From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754786AbeDTMU0 (ORCPT ); Fri, 20 Apr 2018 08:20:26 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:48848 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754705AbeDTMUY (ORCPT ); Fri, 20 Apr 2018 08:20:24 -0400 Date: Fri, 20 Apr 2018 08:20:23 -0400 (EDT) From: Mikulas Patocka X-X-Sender: mpatocka@file01.intranet.prod.int.rdu2.redhat.com To: Matthew Wilcox cc: David Miller , Andrew Morton , linux-mm@kvack.org, eric.dumazet@gmail.com, edumazet@google.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, mst@redhat.com, jasowang@redhat.com, virtualization@lists.linux-foundation.org, dm-devel@redhat.com, Vlastimil Babka Subject: Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM In-Reply-To: <20180420114712.GB10788@bombadil.infradead.org> Message-ID: References: <3e65977e-53cd-bf09-bc4b-0ce40e9091fe@gmail.com> <20180418.134651.2225112489265654270.davem@davemloft.net> <20180420114712.GB10788@bombadil.infradead.org> User-Agent: Alpine 2.02 (LRH 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 20 Apr 2018, Matthew Wilcox wrote: > On Thu, Apr 19, 2018 at 12:12:38PM -0400, Mikulas Patocka wrote: > > Unfortunatelly, some kernel code has bugs - it uses kvmalloc and then > > uses DMA-API on the returned memory or frees it with kfree. Such bugs were > > found in the virtio-net driver, dm-integrity or RHEL7 powerpc-specific > > code. > > Maybe it's time to have the SG code handle vmalloced pages? This is > becoming more and more common with vmapped stacks (and some of our > workarounds are hideous -- allocate 4 bytes with kmalloc because we can't > DMA onto the stack any more?). We already have a few places which do > handle sgs of vmalloced addresses, such as the nx crypto driver: > > if (is_vmalloc_addr(start_addr)) > sg_addr = page_to_phys(vmalloc_to_page(start_addr)) > + offset_in_page(sg_addr); > else > sg_addr = __pa(sg_addr); > > and videobuf: > > pg = vmalloc_to_page(virt); > if (NULL == pg) > goto err; > BUG_ON(page_to_pfn(pg) >= (1 << (32 - PAGE_SHIFT))); > sg_set_page(&sglist[i], pg, PAGE_SIZE, 0); > > Yes, there's the potential that we have to produce two SG entries for a > virtually contiguous region if it crosses a page boundary, and our APIs > aren't set up right to make it happen. But this is something we should > consider fixing ... otherwise we'll end up with dozens of driver hacks. > The videobuf implementation was already copy-and-pasted into the saa7146 > driver, for example. What if the device requires physically contiguous area and the vmalloc area crosses a page? Will you use a bounce buffer? Where do you allocate the bounce buffer from? What if you run out of bounce buffers? Mikulkas