From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752793AbeDLKc7 (ORCPT ); Thu, 12 Apr 2018 06:32:59 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:59028 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751848AbeDLKc6 (ORCPT ); Thu, 12 Apr 2018 06:32:58 -0400 Subject: Re: [PATCH] swiotlb: Fix unexpected swiotlb_alloc_coherent() failures To: Takashi Iwai , Christoph Hellwig Cc: iommu@lists.linux-foundation.org, Konrad Rzeszutek Wilk , linux-kernel@vger.kernel.org, =?UTF-8?Q?Christian_K=c3=b6nig?= References: <20180410170513.22834-1-tiwai@suse.de> <20180410170615.GA27589@lst.de> <40618b9c-786d-7f5f-3f5b-fc1dd8375846@arm.com> <20180410181020.GA28565@lst.de> <20180412060227.GA30248@lst.de> From: Robin Murphy Message-ID: <81b9eae2-11d4-162d-9d3a-12e06eff9fea@arm.com> Date: Thu, 12 Apr 2018 11:32:54 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/04/18 09:27, Takashi Iwai wrote: > On Thu, 12 Apr 2018 10:19:05 +0200, > Takashi Iwai wrote: >> >> On Thu, 12 Apr 2018 10:03:56 +0200, >> Takashi Iwai wrote: >>> >>> On Thu, 12 Apr 2018 08:02:27 +0200, >>> Christoph Hellwig wrote: >>>> >>>> On Wed, Apr 11, 2018 at 09:28:54AM +0200, Takashi Iwai wrote: >>>>>> But we should try a GFP_DMA32 allocation first, so this is a bit >>>>>> surprising. >>>>> >>>>> Hm, do we really try that? >>>>> Through a quick glance, dma_alloc_coherent_gfp_flags() gives GFP_DMA32 >>>>> only when coherent mask <= DMA_BIT_MASK(32); in the case of iwlwifi, >>>>> it's 36bit, so GFP_DMA isn't set. >>>> >>>> Oh, yes - it is using an odd dma mask, and amdgpu seems to use an >>>> just as odd 40-bit dma mask. >>>> >>>>> We had a fallback allocation with GFP_DMA32 in the past, but this >>>>> seems gone long time ago along with cleanups (commit c647c3bb2d16). >>>>> >>>>> But I haven't followed about this topic for long time, so I might have >>>>> missed obviously... >>>> >>>> I think a fallback would be much better here rather than relying on the >>>> limited swiotlb buffer bool. dma_direct_alloc (which in 4.17 is also >>>> used for x86) already has a GFP_DMA fallback, so extending this for >>>> GFP_DMA32 as well would seem reasonable. >>>> >>>> Any volunteers? >>> >>> Below is a quick attempt, totally untested. Actually the retry with >>> GFP_DMA is superfluous for archs without it, so the first patch >>> corrects it. >> >> Gah, scratch this, it doesn't work. A different check is needed... > > The v2 patches are below, replaced with IS_ENABLED(CONFIG_ZONE_DMA). That looks pretty reasonable to me - I'd be tempted to try a factoring out a helper function to avoid the "goto again" logic, but that's hardly crucial. What I'd really love to see is something like the alloc_pages_mask() proposal from years ago to come back such that GFP_DMA32 could die entirely, but I don't know anywhere near enough about the mm layer to consider taking that on myself :( Robin.