[RFC] mm:change meminfo cached calculation

[Andrew Morton]

On Tue, 6 Jan 2015 17:04:33 -0800 (PST) Hugh Dickins <hughd at google.com> wrote:

> On Tue, 6 Jan 2015, Andrew Morton wrote:
> > On Fri, 26 Dec 2014 19:56:49 +0800 "Wang, Yalin" <Yalin.Wang at sonymobile.com> wrote:
> > 
> > > This patch subtract sharedram from cached,
> > > sharedram can only be swap into swap partitions,
> > > they should be treated as swap pages, not as cached pages.
> > > 
> > > ...
> > >
> > > --- a/fs/proc/meminfo.c
> > > +++ b/fs/proc/meminfo.c
> > > @@ -45,7 +45,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
> > >  	committed = percpu_counter_read_positive(&vm_committed_as);
> > >  
> > >  	cached = global_page_state(NR_FILE_PAGES) -
> > > -			total_swapcache_pages() - i.bufferram;
> > > +			total_swapcache_pages() - i.bufferram - i.sharedram;
> > >  	if (cached < 0)
> > >  		cached = 0;
> > 
> > Documentation/filesystems/proc.txt says
> > 
> > :      Cached: in-memory cache for files read from the disk (the
> > :              pagecache).  Doesn't include SwapCached
> > 
> > So yes, I guess it should not include shmem.
> > 
> > And why not do this as well?
> > 
> > 
> > --- a/Documentation/filesystems/proc.txt~mm-change-meminfo-cached-calculation-fix
> > +++ a/Documentation/filesystems/proc.txt
> > @@ -811,7 +811,7 @@ MemAvailable: An estimate of how much me
> >       Buffers: Relatively temporary storage for raw disk blocks
> >                shouldn't get tremendously large (20MB or so)
> >        Cached: in-memory cache for files read from the disk (the
> > -              pagecache).  Doesn't include SwapCached
> > +              pagecache).  Doesn't include SwapCached or Shmem.
> >    SwapCached: Memory that once was swapped out, is swapped back in but
> >                still also is in the swapfile (if memory is needed it
> >                doesn't need to be swapped out AGAIN because it is already
> 
> Whoa.  Changes of this kind would have made good sense about 14 years ago.
> And there's plenty more which would benefit from having anon/shmem/file
> properly distinguished.  But how can we make such a change now,
> breaking everything that has made its own sense of these counts?

That's what I was wondering, but I was having some trouble picking a
situation where it mattered much.  What's the problematic scenario
here?  Userspace that is taking Cached, saying "that was silly" and
subtracting Shmem from it by hand?

I suppose that as nobody knows we should err on the side of caution and
leave this alone.  But the situation is pretty sad - it would be nice
to make the code agree with the documentation at least.