Messages in this thread | | | Date | Wed, 19 Aug 2009 01:27:57 +0900 (JST) | From | KOSAKI Motohiro <> | Subject | Re: [RFC] respect the referenced bit of KVM guest pages? |
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> On Tue, Aug 18, 2009 at 07:00:48PM +0800, Minchan Kim wrote: > > On Tue, Aug 18, 2009 at 7:00 PM, Wu Fengguang<fengguang.wu@intel.com> wrote: > > > On Tue, Aug 18, 2009 at 05:52:47PM +0800, Minchan Kim wrote: > > >> On Tue, 18 Aug 2009 17:31:19 +0800 > > >> Wu Fengguang <fengguang.wu@intel.com> wrote: > > >> > > >> > On Tue, Aug 18, 2009 at 12:17:34PM +0800, Minchan Kim wrote: > > >> > > On Tue, 18 Aug 2009 10:34:38 +0800 > > >> > > Wu Fengguang <fengguang.wu@intel.com> wrote: > > >> > > > > >> > > > Minchan, > > >> > > > > > >> > > > On Mon, Aug 17, 2009 at 10:33:54PM +0800, Minchan Kim wrote: > > >> > > > > On Sun, Aug 16, 2009 at 8:29 PM, Wu Fengguang<fengguang.wu@intel.com> wrote: > > >> > > > > > On Sun, Aug 16, 2009 at 01:15:02PM +0800, Wu Fengguang wrote: > > >> > > > > >> On Sun, Aug 16, 2009 at 11:53:00AM +0800, Rik van Riel wrote: > > >> > > > > >> > Wu Fengguang wrote: > > >> > > > > >> > > On Fri, Aug 07, 2009 at 05:09:55AM +0800, Jeff Dike wrote: > > >> > > > > >> > >> Side question - > > >> > > > > >> > >> Is there a good reason for this to be in shrink_active_list() > > >> > > > > >> > >> as opposed to __isolate_lru_page? > > >> > > > > >> > >> > > >> > > > > >> > >> if (unlikely(!page_evictable(page, NULL))) { > > >> > > > > >> > >> putback_lru_page(page); > > >> > > > > >> > >> continue; > > >> > > > > >> > >> } > > >> > > > > >> > >> > > >> > > > > >> > >> Maybe we want to minimize the amount of code under the lru lock or > > >> > > > > >> > >> avoid duplicate logic in the isolate_page functions. > > >> > > > > >> > > > > >> > > > > >> > > I guess the quick test means to avoid the expensive page_referenced() > > >> > > > > >> > > call that follows it. But that should be mostly one shot cost - the > > >> > > > > >> > > unevictable pages are unlikely to cycle in active/inactive list again > > >> > > > > >> > > and again. > > >> > > > > >> > > > >> > > > > >> > Please read what putback_lru_page does. > > >> > > > > >> > > > >> > > > > >> > It moves the page onto the unevictable list, so that > > >> > > > > >> > it will not end up in this scan again. > > >> > > > > >> > > >> > > > > >> Yes it does. I said 'mostly' because there is a small hole that an > > >> > > > > >> unevictable page may be scanned but still not moved to unevictable > > >> > > > > >> list: when a page is mapped in two places, the first pte has the > > >> > > > > >> referenced bit set, the _second_ VMA has VM_LOCKED bit set, then > > >> > > > > >> page_referenced() will return 1 and shrink_page_list() will move it > > >> > > > > >> into active list instead of unevictable list. Shall we fix this rare > > >> > > > > >> case? > > >> > > > > > > >> > > > > I think it's not a big deal. > > >> > > > > > >> > > > Maybe, otherwise I should bring up this issue long time before :) > > >> > > > > > >> > > > > As you mentioned, it's rare case so there would be few pages in active > > >> > > > > list instead of unevictable list. > > >> > > > > > >> > > > Yes. > > >> > > > > > >> > > > > When next time to scan comes, we can try to move the pages into > > >> > > > > unevictable list, again. > > >> > > > > > >> > > > Will PG_mlocked be set by then? Otherwise the situation is not likely > > >> > > > to change and the VM_LOCKED pages may circulate in active/inactive > > >> > > > list for countless times. > > >> > > > > >> > > PG_mlocked is not important in that case. > > >> > > Important thing is VM_LOCKED vma. > > >> > > I think below annotaion can help you to understand my point. :) > > >> > > > >> > Hmm, it looks like pages under VM_LOCKED vma is guaranteed to have > > >> > PG_mlocked set, and so will be caught by page_evictable(). Is it? > > >> > > >> No. I am sorry for making my point not clear. > > >> I meant following as. > > >> When the next time to scan, > > >> > > >> shrink_page_list > > > -> > > > referenced = page_referenced(page, 1, > > > sc->mem_cgroup, &vm_flags); > > > /* In active use or really unfreeable? Activate it. */ > > > if (sc->order <= PAGE_ALLOC_COSTLY_ORDER && > > > referenced && page_mapping_inuse(page)) > > > goto activate_locked; > > > > > >> -> try_to_unmap > > > ~~~~~~~~~~~~ this line won't be reached if page is found to be > > > referenced in the above lines? > > > > Indeed! In fact, I was worry about that. > > It looks after live lock problem. > > But I think it's very small race window so there isn't any report until now. > > Let's Cced Lee. > > > > If we have to fix it, how about this ? > > This version has small overhead than yours since > > there is less shrink_page_list call than page_referenced. > > Yeah, it looks better. However I still wonder if (VM_LOCKED && !PG_mlocked) > is possible and somehow persistent. Does anyone have the answer? Thanks!
hehe, that's bug. you spotted very good thing IMHO ;) I posted fixed patch. can you see it?
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