linux-kernel - Re: [Report] annoyed dma debug warning "cacheline tracking EEXIST, overlapping mappings aren't supported"

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Message-ID: <20241015045413.GA18058@lst.de>
Date: Tue, 15 Oct 2024 06:54:13 +0200
From: Christoph Hellwig <hch@....de>
To: Robin Murphy <robin.murphy@....com>
Cc: Ming Lei <ming.lei@...hat.com>, Christoph Hellwig <hch@....de>,
	Hannes Reinecke <hare@...e.de>,
	Hamza Mahfooz <someguy@...ective-light.com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-block@...r.kernel.org, io-uring@...r.kernel.org,
	linux-raid@...r.kernel.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [Report] annoyed dma debug warning "cacheline tracking EEXIST,
 overlapping mappings aren't supported"

On Mon, Oct 14, 2024 at 07:09:08PM +0100, Robin Murphy wrote:
>>> The only case I fully understand without looking into the details
>>> is raid1, and that will obviously map the same data multiple times
>>
>> The other cases should be concurrent DIOs on same userspace buffer.
>
> active_cacheline_insert() does already bail out for DMA_TO_DEVICE, so it 
> returning -EEXIST to tickle the warning would seem to genuinely imply these 
> are DMA mappings requesting to *write* the same cacheline concurrently, 
> which is indeed broken in general.

Yes, active_cacheline_insert only complains for FROM_DEVICE or
BIDIRECTIONAL mappings.  I can't see how raid 1 would trigger that
given that it only reads from one leg at a time.

Ming, can you look a bit more into what is happening here?