On Sun, Apr 18, 2021 at 10:12 PM Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx> wrote: > > On Wed, Apr 14, 2021 at 01:09:47PM -0700, Shakeel Butt wrote: > > On Wed, Apr 14, 2021 at 12:42 PM Jesper Dangaard Brouer > > <brouer@xxxxxxxxxx> wrote: > > > > > [...] > > > > > > > > > > Can this page_pool be used for TCP RX zerocopy? If yes then PageType > > > > > can not be used. > > > > > > > > Yes it can, since it's going to be used as your default allocator for > > > > payloads, which might end up on an SKB. > > > > > > I'm not sure we want or should "allow" page_pool be used for TCP RX > > > zerocopy. > > > For several reasons. > > > > > > (1) This implies mapping these pages page to userspace, which AFAIK > > > means using page->mapping and page->index members (right?). > > > > > > > No, only page->_mapcount is used. > > > > I am not sure I like leaving out TCP RX zerocopy. Since we want driver to > adopt the recycling mechanism we should try preserving the current > functionality of the network stack. > > The question is how does it work with the current drivers that already have an > internal page recycling mechanism. > I think the current drivers check page_ref_count(page) to decide to reuse (or not) the already allocated pages. Some examples from the drivers: drivers/net/ethernet/intel/ixgbe/ixgbe_main.c:ixgbe_can_reuse_rx_page() drivers/net/ethernet/intel/igb/igb_main.c:igb_can_reuse_rx_page() drivers/net/ethernet/mellanox/mlx5/core/en_rx.c:mlx5e_rx_cache_get() > > > (2) It feels wrong (security wise) to keep the DMA-mapping (for the > > > device) and also map this page into userspace. > > > > > > > I think this is already the case i.e pages still DMA-mapped and also > > mapped into userspace. > > > > > (3) The page_pool is optimized for refcnt==1 case, and AFAIK TCP-RX > > > zerocopy will bump the refcnt, which means the page_pool will not > > > recycle the page when it see the elevated refcnt (it will instead > > > release its DMA-mapping). > > > > Yes this is right but the userspace might have already consumed and > > unmapped the page before the driver considers to recycle the page. > > Same question here. I'll have a closer look in a few days and make sure we are > not breaking anything wrt zerocopy. > Pages mapped into the userspace have their refcnt elevated, so the page_ref_count() check by the drivers indicates to not reuse such pages. > > > > > > > > (4) I remember vaguely that this code path for (TCP RX zerocopy) uses > > > page->private for tricks. And our patch [3/5] use page->private for > > > storing xdp_mem_info. > > > > > > IMHO when the SKB travel into this TCP RX zerocopy code path, we should > > > call page_pool_release_page() to release its DMA-mapping. > > > > > > > I will let TCP RX zerocopy experts respond to this but from my high > > level code inspection, I didn't see page->private usage. > > Shakeel are you aware of any 'easy' way I can have rx zerocopy running? > I would recommend tools/testing/selftests/net/tcp_mmap.c.
