Messages in this thread | | | Date | Tue, 05 Jun 2012 10:40:17 -0400 | From | KOSAKI Motohiro <> | Subject | Re: [PATCH v9] mm: compaction: handle incorrect MIGRATE_UNMOVABLE type pageblocks |
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(6/5/12 2:05 AM), Minchan Kim wrote: > On 06/05/2012 01:35 PM, KOSAKI Motohiro wrote: > >>>>> Minchan, are you interest this patch? If yes, can you please rewrite >>>>> it? >>>> >>>> Can do it but I want to give credit to Bartlomiej. >>>> Bartlomiej, if you like my patch, could you resend it as formal patch >>>> after you do broad testing? >>>> >>>> Frankly speaking, I don't want to merge it without any data which >>>> prove it's really good for real practice. >>>> >>>> When the patch firstly was submitted, it wasn't complicated so I was >>>> okay at that time but it has been complicated >>>> than my expectation. So if Andrew might pass the decision to me, I'm >>>> totally NACK if author doesn't provide >>>> any real data or VOC of some client. >> >> I agree. And you don't need to bother this patch if you are not interest >> this one. I'm sorry. > > > Never mind. > >> Let's throw it away until the author send us data. >> > > I guess it's hard to make such workload to prove it's useful normally. > But we can't make sure there isn't such workload in the world. > So I hope listen VOC. At least, Mel might require it. > > If anyone doesn't support it, I hope let's add some vmstat like stuff for proving > this patch's effect. If we can't see the benefit through vmstat, we can deprecate > it later.
Eek, bug we can not deprecate the vmstat. I hope to make good decision _before_ inclusion. ;-)
>>> +static bool can_rescue_unmovable_pageblock(struct page *page, bool >>> need_lrulock) >>> +{ >>> + struct zone *zone; >>> + unsigned long pfn, start_pfn, end_pfn; >>> + struct page *start_page, *end_page, *cursor_page; >>> + bool lru_locked = false; >>> + >>> + zone = page_zone(page); >>> + pfn = page_to_pfn(page); >>> + start_pfn = pfn& ~(pageblock_nr_pages - 1); >>> + end_pfn = start_pfn + pageblock_nr_pages - 1; >>> + >>> + start_page = pfn_to_page(start_pfn); >>> + end_page = pfn_to_page(end_pfn); >>> + >>> + for (cursor_page = start_page, pfn = start_pfn; cursor_page<= >>> end_page; >>> + pfn++, cursor_page++) { >>> >>> -/* Returns true if the page is within a block suitable for migration >>> to */ >>> -static bool suitable_migration_target(struct page *page) >>> + if (!pfn_valid_within(pfn)) >>> + continue; >>> + >>> + /* Do not deal with pageblocks that overlap zones */ >>> + if (page_zone(cursor_page) != zone) >>> + goto out; >>> + >>> + if (PageBuddy(cursor_page)) { >>> + unsigned long order = page_order(cursor_page); >>> + >>> + pfn += (1<< order) - 1; >>> + cursor_page += (1<< order) - 1; >>> + continue; >>> + } else if (page_count(cursor_page) == 0) { >>> + continue; >> >> Can we assume freed tail page always have page_count()==0? if yes, why >> do we >> need dangerous PageBuddy(cursor_page) check? ok, but this may be harmless. > > page_count check is for pcp pages.
Right. but my point was, I doubt we can do buddy walk w/o zone->lock.
> Am I missing your point? > > >> But if no, this code is seriously dangerous. think following scenario, >> >> 1) cursor page points free page >> >> +----------------+------------------+ >> | free (order-1) | used (order-1) | >> +----------------+------------------+ >> | >> cursor >> >> 2) moved cursor >> >> +----------------+------------------+ >> | free (order-1) | used (order-1) | >> +----------------+------------------+ >> | >> cursor >> >> 3) neighbor block was freed >> >> >> +----------------+------------------+ >> | free (order-2) | >> +----------------+------------------+ >> | >> cursor >> >> now, cursor points to middle of free block. > >> Anyway, I recommend to avoid dangerous no zone->lock game and change >> can_rescue_unmovable_pageblock() is only called w/ zone->lock. I have > > > > I can't understand your point. > If the page is middle of free block, what's the problem in can_rescue_unmovable_pageblock > at first trial of can_rescue_xxx?
I'm not sure. but other all pfn scanning code carefully avoid to touch a middle of free pages block. (also they take zone->lock anytime)
> I think we can stabilize it in second trial of can_rescue_unmovable_pageblock with zone->lock. > >> no seen any worth to include this high complex for mere minor optimization. > >> > >> >>> + } else if (PageLRU(cursor_page)) { >>> + if (!need_lrulock) >>> + continue; >>> + else if (lru_locked) >>> + continue; >>> + else { >>> + spin_lock(&zone->lru_lock); >> >> Hmm... >> I don't like to take lru_lock. 1) Until now, we carefully avoid to take >> both zone->lock and zone->lru_lock. they are both performance critical >> lock. And I think pageblock migratetype don't need strictly correct. It >> is only optimization of anti fragmentation. Why do we need take it? > > movable_block has unmovable page can make regression of anti-fragmentation. > So I did it. I agree it's a sort of optimization. > If others don't want it at the cost of regression anti-fragmentation, we can remove the lock.
ok.
> >> >> >>> + lru_locked = true; >>> + if (PageLRU(page)) >>> + continue; >>> + } >>> + } >>> + >>> + goto out; >>> + } >>> + >> >> Why don't we need to release lru_lock when returning true. > > > Because my brain has gone. :(
Never mind.
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