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

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Message-ID: <532d6d7e-372c-1524-d015-e15d79fcf11c@sony.com>
Date:   Tue, 20 Apr 2021 09:25:51 +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 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? The value can still be used a relative metric.
You collect the data and see how it change. And when you see
a unexpected change you start to dig in. It fore sure wont tell what line
in your application that has a bug.  But it might be an indicator that
a new game trigger a leak. And it is very well specified, it exactly the
size of mapped dma-buf. For what you use dma-buf you need to know
other parts of the system.