On Wed, Jul 7, 2021 at 12:03 PM Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx> wrote: > > > > Hi, Alexander > > > > > > Thanks for detailed reviewing. > > > > > Likewise! > I'll have a look on the entire conversation in a few days... > > > > > > > > > So this isn't going to work with the current recycling logic. The > > > > expectation there is that we can safely unmap the entire page as soon > > > > as the reference count is greater than 1. > > > > > > Yes, the expectation is changed to we can always recycle the page > > > when the last user has dropped the refcnt that has given to it when > > > the page is not pfmemalloced. > > > > > > 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. I suspect if they both called it at roughly the same time one of them would trigger a NULL pointer dereference since they would both check pp_magic first, and then both set pp to NULL. If run on a system where dma_unmap_page_attrs takes a while it would be very likely to race since pp_magic doesn't get cleared until after the page is unmapped.
