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

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Message-ID: <YH61JJOlLwBwHqVL@dhcp22.suse.cz>
Date:   Tue, 20 Apr 2021 13:04:04 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Peter.Enderborg@...y.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 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?
-- 
Michal Hocko
SUSE Labs