Messages in this thread | | | Date | Tue, 18 Aug 2009 23:03:38 +0900 | Subject | Re: [RFC] respect the referenced bit of KVM guest pages? | From | Minchan Kim <> |
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On Tue, Aug 18, 2009 at 8:11 PM, Wu Fengguang<fengguang.wu@intel.com> wrote: > 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!
I think it's possible. munlock_vma_page pre-clears PG_mlocked of page. And then if isolate_lru_page fail, the page have no PG_mlocked and vma which have VM_LOCKED.
As munlock_vma_page's annotation said, we hope the page will be rescued by try_to_unmap. But As you pointed out, if the page have PG_referenced, it can't reach try_to_unmap so that it will go into the active list.
What are others' opinion ?
> Thanks, > Fengguang
-- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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