On Wed, Mar 24, 2021 at 11:26 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > On Tue, Mar 23, 2021 at 11:42 AM Arjun Roy <arjunroy@xxxxxxxxxx> wrote: > > > [...] > > > > To summarize then, it seems to me that we're on the same page now. > > I'll put together a tentative v3 such that: > > 1. It uses pre-charging, as previously discussed. > > 2. It uses a page flag to delineate pages of a certain networking sort > > (ie. this mechanism). > > 3. It avails itself of up to 4 words of data inside struct page, > > inside the networking specific struct. > > 4. And it sets up this opt-in lifecycle notification for drivers that > > choose to use it, falling back to existing behaviour without. > > > > Arjun, if you don't mind, can you explain how the lifetime of such a > page will look like? > > For example: > > Driver: > page = dev_alloc_page() > /* page has 1 ref */ Yes, this is the case. > dev_map_page(page) > /* I don't think dev_map_page() takes a ref on page, so the ref remains 1. */ > To be clear, do you mean things like DMA setup here? Or specifically what do you mean by dev_map_page? > On incoming traffic the page goes to skb and which then gets assigned > to a struct sock. Does the kernel increase refcnt of the page on these > operations? > Adding a page to an skb will mean that, when the skb is cleaned up, a page ref is dropped: https://github.com/torvalds/linux/blob/master/net/core/skbuff.c#L666 So a driver may bump the refcount for the page, before adding it to the skb: https://github.com/torvalds/linux/blob/master/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c#L442 > The page gets mapped into user space which increments its refcnt. > Yes. > After processing the data, the application unmaps the page and its > refcnt will be decremented. > Yes. > __put_page() will be called when refcnt reaches 0, so, the initial > refcnt which the driver has acquired, has to be transferred to the > next layer. So, I am trying to understand how that will work? Ah, I see - there was a miscommunication. Johannes mentioned __put_page() but I read put_page(). That is where I was planning on adding the interposition for these network pages. So in put_page(), if it turns out it's a network page, we do our handling then as I described in prior emails. Sorry for the confusion. Thanks, -Arjun
