On Thu, Dec 2, 2010 at 9:29 AM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > On Wed, 2010-12-01 at 23:58 +0800, Minchan Kim wrote: >> On Wed, Dec 01, 2010 at 06:44:27PM +0900, KOSAKI Motohiro wrote: >> > > T0: Task1 wakeup_kswapd(order=3) >> > > T1: kswapd enters balance_pgdat >> > > T2: Task2 wakeup_kswapd(order=2), because pages reclaimed by kswapd are used >> > > quickly >> > > T3: kswapd exits balance_pgdat. kswapd will do check. Now new order=2, >> > > pgdat->kswapd_max_order will become 0, but order=3, if sleeping_prematurely, >> > > then order will become pgdat->kswapd_max_order(0), while at this time the >> > > order should 2 >> > > This isn't a big deal, but we do have a small window the order is wrong. >> > > >> > > Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx> >> > > >> > > diff --git a/mm/vmscan.c b/mm/vmscan.c >> > > index d31d7ce..15cd0d2 100644 >> > > --- a/mm/vmscan.c >> > > +++ b/mm/vmscan.c >> > > @@ -2450,7 +2450,7 @@ static int kswapd(void *p) >> > > } >> > > } >> > > >> > > - order = pgdat->kswapd_max_order; >> > > + order = max_t(unsigned long, new_order, pgdat->kswapd_max_order); >> > > } >> > > finish_wait(&pgdat->kswapd_wait, &wait); >> > >> > Good catch! >> > >> > But unfortunatelly, the code is not correct. At least, don't fit corrent >> > design. >> > >> > 1) if "order < new_order" condition is false, we already decided to don't >> > use new_order. So, we shouldn't use new_order after kswapd_try_to_sleep() >> > 2) if sleeping_prematurely() return false, it probably mean >> > zone_watermark_ok_safe(zone, order, high_wmark) return false. >> > therefore, we have to retry reclaim by using old 'order' parameter. >> >> Good catch, too. >> >> In Shaohua's scenario, if Task1 gets the order-3 page after kswapd's reclaiming, >> it's no problem. >> But if Task1 doesn't get the order-3 page and others used the order-3 page for Task1, >> Kswapd have to reclaim order-3 for Task1, again. > why? it's just a possibility. Task1 might get its pages too. If Task1 > doesn't get its pages, it will wakeup kswapd too with its order. > >> In addtion, new order is always less than old order in that context. >> so big order page reclaim makes much safe for low order pages. > big order page reclaim makes we have more chances to reclaim useful > pages by lumpy, why it's safe? For example, It assume tat Task1 continues to fail get the order-3 page of GFP_ATOMIC since other tasks continues to allocate order-2 pages so that they steal pages. Then, your patch makes continue to reclaim order-2 page in this scenario. Task1 never get the order-3 pages if it doesn't have a merge luck. It's kind of live lock(But it's just unlikely theory). But KOSAKI's approach can make sure reclaim order-3 pages so it can meet requirement about both order-2,3. In this context, I said _safety_. Of course, it could discard useful pages if Task1 get a pages. I think it's a trade-off. We should determine the policy. I biased safety of GFP_ATOMIC. > Thanks, > Shaohua > > -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href