Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 9 Apr 2010 15:47:30 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma() |
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On Fri, 9 Apr 2010 15:34:33 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> > On Fri, 9 Apr 2010 13:16:41 +1000 > > Nick Piggin <npiggin@suse.de> wrote: > > > > > On Thu, Apr 08, 2010 at 09:17:39PM +0200, Peter Zijlstra wrote: > > > > There is nothing preventing the anon_vma from being detached while we > > > > are spinning to acquire the lock. Most (all?) current users end up > > > > calling something like vma_address(page, vma) on it, which has a > > > > fairly good chance of weeding out wonky vmas. > > > > > > > > However suppose the anon_vma got freed and re-used while we were > > > > waiting to acquire the lock, and the new anon_vma fits with the > > > > page->index (because that is the only thing vma_address() uses to > > > > determine if the page fits in a particular vma, we could end up > > > > traversing faulty anon_vma chains. > > > > > > > > Close this hole for good by re-validating that page->mapping still > > > > holds the very same anon_vma pointer after we acquire the lock, if not > > > > be utterly paranoid and retry the whole operation (which will very > > > > likely bail, because it's unlikely the page got attached to a different > > > > anon_vma in the meantime). > > > > > > Hm, looks like a bugfix? How was this supposed to be safe? > > > > > IIUC. > > > > Before Rik's change to anon_vma, once page->mapping is set as anon_vma | 0x1, > > it's not modified until the page is freed. > > After the patch, do_wp_page() overwrite page->mapping when it reuse existing > > page. > > Why? > IIUC. page->mapping dereference in page_lock_anon_vma() makes four story. > > 1. the anon_vma is valid > -> do page_referenced_one(). > 2. the anon_vma is invalid and freed to buddy > -> bail out by page_mapped(), no touch anon_vma > 3. the anon_vma is kfreed, and not reused > -> bail out by page_mapped() > 4. the anon_vma is kfreed, but reused as another anon_vma > -> bail out by page_check_address() > > Now we have to consider 5th story. > > 5. the anon_vma is exchanged another anon_vma by do_wp_page. > -> bail out by above bailing out stuff. > > > I agree peter's patch makes sense. but I don't think Rik's patch change > locking rule. >
Hmm, I think following.
Assume a page is ANON and SwapCache, and it has only one reference. Consider it's read-only mapped and cause do_wp_page(). page_mapcount(page) == 1 here.
CPU0 CPU1
1. do_wp_page() 2. ..... 3. replace anon_vma. anon_vma = lock_page_anon_vma()
So, lock_page_anon_vma() may have lock on wrong anon_vma, here.(mapcount=1)
4. modify pte to writable. do something...
After lock, in CPU1, a pte of estimated address by vma_address(vma, page) containes pfn of the page and page_check_address() will success.
I'm not sure how this is dangerouns. But it's possible that CPU1 cannot notice there was anon_vma replacement. And modifies pte withoug holding anon vma's lock which the code believes it's holded.
Thanks, -Kame
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