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

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Message-ID: <YH2a9YfRBlfNnF+u@dhcp22.suse.cz>
Date:   Mon, 19 Apr 2021 17:00:05 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Peter.Enderborg@...y.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 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?

> 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.

-- 
Michal Hocko
SUSE Labs