Messages in this thread Patch in this message | | | Date | Tue, 18 Aug 2009 19:11:25 +0800 | From | Wu Fengguang <> | 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!
Thanks, Fengguang
> > diff --git a/mm/rmap.c b/mm/rmap.c > index ed63894..283266c 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -358,6 +358,7 @@ static int page_referenced_one(struct page *page, > */ > if (vma->vm_flags & VM_LOCKED) { > *mapcount = 1; /* break early from loop */ > + *vm_flags |= VM_LOCKED; > goto out_unmap; > } > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index d224b28..d156e1d 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -632,7 +632,8 @@ static unsigned long shrink_page_list(struct > list_head *page_list, > 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)) > + referenced && page_mapping_inuse(page) > + && !(vm_flags & VM_LOCKED)) > goto activate_locked; > > > > > > > Thanks, > > Fengguang > > > >> -> try_to_unmap_xxx > >> -> if (vma->vm_flags & VM_LOCKED) > >> -> try_to_mlock_page > >> -> TestSetPageMlocked > >> -> putback_lru_page > >> > >> So at last, the page will be located in unevictable list. > >> > >> > Then I was worrying about a null problem. Sorry for the confusion! > >> > > >> > Thanks, > >> > Fengguang > >> > > >> > > ---- > >> > > > >> > > /* > >> > > * called from munlock()/munmap() path with page supposedly on the LRU. > >> > > * > >> > > * Note: unlike mlock_vma_page(), we can't just clear the PageMlocked > >> > > * [in try_to_munlock()] and then attempt to isolate the page. We must > >> > > * isolate the page to keep others from messing with its unevictable > >> > > * and mlocked state while trying to munlock. However, we pre-clear the > >> > > * mlocked state anyway as we might lose the isolation race and we might > >> > > * not get another chance to clear PageMlocked. If we successfully > >> > > * isolate the page and try_to_munlock() detects other VM_LOCKED vmas > >> > > * mapping the page, it will restore the PageMlocked state, unless the page > >> > > * is mapped in a non-linear vma. So, we go ahead and SetPageMlocked(), > >> > > * perhaps redundantly. > >> > > * If we lose the isolation race, and the page is mapped by other VM_LOCKED > >> > > * vmas, we'll detect this in vmscan--via try_to_munlock() or try_to_unmap() > >> > > * either of which will restore the PageMlocked state by calling > >> > > * mlock_vma_page() above, if it can grab the vma's mmap sem. > >> > > */ > >> > > static void munlock_vma_page(struct page *page) > >> > > { > >> > > ... > >> > > > >> > > -- > >> > > Kind regards, > >> > > Minchan Kim > >> > >> > >> -- > >> Kind regards, > >> Minchan Kim > > > > > > -- > 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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