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

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, May 19, 2011 at 9:28 AM, Hugh Dickins <hughd@google.com> wrote:
> 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.

Okay. I don't feel it strongly now.
Then, could you repost your patch with corrected description about
this behavior change which is a bug or inconsistency whatever. :)


>
> Hugh
>



-- 
Kind regards,
Minchan Kim

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