Re: committed memory, mmaps and shms

[Michal Hocko <mhocko@suse.cz>]

On Thu 12-03-15 11:54:22, Marcos Dione wrote:
> On Thu, Mar 12, 2015 at 08:40:53AM -0400, Michal Hocko wrote:
> > [CCing MM maling list]
> 
>     Shall we completely migrate the rest of the conversation there?

It is usually better to keep lkml on the cc list for a larger audience.
 
> > On Wed 11-03-15 19:10:44, Marcos Dione wrote:
[...]
> > > $ free
> > >              total       used       free     shared    buffers     cached
> > > Mem:     396895176  395956332     938844          0       8972  356409952
> > > -/+ buffers/cache:   39537408  357357768
> > > Swap:      8385788    8385788          0
> > > 
> > >     This reports 378GiB of RAM, 377 used; of those 8MiB in buffers,
> > > 339GiB in cache, leaving only 38Gib for processes (for some reason this
> > 
> > I am not sure I understand your math here. 339G in the cache should be
> > reclaimable (be careful about the shmem though). It is the rest which
> > might be harder to reclaim.
> 
>     These 38GiB I mention is the rest of 378 available minus 339 in
> cache. To me this difference represents the sum of the resident
> anonymous memory malloc'ed by all processes. Unless there's some othr
> kind of pages accounted in 'Used'.

The kernel needs memory as well for its internal data structures
(stacks, page tables, slab objects, memory used by drivers and what not).
 
> > shmem (tmpfs) is a in memory filesystem. Pages backing shmem mappings
> > are maintained in the page cache. Their backing storage is swap as you
> > said. So from a conceptual point of vew this makes a lot of sense. 
> 
>     Now it's completely clear, thanks.
> 
> > > * Why 'pure' mmalloc'ed memory is ever reported? Does it make sense to
> > >   talk about it?
> > 
> > This is simply private anonymous memory. And you can see it as such in
> > /proc/<pid>/[s]maps
> 
>     Yes, but my question was more on the lines of 'why free or
> /proc/meminfo do not show it'. Maybe it's just that it's difficult to
> define (like I said, "sum of resident anonymous..." &c) or nobody really
> cares about this. Maybe I shouldn't either.

meminfo is exporting this information as AnonPages.

[...]
> > > * What is actually counted in Committed_AS? Does it count shms or mmaps?
> > >   How?
> > 
> > This depends on the overcommit configuration. See
> > Documentation/sysctl/vm.txt for more information.
> 
>     I understand what /proc/sys/vm/overcommit_memory is for; what I
> don't understand is what exactly counted in the Committed_AS line in
> /proc/meminfo.

It accounts all the address space reservations - e.g. mmap(len), len
will get added. The things are slightly more complicated but start
looking at callers of security_vm_enough_memory_mm should give you an
idea what everything is included.
How is this number used depends on the overcommit mode.
__vm_enough_memory would give you a better picture.

> I also read Documentation/vm/overcommit-accounting

What would help you to understand it better?

-- 
Michal Hocko
SUSE Labs

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