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

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:

quoted

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?

quoted

 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it's outdated, and doesn't include Mlocked, Unevictable or KernelStack

Fixing the outdated docs and adding more detailed explanation is obviously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.