linux-kernel - Re: [PATCH v4] dma-buf: Add DmaBufTotal counter in meminfo

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Date:   Mon, 19 Apr 2021 15:19:03 +0000
From:   <Peter.Enderborg@...y.com>
To:     <mhocko@...e.com>
CC:     <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <sumit.semwal@...aro.org>, <christian.koenig@....com>,
        <adobriyan@...il.com>, <akpm@...ux-foundation.org>,
        <songmuchun@...edance.com>, <guro@...com>, <shakeelb@...gle.com>,
        <neilb@...e.de>, <samitolvanen@...gle.com>, <rppt@...nel.org>,
        <linux-media@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <linaro-mm-sig@...ts.linaro.org>, <willy@...radead.org>
Subject: Re: [PATCH v4] dma-buf: Add DmaBufTotal counter in meminfo

On 4/19/21 5:00 PM, Michal Hocko wrote:
> On Mon 19-04-21 12:41:58, Peter.Enderborg@...y.com wrote:
>> On 4/19/21 2:16 PM, Michal Hocko wrote:
>>> On Sat 17-04-21 12:40:32, Peter Enderborg wrote:
>>>> This adds a total used dma-buf memory. Details
>>>> can be found in debugfs, however it is not for everyone
>>>> and not always available. dma-buf are indirect allocated by
>>>> userspace. So with this value we can monitor and detect
>>>> userspace applications that have problems.
>>> The changelog would benefit from more background on why this is needed,
>>> and who is the primary consumer of that value.
>>>
>>> I cannot really comment on the dma-buf internals but I have two remarks.
>>> Documentation/filesystems/proc.rst needs an update with the counter
>>> explanation and secondly is this information useful for OOM situations
>>> analysis? If yes then show_mem should dump the value as well.
>>>
>>> From the implementation point of view, is there any reason why this
>>> hasn't used the existing global_node_page_state infrastructure?
>> I fix doc in next version.  Im not sure what you expect the commit message to include.
> As I've said. Usual justification covers answers to following questions
> 	- Why do we need it?
> 	- Why the existing data is insuficient?
> 	- Who is supposed to use the data and for what?
>
> I can see an answer for the first two questions (because this can be a
> lot of memory and the existing infrastructure is not production suitable
> - debugfs). But the changelog doesn't really explain who is going to use
> the new data. Is this a monitoring to raise an early alarm when the
> value grows? Is this for debugging misbehaving drivers? How is it
> valuable for those?
>
>> The function of the meminfo is: (From Documentation/filesystems/proc.rst)
>>
>> "Provides information about distribution and utilization of memory."
> True. Yet we do not export any random counters, do we?
>
>> Im not the designed of dma-buf, I think  global_node_page_state as a kernel
>> internal.
> It provides a node specific and optimized counters. Is this a good fit
> with your new counter? Or the NUMA locality is of no importance?

Sounds good to me, if Christian Koenig think it is good, I will use that.
It is only virtio in drivers that use the global_node_page_state if
that matters.


>
>> dma-buf is a device driver that provides a function so I might be
>> on the outside. However I also see that it might be relevant for a OOM.
>> It is memory that can be freed by killing userspace processes.
>>
>> The show_mem thing. Should it be a separate patch?
> This is up to you but if you want to expose the counter then send it in
> one series.
>