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Subject: Re: DPAA2 triggers, [PATCH] dma debug: report -EEXIST errors in
 add_dma_entry
To:     Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Hamza Mahfooz <someguy@effective-light.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>
Cc:     Karsten Graul <kgraul@linux.ibm.com>,
        Ioana Ciornei <ioana.ciornei@nxp.com>,
        Jeremy Linton <jeremy.linton@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>
References: <20210518125443.34148-1-someguy@effective-light.com>
 <fd67fbac-64bf-f0ea-01e1-5938ccfab9d0@arm.com>
 <20210914154504.z6vqxuh3byqwgfzx@skbuf>
 <185e7ee4-3749-4ccb-6d2e-da6bc8f30c04@linux.ibm.com>
 <20211001145256.0323957a@thinkpad> <20211006151043.61fe9613@thinkpad>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <4a96b583-1119-8b26-cc85-f77a6b4550a2@arm.com>
Date:   Wed, 6 Oct 2021 15:23:36 +0100
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On 2021-10-06 14:10, Gerald Schaefer wrote:
> On Fri, 1 Oct 2021 14:52:56 +0200
> Gerald Schaefer <gerald.schaefer@linux.ibm.com> wrote:
> 
>> On Thu, 30 Sep 2021 15:37:33 +0200
>> Karsten Graul <kgraul@linux.ibm.com> wrote:
>>
>>> On 14/09/2021 17:45, Ioana Ciornei wrote:
>>>> On Wed, Sep 08, 2021 at 10:33:26PM -0500, Jeremy Linton wrote:
>>>>> +DPAA2, netdev maintainers
>>>>> Hi,
>>>>>
>>>>> On 5/18/21 7:54 AM, Hamza Mahfooz wrote:
>>>>>> Since, overlapping mappings are not supported by the DMA API we should
>>>>>> report an error if active_cacheline_insert returns -EEXIST.
>>>>>
>>>>> It seems this patch found a victim. I was trying to run iperf3 on a
>>>>> honeycomb (5.14.0, fedora 35) and the console is blasting this error message
>>>>> at 100% cpu. So, I changed it to a WARN_ONCE() to get the call trace, which
>>>>> is attached below.
>>>>>
>>>>
>>>> These frags are allocated by the stack, transformed into a scatterlist
>>>> by skb_to_sgvec and then DMA mapped with dma_map_sg. It was not the
>>>> dpaa2-eth's decision to use two fragments from the same page (that will
>>>> also end un in the same cacheline) in two different in-flight skbs.
>>>>
>>>> Is this behavior normal?
>>>>
>>>
>>> We see the same problem here and it started with 5.15-rc2 in our nightly CI runs.
>>> The CI has panic_on_warn enabled so we see the panic every day now.
>>
>> Adding a WARN for a case that be detected false-positive seems not
>> acceptable, exactly for this reason (kernel panic on unaffected
>> systems).
>>
>> So I guess it boils down to the question if the behavior that Ioana
>> described is legit behavior, on a system that is dma coherent. We
>> are apparently hitting the same scenario, although it could not yet be
>> reproduced with debug printks for some reason.
>>
>> If the answer is yes, than please remove at lease the WARN, so that
>> it will not make systems crash that behave valid, and have
>> panic_on_warn set. Even a normal printk feels wrong to me in that
>> case, it really sounds rather like you want to fix / better refine
>> the overlap check, if you want to report anything here.
> 
> Dan, Christoph, any opinion?
> 
> So far it all looks a lot like a false positive, so could you please
> see that those patches get reverted? I do wonder a bit why this is
> not an issue for others, we surely cannot be the only ones running
> CI with panic_on_warn.

What convinces you it's a false-positive? I'm hardly familiar with most 
of that callstack, but it appears to be related to mlx5, and I know that 
exists on expansion cards which could be plugged into a system with 
non-coherent PCIe where partial cacheline overlap *would* be a real 
issue. Of course it's dubious that there are many real use-cases for 
plugging a NIC with a 4-figure price tag into a little i.MX8 or 
whatever, but the point is that it *should* still work correctly.

> We would need to disable DEBUG_DMA if this WARN stays in, which
> would be a shame. Of course, in theory, this might also indicate
> some real bug, but there really is no sign of that so far.

The whole point of DMA debug is to flag up things that you *do* get away 
with on the vast majority of systems, precisely because most testing 
happens on those systems rather than more esoteric embedded setups. Say 
your system only uses dma-direct and a driver starts triggering the 
warning for not calling dma_mapping_error(), would you argue for 
removing that warning as well since dma_map_single() can't fail on your 
machine so it's "not a bug"?

> Having multiple sg elements in the same page (or cacheline) is
> valid, correct? And this is also not a decision of the driver
> IIUC, so if it was bug, it should be addressed in common code,
> correct?

According to the streaming DMA API documentation, it is *not* valid:

".. warning::

   Memory coherency operates at a granularity called the cache
   line width.  In order for memory mapped by this API to operate
   correctly, the mapped region must begin exactly on a cache line
   boundary and end exactly on one (to prevent two separately mapped
   regions from sharing a single cache line).  Since the cache line size
   may not be known at compile time, the API will not enforce this
   requirement.  Therefore, it is recommended that driver writers who
   don't take special care to determine the cache line size at run time
   only map virtual regions that begin and end on page boundaries (which
   are guaranteed also to be cache line boundaries)."

>> BTW, there is already a WARN in the add_dma_entry() path, related
>> to cachlline overlap and -EEXIST:
>>
>> add_dma_entry() -> active_cacheline_insert() -> -EEXIST ->
>> active_cacheline_inc_overlap()
>>
>> That will only trigger when "overlap > ACTIVE_CACHELINE_MAX_OVERLAP".
>> Not familiar with that code, but it seems that there are now two
>> warnings for more or less the same, and the new warning is much more
>> prone to false-positives.
>>
>> How do these 2 warnings relate, are they both really necessary?
>> I think the new warning was only introduced because of some old
>> TODO comment in add_dma_entry(), see commit 2b4bbc6231d78
>> ("dma-debug: report -EEXIST errors in add_dma_entry").

AFAICS they are different things. I believe the new warning is supposed 
to be for the fundementally incorrect API usage (as above) of mapping 
different regions overlapping within the same cacheline. The existing 
one is about dma-debug losing internal consistency when tracking the 
*same* region being mapped multiple times, which is a legal thing to do 
- e.g. buffer sharing between devices - but if anyone's doing it to 
excess that's almost certainly a bug (i.e. they probably intended to 
unmap it in between but missed that out).

Robin.

>> That comment was initially added by Dan long time ago, and he
>> added several fix-ups for overlap detection after that, including
>> the "overlap > ACTIVE_CACHELINE_MAX_OVERLAP" stuff in
>> active_cacheline_inc_overlap(). So could it be that the TODO
>> comment was simply not valid any more, and better be removed
>> instead of adding new / double warnings, that also generate
>> false-positives and kernel crashes?
>