Hi KOSAKI,
On 01/04/2012 10:38 AM, KOSAKI Motohiro wrote:
>
>>> @@ -704,10 +747,23 @@ static void ____pagevec_lru_add_fn(struct page
>>> *page, void *arg)
>>> VM_BUG_ON(PageLRU(page));
>>>
>>> SetPageLRU(page);
>>> - if (active)
>>> - SetPageActive(page);
>>> - update_page_reclaim_stat(zone, page, file, active);
>>> - add_page_to_lru_list(zone, page, lru);
>>> + redo:
>>> + if (page_evictable(page, NULL)) {
>>> + if (active)
>>> + SetPageActive(page);
>>> + update_page_reclaim_stat(zone, page, file, active);
>>> + add_page_to_lru_list(zone, page, lru);
>>> + } else {
>>> + SetPageUnevictable(page);
>>> + add_page_to_lru_list(zone, page, LRU_UNEVICTABLE);
>>> + smp_mb();
>>
>> Why do we need barrier in here? Please comment it.
>
> To cut-n-paste a comment from putback_lru_page() is good idea? :)
>
> + /*
> + * When racing with an mlock clearing (page is
> + * unlocked), make sure that if the other thread does
> + * not observe our setting of PG_lru and fails
> + * isolation, we see PG_mlocked cleared below and move
> + * the page back to the evictable list.
> + *
> + * The other side is TestClearPageMlocked().
> + */
> + smp_mb();
>
>
>
>>> + if (page_evictable(page, NULL)) {
>>> + del_page_from_lru_list(zone, page, LRU_UNEVICTABLE);
>>> + ClearPageUnevictable(page);
>>> + goto redo;
>>> + }
>>> + }
>>
>> I am not sure it's a good idea.
>> mlock is very rare event but ____pagevec_lru_add_fn is called frequently.
>> We are adding more overhead in ____pagevec_lru_add_fn.
>> Is it valuable?
>
> dunno.
>
> Personally, I think tao's case is too artificial and I haven't observed
> any real world application do such crazy mlock/munlock repeatness. But
> he said he has a such application.
ok, I will talk more about our application here.
So it is backend of a php. And for every user request, we will have to
call libmcrypt(http://sourceforge.net/projects/mcrypt/) several times to
encrypt some information, and libmcrypt will use mlock/munlock. As a
server can finish many requests in one second, so the total
mlock/munlock counts will sum up to around 2000 and it really means some
for us.
>
> If my remember is correct, ltp or some test suite depend on current
> meminfo synching behavior. then I'm afraid simple removing bring us
> new annoying bug report.
So this is the only side effect for removing the lru_add_drain_all from
mlock/mlockall right? Is there any other know issues?
I have read Andrew's comment, and if we have decided to remove all these
lru_* stuff, it seems that we have a long way to go before this issue
can be completed resolved. So I will remove it from our production
kernel first and wait for your final cleanup. Great thanks for your time
and kindly help.
Thanks
Tao
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