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Date:   Thu, 19 Jan 2023 11:12:11 -0500
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Subject: Re: [PATCH v4 4/7] s390/pci: Use dma-iommu layer
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To:     Niklas Schnelle <schnelle@linux.ibm.com>,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Wenjia Zhang <wenjia@linux.ibm.com>
Cc:     Gerd Bayer <gbayer@linux.ibm.com>,
        Pierre Morel <pmorel@linux.ibm.com>, iommu@lists.linux.dev,
        linux-s390@vger.kernel.org, borntraeger@linux.ibm.com,
        hca@linux.ibm.com, gor@linux.ibm.com,
        gerald.schaefer@linux.ibm.com, agordeev@linux.ibm.com,
        svens@linux.ibm.com, linux-kernel@vger.kernel.org,
        Julian Ruess <julianr@linux.ibm.com>
References: <20230104120543.308933-1-schnelle@linux.ibm.com>
 <20230104120543.308933-5-schnelle@linux.ibm.com>
 <71b9e85d-960f-7403-0113-135746127f3b@linux.ibm.com>
 <a623049fb595480ffdf1130bbe800d79705cef03.camel@linux.ibm.com>
 <50c22529-e0f4-9124-fdc0-c8da49300b18@linux.ibm.com>
 <0ce7d96b8447a293b147976e5993cb053feb9c52.camel@linux.ibm.com>
From:   Matthew Rosato <mjrosato@linux.ibm.com>
In-Reply-To: <0ce7d96b8447a293b147976e5993cb053feb9c52.camel@linux.ibm.com>
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On 1/19/23 11:04 AM, Niklas Schnelle wrote:
> On Thu, 2023-01-19 at 10:59 -0500, Matthew Rosato wrote:
>> On 1/19/23 6:03 AM, Niklas Schnelle wrote:
>>>>>>>>>>>>>>>>>>>  
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>  static char *pci_sw_names[] = {
>>>>>>>>>>>>>>>>>>> - "Allocated pages",
>>>>>>>>>>>>>>>>>>> +/* TODO "Allocated pages", */
>>>>>>>>>>>
>>>>>>>>>>> ? Forgot to finish this?
>>>
>>> Definitely forgot to remove the TODO. I think my latest plan was to
>>> just remove this counter. With the DMA API conversion the
>>> dma_map_ops.alloc and dma_map_ops.free move to common code and I don't
>>> see how we could differentiate these from map/unmap on our side. I'm
>>> not sure how helpful this counter really is either. If you're
>>> interested in how many pages are mapped long term I think it makes more
>>> sense to look at the difference between mapped and unmapped pages. What
>>> do you think?
>>>>>>>>>>
>>
>> Sounds reasonable to me, but I also note that without this series, when viewing statistics for a device, mapped - unmapped != allocated.  Maybe allocated pages was already broken, or is it taking into account something else that mapped - unmapped would not (maybe mapping the same page multiple times)?
>>
>>
> 
> Allocated Pages only counts the memory allocated via dma_map_ops.alloc
> so it would not count long term mappings of memory the driver allocated
> differently and then mapped for long term use.

Oh, right, I see it now.

Seems to me then that mapped-unmapped is more indicative of the actual footprint anyway so in the absence of an obvious analogue I'm fine with just getting rid of it.