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[156.34.48.30]) by smtp.gmail.com with ESMTPSA id b12sm974336qtj.12.2020.11.05.06.22.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Nov 2020 06:22:07 -0800 (PST) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1kag9O-00HNUk-MI; Thu, 05 Nov 2020 10:22:06 -0400 Date: Thu, 5 Nov 2020 10:22:06 -0400 From: Jason Gunthorpe To: "Xiong, Jianxin" Cc: "linux-rdma@vger.kernel.org" , "dri-devel@lists.freedesktop.org" , Doug Ledford , Leon Romanovsky , Sumit Semwal , Christian Koenig , "Vetter, Daniel" Subject: Re: [PATCH v7 4/5] RDMA/mlx5: Support dma-buf based userspace memory region Message-ID: <20201105142206.GA36674@ziepe.ca> References: <1604527595-39736-1-git-send-email-jianxin.xiong@intel.com> <1604527595-39736-5-git-send-email-jianxin.xiong@intel.com> <20201105000721.GY36674@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-rdma@vger.kernel.org On Thu, Nov 05, 2020 at 12:36:25AM +0000, Xiong, Jianxin wrote: > > From: Jason Gunthorpe > > Sent: Wednesday, November 04, 2020 4:07 PM > > To: Xiong, Jianxin > > Cc: linux-rdma@vger.kernel.org; dri-devel@lists.freedesktop.org; Doug Ledford ; Leon Romanovsky > > ; Sumit Semwal ; Christian Koenig ; Vetter, Daniel > > > > Subject: Re: [PATCH v7 4/5] RDMA/mlx5: Support dma-buf based userspace memory region > > > > On Wed, Nov 04, 2020 at 02:06:34PM -0800, Jianxin Xiong wrote: > > > + umem = ib_umem_dmabuf_get(&dev->ib_dev, offset, length, fd, access_flags, > > > + &mlx5_ib_dmabuf_attach_ops); > > > + if (IS_ERR(umem)) { > > > + mlx5_ib_dbg(dev, "umem get failed (%ld)\n", PTR_ERR(umem)); > > > + return ERR_PTR(PTR_ERR(umem)); > > > + } > > > + > > > + mr = alloc_mr_from_cache(pd, umem, virt_addr, access_flags); > > > > It is very subtle, but this calls mlx5_umem_find_best_pgsz() which calls ib_umem_find_best_pgsz() which goes over the SGL to determine > > the page size to use. > > > > When this is called here, the umem sglist is still NULL because dma_buf_map_attachment() > is not called until a page fault occurs. In patch 1/5, the function ib_umem_find_best_pgsz() > has been modified to always return PAGE_SIZE for dma-buf based MR. Oh.. That isn't a good idea. ib_umem_find_best_pgsz() must be run on any SGL list to validate it against the constraints, making it un-runable for the dmabuf case means we can never support large page size or even validate that the SGL is properly formed. So I think this need to change the alloc_mr_from_cache() to early exit for dma_buf ones And it still need to call ib_umem_find_best_pgsz() but just check the page size. > > Edit the last SGE to have a reduced length > > Do you still think modifying the SGL in place needed given the above > explanation? I do see some benefits of doing so -- hiding the > discrepancy of sgl and addr/length from the device drivers and avoid > special handling in the code that use the sgl. Yes, a umem SGL should always be properly formed or I will have a meltdown trying to keep all the drivers working :\ Jason