Re: [PATCH mmotm] add the pagefault count into memcg stats: shmem fix

[Hugh Dickins <hughd@google.com>]

On Thu, 19 May 2011, KAMEZAWA Hiroyuki wrote:
> On Wed, 18 May 2011 11:25:48 -0700 (PDT)
> Hugh Dickins <hughd@google.com> wrote:
> 
> > On Wed, 18 May 2011, Daisuke Nishimura wrote:
> > > On Tue, 17 May 2011 11:24:40 -0700 (PDT)
> > > Hugh Dickins <hughd@google.com> wrote:
> > > 
> > > > mem_cgroup_count_vm_event() should update the PGMAJFAULT count for the
> > > > target mm, not for current mm (but of course they're usually the same).
> > > > 
> > > hmm, why ?
> > > In shmem_getpage(), we charge the page to the memcg where current mm belongs to,
> > 
> > (In the case when it's this fault which is creating the page.
> > Just as when filemap_fault() reads in the page, add_to_page_cache
> > will charge it to the current->mm's memcg, yes.  Arguably correct.)
> > 
> > > so I think counting vm events of the memcg is right.
> > 
> > It should be consistent with which task gets the maj_flt++, and
> > it should be consistent with filemap_fault(), and it should be a
> > subset of what's counted by mem_cgroup_count_vm_event(mm, PGFAULT).
> > 
> > In each case, those work on target mm rather than current->mm.
> > 
> 
> Hmm, I have no strong opinion on this but yes, it makes sense to account
> PGMAJFLT to the process whose mm->maj_flt++.

mm->maj_flt++ would be yet another story!  But it's tsk->maj_flt++.

> BTW,  do you think memcg should
> account shmem into vma->vm_mm rather than current->mm ? When vma->vm_mm
> is different from current ? At get_user_pages() + MAJFLT ?

If what we have at present works well enough, I don't think we should
risk breaking it with a funny change like that.

Is there a reason to treat shmem differently from filemap there?
You can argue that if it's shm then yes, but if it's tmpfs then no.

I suppose shmem.c does usually know the difference (by VM_NORESERVE),
but does not export it; and I'd prefer to keep it that way.

Unless we've got a real bug to fix here, let's not mess with it.

Hugh

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