Messages in this thread Patch in this message | | | Date | Tue, 18 Aug 2009 20:00:48 +0900 | Subject | Re: [RFC] respect the referenced bit of KVM guest pages? | From | Minchan Kim <> |
| |
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.
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/
| |