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

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Message-ID: <022cd9d9-8d0a-d1c4-1651-bc5e126b4760@sony.com>
Date:   Tue, 20 Apr 2021 11:24:26 +0000
From:   <Peter.Enderborg@...y.com>
To:     <mhocko@...e.com>
CC:     <christian.koenig@....com>, <linux-kernel@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <sumit.semwal@...aro.org>,
        <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 4/20/21 1:04 PM, Michal Hocko wrote:
> On Tue 20-04-21 09:25:51, Peter.Enderborg@...y.com wrote:
>> On 4/20/21 11:12 AM, Michal Hocko wrote:
>>> On Tue 20-04-21 09:02:57, Peter.Enderborg@...y.com wrote:
>>>>>> But that isn't really system memory at all, it's just allocated device
>>>>>> memory.
>>>>> OK, that was not really clear to me. So this is not really accounted to
>>>>> MemTotal? If that is really the case then reporting it into the oom
>>>>> report is completely pointless and I am not even sure /proc/meminfo is
>>>>> the right interface either. It would just add more confusion I am
>>>>> afraid.
>>>>>  
>>>> Why is it confusing? Documentation is quite clear:
>>> Because a single counter without a wider context cannot be put into any
>>> reasonable context. There is no notion of the total amount of device
>>> memory usable for dma-buf. As Christian explained some of it can be RAM
>>> based. So a single number is rather pointless on its own in many cases.
>>>
>>> Or let me just ask. What can you tell from dma-bud: $FOO kB in its
>>> current form?
>> It is better to be blind?
> No it is better to have a sensible counter that can be reasoned about.
> So far you are only claiming that having something is better than
> nothing and I would agree with you if that was a debugging one off
> interface. But /proc/meminfo and other proc files have to be maintained
> with future portability in mind. This is not a dumping ground for _some_
> counters that might be interesting at the _current_ moment. E.g. what
> happens if somebody wants to have a per device resp. memory based
> dma-buf data? Are you going to change the semantic or add another
> 2 counters?

This is the DmaBufTotal. It is the upper limit. If is not there is  is something else.

And when we have a better resolution on measuring it, it would make sense
to add a DmaBufVram, DmaBufMemGC or what ever we can pickup.

This is what we can measure today.