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([2620:15c:2cd:203:5cdc:422c:7b28:ebb5]) by smtp.gmail.com with ESMTPSA id n83-v6sm318808pfk.19.2018.09.24.13.54.02 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 24 Sep 2018 13:54:03 -0700 (PDT) Message-ID: <1537822441.195115.32.camel@acm.org> Subject: Re: block: DMA alignment of IO buffer allocated from slab From: Bart Van Assche To: Matthew Wilcox Cc: Andrey Ryabinin , Ming Lei , Vitaly Kuznetsov , Christoph Hellwig , Ming Lei , linux-block , linux-mm , Linux FS Devel , "open list:XFS FILESYSTEM" , Dave Chinner , Linux Kernel Mailing List , Jens Axboe , Christoph Lameter , Linus Torvalds , Greg Kroah-Hartman Date: Mon, 24 Sep 2018 13:54:01 -0700 In-Reply-To: <20180924204148.GA2542@bombadil.infradead.org> References: <38c03920-0fd0-0a39-2a6e-70cd8cb4ef34@virtuozzo.com> <20a20568-5089-541d-3cee-546e549a0bc8@acm.org> <12eee877-affa-c822-c9d5-fda3aa0a50da@virtuozzo.com> <1537801706.195115.7.camel@acm.org> <1537804720.195115.9.camel@acm.org> <10c706fd-2252-f11b-312e-ae0d97d9a538@virtuozzo.com> <1537805984.195115.14.camel@acm.org> <20180924185753.GA32269@bombadil.infradead.org> <1537818978.195115.25.camel@acm.org> <20180924204148.GA2542@bombadil.infradead.org> Content-Type: text/plain; charset="UTF-7" X-Mailer: Evolution 3.26.2-1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2018-09-24 at 13:41 -0700, Matthew Wilcox wrote: +AD4 On Mon, Sep 24, 2018 at 12:56:18PM -0700, Bart Van Assche wrote: +AD4 +AD4 On Mon, 2018-09-24 at 11:57 -0700, Matthew Wilcox wrote: +AD4 +AD4 +AD4 You're not supposed to use kmalloc memory for DMA. This is why we have +AD4 +AD4 +AD4 dma+AF8-alloc+AF8-coherent() and friends. +AD4 +AD4 +AD4 +AD4 Are you claiming that all drivers that use DMA should use coherent DMA only? If +AD4 +AD4 coherent DMA is the only DMA style that should be used, why do the following +AD4 +AD4 function pointers exist in struct dma+AF8-map+AF8-ops? +AD4 +AD4 Good job snipping the part of my reply which addressed this. Go read +AD4 DMA-API.txt yourself. Carefully. The snipped part did not contradict your claim that +ACI-You're not supposed to use kmalloc memory for DMA.+ACI In the DMA-API.txt document however there are multiple explicit statements that support allocating memory for DMA with kmalloc(). Here is one example from the DMA-API.txt section about dma+AF8-map+AF8-single(): Not all memory regions in a machine can be mapped by this API. Further, contiguous kernel virtual space may not be contiguous as physical memory. Since this API does not provide any scatter/gather capability, it will fail if the user tries to map a non-physically contiguous piece of memory. For this reason, memory to be mapped by this API should be obtained from sources which guarantee it to be physically contiguous (like kmalloc). Bart.