Re: [PATCH v2] docs: proc.rst: meminfo: briefly describe gaps in memory accounting

From: Mike Rapoport
Date: Tue Apr 20 2021 - 10:52:03 EST


On Tue, Apr 20, 2021 at 02:24:30PM +0100, Matthew Wilcox wrote:
> On Tue, Apr 20, 2021 at 03:13:54PM +0300, Mike Rapoport wrote:
> > Add a paragraph that explains that it may happen that the counters in
> > /proc/meminfo do not add up to the overall memory usage.
>
> ... that is, the sum may be lower because memory is allocated for other
> purposes that is not reported here, right?
>
> Is it ever possible for it to be higher? Maybe due to a race when
> sampling the counters?
>
> > Provides information about distribution and utilization of memory. This
> > -varies by architecture and compile options. The following is from a
> > -16GB PIII, which has highmem enabled. You may not have all of these fields.
> > +varies by architecture and compile options. Please note that it may happen
> > +that the memory accounted here does not add up to the overall memory usage
> > +and the difference for some workloads can be substantial. In many cases there
> > +are other means to find out additional memory using subsystem specific
> > +interfaces, for instance /proc/net/sockstat for TCP memory allocations.
>
> How about just:
>
> +varies by architecture and compile options. The memory reported here
> +may not add up to the overall memory usage and the difference for some
> +workloads can be substantial. [...]

I like this. I also for adding a sentence about overlap in the counters:

+varies by architecture and compile options. Some of the counters reported
+here overlap. The memory reported by the non overlapping counters may not
+add up to the overall memory usage and the difference for some workloads
can be substantial. [...]

> But I'd like to be a bit more explicit about the reason, hence my question
> above to be sure I understand.
>
> It's also not entirely clear which of the fields in meminfo can be
> usefully summed. VmallocTotal is larger than MemTotal, for example.
> But I know that KernelStack is allocated through vmalloc these days,
> and I don't know whether VmallocUsed includes KernelStack or whether I
> can sum them. Similarly, is Mlocked a subset of Unevictable?
>
> There is some attempt at explaining how these numbers fit together, but
> it's outdated, and doesn't include Mlocked, Unevictable or KernelStack

Fixing the outdated docs and adding more detailed explanation is obviously
welcome, but it's beyond the scope of the current patch.

--
Sincerely yours,
Mike.