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SubjectRe: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma()
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.
==
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.



>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > ---
> > mm/rmap.c | 7 +++++++
> > 1 file changed, 7 insertions(+)
> >
> > Index: linux-2.6/mm/rmap.c
> > ===================================================================
> > --- linux-2.6.orig/mm/rmap.c
> > +++ linux-2.6/mm/rmap.c
> > @@ -294,6 +294,7 @@ struct anon_vma *page_lock_anon_vma(stru
> > unsigned long anon_mapping;
> >
> > rcu_read_lock();
> > +again:
> > anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
> > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
> > goto out;
> > @@ -302,6 +303,12 @@ struct anon_vma *page_lock_anon_vma(stru
> >
> > anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
> > spin_lock(&anon_vma->lock);
> > +
> > + if (page_rmapping(page) != anon_vma) {
>
> very unlikely()?
>
I think so.

Thanks,
-Kame



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