On Thu, Jul 08, 2021 at 08:29:56AM -0700, Alexander Duyck wrote: > On Thu, Jul 8, 2021 at 8:17 AM Ilias Apalodimas > <ilias.apalodimas@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > The above expectation is based on that the last user will always > > > > > > > > > call page_pool_put_full_page() in order to do the recycling or do > > > > > > > > > the resource cleanup(dma unmaping..etc). > > > > > > > > > > > > > > > > > > As the skb_free_head() and skb_release_data() have both checked the > > > > > > > > > skb->pp_recycle to call the page_pool_put_full_page() if needed, I > > > > > > > > > think we are safe for most case, the one case I am not so sure above > > > > > > > > > is the rx zero copy, which seems to also bump up the refcnt before > > > > > > > > > mapping the page to user space, we might need to ensure rx zero copy > > > > > > > > > is not the last user of the page or if it is the last user, make sure > > > > > > > > > it calls page_pool_put_full_page() too. > > > > > > > > > > > > > > > > Yes, but the skb->pp_recycle value is per skb, not per page. So my > > > > > > > > concern is that carrying around that value can be problematic as there > > > > > > > > are a number of possible cases where the pages might be > > > > > > > > unintentionally recycled. All it would take is for a packet to get > > > > > > > > cloned a few times and then somebody starts using pskb_expand_head and > > > > > > > > you would have multiple cases, possibly simultaneously, of entities > > > > > > > > trying to free the page. I just worry it opens us up to a number of > > > > > > > > possible races. > > > > > > > > > > > > > > Maybe I missde something, but I thought the cloned SKBs would never trigger > > > > > > > the recycling path, since they are protected by the atomic dataref check in > > > > > > > skb_release_data(). What am I missing? > > > > > > > > > > > > Are you talking about the head frag? So normally a clone wouldn't > > > > > > cause an issue because the head isn't changed. In the case of the > > > > > > head_frag we should be safe since pskb_expand_head will just kmalloc > > > > > > the new head and clears head_frag so it won't trigger > > > > > > page_pool_return_skb_page on the head_frag since the dataref just goes > > > > > > from 2 to 1. > > > > > > > > > > > > The problem is that pskb_expand_head memcopies the page frags over and > > > > > > takes a reference on the pages. At that point you would have two skbs > > > > > > both pointing to the same set of pages and each one ready to call > > > > > > page_pool_return_skb_page on the pages at any time and possibly racing > > > > > > with the other. > > > > > > > > > > Ok let me make sure I get the idea properly. > > > > > When pskb_expand_head is called, the new dataref will be 1, but the > > > > > head_frag will be set to 0, in which case the recycling code won't be > > > > > called for that skb. > > > > > So you are mostly worried about a race within the context of > > > > > pskb_expand_skb() between copying the frags, releasing the previous head > > > > > and preparing the new one (on a cloned skb)? > > > > > > > > The race is between freeing the two skbs. So the original and the > > > > clone w/ the expanded head will have separate instances of the page. I > > > > am pretty certain there is a race if the two of them start trying to > > > > free the page frags at the same time. > > > > > > > > > > Right, I completely forgot calling __skb_frag_unref() before releasing the > > > head ... > > > You are right, this will be a race. Let me go back to the original mail > > > thread and see what we can do > > > > > > > What do you think about resetting pp_recycle bit on pskb_expand_head()? > > I assume you mean specifically in the cloned case? > Yes. Even if we do it unconditionally we'll just loose non-cloned buffers from the recycling. I'll send a patch later today. > > If my memory serves me right Eric wanted that from the beginning. Then the > > cloned/expanded SKB won't trigger the recycling. If that skb hits the free > > path first, we'll end up recycling the fragments eventually. If the > > original one goes first, we'll just unmap the page(s) and freeing the cloned > > one will free all the remaining buffers. > > I *think* that should be fine. Effectively what we are doing is making > it so that if the original skb is freed first the pages are released, > and if it is released after the clone/expended skb then it can be > recycled. Exactly > > The issue is we have to maintain it so that there will be exactly one > caller of the recycling function for the pages. So any spot where we > are updating skb->head we will have to see if there is a clone and if > so we have to clear the pp_recycle flag on our skb so that it doesn't > try to recycle the page frags as well. Correct. I'll keep looking around in case there's something less fragile we can do Thanks /Ilias
