Messages in this thread | | | From | KOSAKI Motohiro <> | Subject | Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma() | Date | Fri, 9 Apr 2010 15:34:33 +0900 (JST) |
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> 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.
> > == > static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma, > unsigned long address, pte_t *page_table, pmd_t *pmd, > spinlock_t *ptl, pte_t orig_pte) > { > .... > if (PageAnon(old_page) && !PageKsm(old_page)) { > if (!trylock_page(old_page)) { > page_cache_get(old_page); > .... > reuse = reuse_swap_page(old_page); > if (reuse) > /* > * The page is all ours. Move it to our anon_vma so > * the rmap code will not search our parent or siblings. > * Protected against the rmap code by the page lock. > */ > page_move_anon_rmap(old_page, vma, address); ----(*) > } > === > (*) is new. > > Then, this new check makes sense in the current kernel.
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