On Tue, 1 Jun 2021 11:27:12 +0800, Jason Wang <[email protected]> wrote:
>
> 在 2021/6/1 上午11:08, Xuan Zhuo 写道:
> > On Tue, 1 Jun 2021 11:03:37 +0800, Jason Wang <[email protected]> wrote:
> >> 在 2021/5/31 下午6:58, Xuan Zhuo 写道:
> >>> On Mon, 31 May 2021 14:10:55 +0800, Jason Wang <[email protected]>
> >>> wrote:
> >>>> 在 2021/5/14 下午11:16, Xuan Zhuo 写道:
> >>>>> In the case of merge, the page passed into page_to_skb() may be a head
> >>>>> page, not the page where the current data is located.
> >>>> I don't get how this can happen?
> >>>>
> >>>> Maybe you can explain a little bit more?
> >>>>
> >>>> receive_mergeable() call page_to_skb() in two places:
> >>>>
> >>>> 1) XDP_PASS for linearized page , in this case we use xdp_page
> >>>> 2) page_to_skb() for "normal" page, in this case the page contains the
> >>>> data
> >>> The offset may be greater than PAGE_SIZE, because page is obtained by
> >>> virt_to_head_page(), not the page where buf is located. And "offset" is
> >>> the offset
> >>> of buf relative to page.
> >>>
> >>> tailroom = truesize - len - offset;
> >>>
> >>> In this case, the tailroom must be less than 0. Although there may be
> >>> enough
> >>> content on this page to save skb_shared_info.
> >>
> >> Interesting, I think we don't use compound pages for virtio-net. (We
> >> don't define SKB_FRAG_PAGE_ORDER).
> >>
> >> Am I wrong?
> >
> > It seems to me that it seems to be a fixed setting, not for us to configure
> > independently
>
>
> Looks like you are right.
>
> See comments below.
>
>
> >
> > Thanks.
> >
> > ==========================================
> >
> > net/sock.c
> >
> > #define SKB_FRAG_PAGE_ORDER get_order(32768)
> > DEFINE_STATIC_KEY_FALSE(net_high_order_alloc_disable_key);
> >
> > /**
> > * skb_page_frag_refill - check that a page_frag contains enough room
> > * @sz: minimum size of the fragment we want to get
> > * @pfrag: pointer to page_frag
> > * @gfp: priority for memory allocation
> > *
> > * Note: While this allocator tries to use high order pages, there is
> > * no guarantee that allocations succeed. Therefore, @sz MUST be
> > * less or equal than PAGE_SIZE.
> > */
> > bool skb_page_frag_refill(unsigned int sz, struct page_frag *pfrag, gfp_t
> > gfp)
> > {
> > if (pfrag->page) {
> > if (page_ref_count(pfrag->page) == 1) {
> > pfrag->offset = 0;
> > return true;
> > }
> > if (pfrag->offset + sz <= pfrag->size)
> > return true;
> > put_page(pfrag->page);
> > }
> >
> > pfrag->offset = 0;
> > if (SKB_FRAG_PAGE_ORDER &&
> > !static_branch_unlikely(&net_high_order_alloc_disable_key)) {
> > /* Avoid direct reclaim but allow kswapd to wake */
> > pfrag->page = alloc_pages((gfp & ~__GFP_DIRECT_RECLAIM) |
> > __GFP_COMP | __GFP_NOWARN |
> > __GFP_NORETRY,
> > SKB_FRAG_PAGE_ORDER);
> > if (likely(pfrag->page)) {
> > pfrag->size = PAGE_SIZE << SKB_FRAG_PAGE_ORDER;
> > return true;
> > }
> > }
> > pfrag->page = alloc_page(gfp);
> > if (likely(pfrag->page)) {
> > pfrag->size = PAGE_SIZE;
> > return true;
> > }
> > return false;
> > }
> > EXPORT_SYMBOL(skb_page_frag_refill);
> >
> >
> >> Thanks
> >>
> >>
> >>> Thanks.
> >>>
> >>>> Thanks
> >>>>
> >>>>
> >>>>> So when trying to
> >>>>> get the buf where the data is located, you should directly use the
> >>>>> pointer(p) to get the address corresponding to the page.
> >>>>>
> >>>>> At the same time, the offset of the data in the page should also be
> >>>>> obtained using offset_in_page().
> >>>>>
> >>>>> This patch solves this problem. But if you don’t use this patch, the
> >>>>> original code can also run, because if the page is not the page of the
> >>>>> current data, the calculated tailroom will be less than 0, and will not
> >>>>> enter the logic of build_skb() . The significance of this patch is to
> >>>>> modify this logical problem, allowing more situations to use
> >>>>> build_skb().
> >>>>>
> >>>>> Signed-off-by: Xuan Zhuo <[email protected]>
> >>>>> Acked-by: Michael S. Tsirkin <[email protected]>
> >>>>> ---
> >>>>> drivers/net/virtio_net.c | 8 ++++++--
> >>>>> 1 file changed, 6 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >>>>> index 3e46c12dde08..073fec4c0df1 100644
> >>>>> --- a/drivers/net/virtio_net.c
> >>>>> +++ b/drivers/net/virtio_net.c
> >>>>> @@ -407,8 +407,12 @@ static struct sk_buff *page_to_skb(struct
> >>>>> virtnet_info *vi,
> >>>>> * see add_recvbuf_mergeable() + get_mergeable_buf_len()
> >>>>> */
> >>>>> truesize = PAGE_SIZE;
> >>>>> - tailroom = truesize - len - offset;
> >>>>> - buf = page_address(page);
> >>>>> +
> >>>>> + /* page maybe head page, so we should get the buf by p,
> >>>>> not the
> >>>>> + * page
> >>>>> + */
> >>>>> + tailroom = truesize - len - offset_in_page(p);
>
>
> I wonder why offset_in_page(p) is correct? I guess it should be:
>
> tailroom = truesize - len - headroom;
>
> The reason is that the buffer is not necessarily allocated at the page
> boundary.
In my understanding, offset_in_page(p) is the offset of p in the page where it
is located. In this case, the two should be equal. This has nothing to do with
which page is allocated.
Of course I think using headroom is a good idea, and it is semantically better.
Thanks.
>
> Thanks
>
>
> >>>>> + buf = (char *)((unsigned long)p & PAGE_MASK);
> >>>>> } else {
> >>>>> tailroom = truesize - len;
> >>>>> buf = p;
>
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