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Message-ID: <f77a41bd-9df7-87c0-9b3c-e9e1a5deaa63@arm.com>
Date:   Fri, 16 Dec 2022 12:52:56 +0000
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Subject: Re: [6.2][regression] after commit
 ffcb754584603adf7039d7972564fbf6febdc542 all sound devices disappeared (due
 BUG at mm/page_alloc.c:3592!)
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To:     Christoph Hellwig <hch@lst.de>
Cc:     Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>,
        Takashi Iwai <tiwai@suse.de>, alsa-devel@alsa-project.org,
        m.szyprowski@samsung.com,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        iommu@lists.linux.dev
References: <CABXGCsPnpu0TGHnvXM1we7q1t3tJAOYW2rA=AMvf7ZahcYvpRQ@mail.gmail.com>
 <20221216064645.GA24446@lst.de>
 <a76cb262-a24f-e113-5680-2025aa143ee6@arm.com> <20221216121519.GA5710@lst.de>
From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <20221216121519.GA5710@lst.de>
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On 2022-12-16 12:15, Christoph Hellwig wrote:
> On Fri, Dec 16, 2022 at 11:40:57AM +0000, Robin Murphy wrote:
>> On 2022-12-16 06:46, Christoph Hellwig wrote:
>>> Ok, it seems like the sound noncontig alloc code that I already
>>> commented on as potentially bogus GFP_GOMP mapping trips this.  I think
>>> for now the right thing would be to revert the hunk in dma-iommu.c
>>> (see patch below).  The other thing to try would be to remove both
>>> uses GFP_COMP in sound/core/memalloc.c, which should have the same
>>> effect.
>>
>> Or we explicitly strip the flag in dma_alloc_noncontiguous() (and maybe
>> dma_alloc_pages() as well) for consistency with dma_alloc_attrs(). That
>> seems like it might be the most robust option.
> 
> In the long run warning there and returning an error seems like the
> right thing to do, yes. I'm just a little worried doing this right now
> after the merge window.

Fair point, I guess nobody else actually implements 
dma_alloc_noncontiguous(), and dma_alloc_pages() seems a bit of a grey 
area since it is more of an explicit page allocator. So yeah, just 
restoring iommu-dma (perhaps with a mild VM_WARN_ON?) seems like a 
sufficiently safe and sensible fix for the short term. You can have my 
pre-emptive ack for that.

Cheers,
Robin.