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. > (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. > > (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.
