On Wed, 14 Apr 2021 13:09:47 -0700 Shakeel Butt <shakeelb@xxxxxxxxxx> 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. Good to know. I will admit that I don't fully understand the usage of page->mapping and page->index members. > > (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. True, other drivers are doing the same. > > (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. That is a good point. So, there is a race window where it is possible to gain recycling. It seems my page_pool co-maintainer Ilias is interested in taking up the challenge to get this working with TCP RX zerocopy. So, lets see how this is doable. > > > > (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. I trust when Eric says page->private isn't used in this code path. So, it might actually be possible :-) I will challenge Ilias and Matteo to pull this off. (But I know that both of them are busy for personal reasons, so be patient with them). -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
