uses the DMA APIs wrapped by virtio core. Therefo= re, > > > > > > we can only prevent the page pool from performing DMA operat= ions, 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 p= utting > > > > > > the page back to the page pool. (This is actually a waste, w= e 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 p= ool is > > > > > > unsafe behavior. > > > > > > > > > > > > More: > > > > > > https://lore.kernel.org/all/CACGkMEu=3DAok9z2imB_c5qVuujSh= =3Dvjj1kx12fy9N7hqyi+M5Ow@mail.gmail.com/ > > > > > > > > > > > > Signed-off-by: Xuan Zhuo > > > > > > --- > > > > > > 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 add= r, u32 len) > > > > > > +{ > > > > > > + sg->dma_address =3D addr; > > > > > > + sg->length =3D 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 t= he 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 =3D lower_32_bits(addr); > > > > > > + p->pp_magic =3D upper_32_bits(addr); > > > > > > > > > > And this uses three fields on page_pool which I'm not sure the ot= her > > > > > 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 introd= ucing > > > > > a new flag with dma callbacks? > > > > > > > > If we use page pool, how can we chain the pages allocated for a pac= ket? > > > > > > 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 prot= ected by > > > > * lruvec->lru_lock. Sometimes used as a g= eneric list > > > > * by the page owner. > > > > */ > > > > union { > > > > struct list_head lru; > > > > > > > > /* Or, for the Unevictable "LRU lis= t" slot */ > > > > struct { > > > > /* Always even, to negate P= ageTail */ > > > > 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 offs= et within mapping. */ > > > > unsigned long share; /* share co= unt for fsdax */ > > > > }; > > > > /** > > > > * @private: Mapping-private opaque data. > > > > * Usually used for buffer_heads if PagePri= vate. > > > > * Used for swp_entry_t if PageSwapCache. > > > > * Indicates order in the buddy system if P= ageBuddy. > > > > */ > > > > 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 se= t did. > > The size of the private space inside the page is 20(32bit)/40(64bit) b= ytes. > > That is enough for us. > > > > If you worry about the change of the pp structure, we can use the "priv= ate" 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. > > > > > The min size of "struct list_head lru" is 8 bytes, that is enough for t= he > > dma_addr_t. > > > > We can do this more simper: > > > > static void page_chain_set_dma(struct page *p, dma_addr_t dma) > > { > > BUILD_BUG_ON(sizeof(p->lru)) < sizeof(dma)); > > > > dma_addr_t *addr; > > > > addr =3D &page->lru; > > > > *addr =3D dma; > > } > > So we had this in the kernel code. > > /* > * Five words (20/40 bytes) are available in this union. > * WARNING: bit 0 of the first word is used for PageTail(). That > * means the other users of this union MUST NOT use the bit to > * avoid collision and false-positive PageTail(). > */ > > And by looking at the discussion that introduces the pp_magic, reusing > fields seems to be tricky as we may end up with side effects of > aliasing fields in page structure. Technically, we can invent new > structures in the union, but it might not be worth it to bother. > > So I think we can leave the fallback code and revisit this issue in the f= uture. > > Thanks > > > > > Thanks. > > > > > > > > > > > > Thanks > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > >