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

[Hugh Dickins <hughd@google.com>]

On Thu, 19 May 2011, Minchan Kim wrote:
> On Wed, May 18, 2011 at 3:24 AM, 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).
> >
> > We don't know the target mm in shmem_getpage(), so do it at the outer
> > level in shmem_fault(); and it's easier to follow if we move the
> > count_vm_event(PGMAJFAULT) there too.
> >
> > Hah, it was using __count_vm_event() before, sneaking that update into
> > the unpreemptible section under info->lock: well, it comes to the same
> > on x86 at least, and I still think it's best to keep these together.
> >
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> 
> It's good to me but I have a nitpick.
> 
> You are changing behavior a bit.
> Old behavior is to account FAULT although the operation got failed.
> But new one is to not account it.
> I think we have to account it regardless of whether it is successful or not.
> That's because it is fact fault happens.

That's a good catch: something I didn't think of at all.

However, it looks as if the patch remains correct, and is fixing
a bug (or inconsistency) that we hadn't noticed before.

If you look through filemap_fault() or do_swap_page() (or even
ncp_file_mmap_fault(), though I don't take that one as canonical!),
they clearly do not count the major fault on error (except in the
case where VM_FAULT_MAJOR needs VM_FAULT_RETRY, then gets
VM_FAULT_ERROR on the retry).

So, shmem.c was the odd one out before.  If you feel very strongly
about it ("it is fact fault happens") you could submit a patch to
change them all - but I think just leave them as is.

Hugh

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