On Wed, Jan 30, 2019 at 09:02:08AM +0100, Christoph Hellwig wrote: > On Tue, Jan 29, 2019 at 08:58:35PM +0000, Jason Gunthorpe wrote: > > On Tue, Jan 29, 2019 at 01:39:49PM -0700, Logan Gunthorpe wrote: > > > > > implement the mapping. And I don't think we should have 'special' vma's > > > for this (though we may need something to ensure we don't get mapping > > > requests mixed with different types of pages...). > > > > I think Jerome explained the point here is to have a 'special vma' > > rather than a 'special struct page' as, really, we don't need a > > struct page at all to make this work. > > > > If I recall your earlier attempts at adding struct page for BAR > > memory, it ran aground on issues related to O_DIRECT/sgls, etc, etc. > > Struct page is what makes O_DIRECT work, using sgls or biovecs, etc on > it work. Without struct page none of the above can work at all. That > is why we use struct page for backing BARs in the existing P2P code. > Not that I'm a particular fan of creating struct page for this device > memory, but without major invasive surgery to large parts of the kernel > it is the only way to make it work. I don't think anyone is interested in O_DIRECT/etc for RDMA doorbell pages. .. and again, I recall Logan already attempted to mix non-CPU memory into sgls and it was a disaster. You pointed out that one cannot just put iomem addressed into a SGL without auditing basically the entire block stack to prove that nothing uses iomem without an iomem accessor. I recall that proposal veered into a direction where the block layer would just fail very early if there was iomem in the sgl, so generally no O_DIRECT support anyhow. We already accepted the P2P stuff from Logan as essentially a giant special case - it only works with RDMA and only because RDMA MR was hacked up with a special p2p callback. I don't see why a special case with a VMA is really that different. If someone figures out the struct page path down the road it can obviously be harmonized with this VMA approach pretty easily. Jason _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
