Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752941AbdF0Mou (ORCPT ); Tue, 27 Jun 2017 08:44:50 -0400 Received: from mail-yb0-f169.google.com ([209.85.213.169]:36699 "EHLO mail-yb0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752413AbdF0Mon (ORCPT ); Tue, 27 Jun 2017 08:44:43 -0400 MIME-Version: 1.0 In-Reply-To: <0a6dceb1-ffe6-b213-eb98-fd897b62febd@arm.com> References: <20170627072812.15316-1-tfiga@chromium.org> <0a6dceb1-ffe6-b213-eb98-fd897b62febd@arm.com> From: Tomasz Figa Date: Tue, 27 Jun 2017 21:44:20 +0900 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 1/2] iommu/dma: Respect __GFP_DMA and __GFP_DMA32 in incoming GFP flags To: Robin Murphy Cc: "open list:IOMMU DRIVERS" , "linux-kernel@vger.kernel.org" , Joerg Roedel Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3715 Lines: 95 On Tue, Jun 27, 2017 at 9:26 PM, Robin Murphy wrote: > On 27/06/17 12:17, Tomasz Figa wrote: >> On Tue, Jun 27, 2017 at 8:01 PM, Robin Murphy wrote: >>> On 27/06/17 08:28, Tomasz Figa wrote: >>>> Current implementation of __iommu_dma_alloc_pages() keeps adding >>>> __GFP_HIGHMEM to GFP flags regardless of whether other zone flags are >>>> already included in the incoming flags. If __GFP_DMA or __GFP_DMA32 is >>>> set at the same time as __GFP_HIGHMEM, the allocation fails due to >>>> invalid zone flag combination. >>>> >>>> Fix this by checking for __GFP_DMA and __GFP_DMA32 in incoming GFP flags >>>> and adding __GFP_HIGHMEM only if they are not present. >>> >>> Wouldn't it make more sense to strip off the ZONE_DMA* related flags, >>> since the whole point here is that we don't care where the pages come from? >> >> I guess for my use case it wouldn't break things, as I strip them in >> my DMA mapping implementation right now (+/- one minor detail below). >> >> However I recall existing IOMMUs that could only use pages from within >> the 32-bit address space (e.g. Tegra X1). > > In general, iommu-dma can't really support IOMMUs which can't reach the > entirety of kernel memory - there's no easy way to determine what such a > limit is if it exists, nor necessarily enforce it, and either way the > streaming API callbacks are pretty much dead in the water. Right. Especially with various user pointer sources it doesn't sound very realistic to enforce that all memory comes from __GFP_DMA(32) zone. On the other hand, support for user pointer is optional in subsystems such as V4L2 and drivers for such disabled hardware could opt out. Then at least dma_alloc can be made working, giving some level of usability, IMHO higher than no IOMMU and contiguous memory allocated from CMA. > >> Also the IOMMU I'm working >> on is a part of a PCI device and it might actually prefer 32-bit >> addressable memory as well (to avoid DAC addressing; I still need to >> evaluate this). With this said, maybe it could actually make sense to >> leave the choice to the DMA mapping implementation? > > I think you're right - we're just not in a position to make any decision > at this level, so we probably do have to do this for robustness. I would > like to fix the longstanding dodgy comment, though, to clarify that > "IOMMU can map any pages" is only an assumption, and particularly one > which is invalidated by the presence of GFP_DMA flags. Just to make sure, should I resent with the commit updated? If so, what would be your preference on the wording? Best regards, Tomasz > > Robin. > >> >> Best regards, >> Tomasz >> >>> >>> Robin. >>> >>>> Signed-off-by: Tomasz Figa >>>> --- >>>> drivers/iommu/dma-iommu.c | 10 ++++++++-- >>>> 1 file changed, 8 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c >>>> index 9d1cebe7f6cb..29965a092a69 100644 >>>> --- a/drivers/iommu/dma-iommu.c >>>> +++ b/drivers/iommu/dma-iommu.c >>>> @@ -445,8 +445,14 @@ static struct page **__iommu_dma_alloc_pages(unsigned int count, >>>> if (!pages) >>>> return NULL; >>>> >>>> - /* IOMMU can map any pages, so himem can also be used here */ >>>> - gfp |= __GFP_NOWARN | __GFP_HIGHMEM; >>>> + /* >>>> + * IOMMU can map any pages, so himem can also be used here, >>>> + * unless another DMA zone is explicitly requested. >>>> + */ >>>> + if (!(gfp & (__GFP_DMA | __GFP_DMA32))) >>>> + gfp |= __GFP_HIGHMEM; >>>> + >>>> + gfp |= __GFP_NOWARN; >>>> >>>> while (count) { >>>> struct page *page = NULL; >>>> >>> >