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

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Message-ID: <7411ae1d-5e36-46da-99cf-c485ebdb31bc@arm.com>
Date: Mon, 14 Oct 2024 19:09:08 +0100
From: Robin Murphy <robin.murphy@....com>
To: Ming Lei <ming.lei@...hat.com>, Christoph Hellwig <hch@....de>
Cc: 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 14/10/2024 8:58 am, Ming Lei wrote:
> On Mon, Oct 14, 2024 at 09:41:51AM +0200, Christoph Hellwig wrote:
>> On Mon, Oct 14, 2024 at 09:23:14AM +0200, Hannes Reinecke wrote:
>>>> 3) some storage utilities
>>>> - dm thin provisioning utility of thin_check
>>>> - `dt`(https://github.com/RobinTMiller/dt)
>>>>
>>>> I looks like same user buffer is used in more than 1 dio.
>>>>
>>>> 4) some self cooked test code which does same thing with 1)
>>>>
>>>> In storage stack, the buffer provider is far away from the actual DMA
>>>> controller operating code, which doesn't have the knowledge if
>>>> DMA_ATTR_SKIP_CPU_SYNC should be set.
>>>>
>>>> And suggestions for avoiding this noise?
>>>>
>>> Can you check if this is the NULL page? Operations like 'discard' will
>>> create bios with several bvecs all pointing to the same NULL page.
>>> That would be the most obvious culprit.
>>
>> 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.

Thanks,
Robin.