On Sat, 10 Apr 2021 21:27:31 +0300 Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx> wrote: > On Sat, Apr 10, 2021 at 10:42:30AM -0700, Shakeel Butt wrote: > > > On Sat, Apr 10, 2021 at 9:16 AM Ilias Apalodimas > > <ilias.apalodimas@xxxxxxxxxx> wrote: > > > > > > Hi Matthew > > > > > > On Sat, Apr 10, 2021 at 04:48:24PM +0100, Matthew Wilcox wrote: > > > > On Sat, Apr 10, 2021 at 12:37:58AM +0200, Matteo Croce wrote: > > > > > This is needed by the page_pool to avoid recycling a page not allocated > > > > > via page_pool. > > > > > > > > Is the PageType mechanism more appropriate to your needs? It wouldn't > > > > be if you use page->_mapcount (ie mapping it to userspace). > > > > > > Interesting! > > > Please keep in mind this was written ~2018 and was stale on my branches for > > > quite some time. So back then I did try to use PageType, but had not free > > > bits. Looking at it again though, it's cleaned up. So yes I think this can > > > be much much cleaner. Should we go and define a new PG_pagepool? > > > > > > > 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?). (2) It feels wrong (security wise) to keep the DMA-mapping (for the device) and also map this page 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). (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. > > [1] https://lore.kernel.org/linux-mm/20210316013003.25271-1-arjunroy.kdev@xxxxxxxxx/ -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
