Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

Mel Gorman
2010-05-06T09:54:42Z

On Thu, May 06, 2010 at 06:47:12PM +0900, Minchan Kim wrote:
> On Thu, May 6, 2010 at 9:22 AM, Mel Gorman <mel@csn.ul.ie> wrote:
> > On Wed, May 05, 2010 at 11:02:25AM -0700, Linus Torvalds wrote:
> >>
> >>
> >> On Wed, 5 May 2010, Mel Gorman wrote:
> >> >
> >> > If the same_vma list is properly ordered then maybe something like the
> >> > following is allowed?
> >>
> >> Heh. This is the same logic I just sent out. However:
> >>
> >> > +   anon_vma = page_rmapping(page);
> >> > +   if (!anon_vma)
> >> > +           return NULL;
> >> > +
> >> > +   spin_lock(&anon_vma->lock);
> >>
> >> RCU should guarantee that this spin_lock() is valid, but:
> >>
> >> > +   /*
> >> > +    * Get the oldest anon_vma on the list by depending on the ordering
> >> > +    * of the same_vma list setup by __page_set_anon_rmap
> >> > +    */
> >> > +   avc = list_entry(&anon_vma->head, struct anon_vma_chain, same_anon_vma);
> >>
> >> We're not guaranteed that the 'anon_vma->head' list is non-empty.
> >>
> >> Somebody could have freed the list and the anon_vma and we have a stale
> >> 'page->anon_vma' (that has just not been _released_ yet).
> >>
> >> And shouldn't that be 'list_first_entry'? Or &anon_vma->head.next?
> >>
> >> How did that line actually work for you? Or was it just a "it boots", but
> >> no actual testing of the rmap walk?
> >>
> >
> > This is what I just started testing on a 4-core machine. Lockdep didn't
> > complain but there are two potential sources of badness in anon_vma_lock_root
> > marked with XXX. The second is the most important because I can't see how the
> > local and root anon_vma locks can be safely swapped - i.e. release local and
> > get the root without the root disappearing. I haven't considered the other
> > possibilities yet such as always locking the root anon_vma. Going to
> > sleep on it.
> >
> > Any comments?
>
> <snip>
> > +/* Given an anon_vma, find the root of the chain, lock it and return the root */
> > +struct anon_vma *anon_vma_lock_root(struct anon_vma *anon_vma)
> > +{
> > +       struct anon_vma *root_anon_vma;
> > +       struct anon_vma_chain *avc, *root_avc;
> > +       struct vm_area_struct *vma;
> > +
> > +       /* Lock the same_anon_vma list and make sure we are on a chain */
> > +       spin_lock(&anon_vma->lock);
> > +       if (list_empty(&anon_vma->head)) {
> > +               spin_unlock(&anon_vma->lock);
> > +               return NULL;
> > +       }
> > +
> > +       /*
> > +        * Get the root anon_vma on the list by depending on the ordering
> > +        * of the same_vma list setup by __page_set_anon_rmap. Basically
> > +        * we are doing
> > +        *
> > +        * local anon_vma -> local vma -> deepest vma -> anon_vma
> > +        */
> > +       avc = list_first_entry(&anon_vma->head, struct anon_vma_chain, same_anon_vma);
>
> Dumb question.
>
> I can't understand why we should use list_first_entry.
>
> I looked over the code.
> anon_vma_chain_link uses list_add_tail so I think that's right.
> But anon_vma_prepare uses list_add. So it's not consistent.
> How do we make sure list_first_entry returns deepest vma?
>

list_first_entry is not getting the root (what you called deepest but lets
pick a name and stick with it or this will be worse than it already is). That
list_first entry is what gets us from

local anon_vma -> avc for the local anon_vma -> local vma

It's the ordering of the same_vma list hanging off the local_vma that is
important according to this comment in __page_set_anon_rmap

/*
* The page may be shared between multiple processes.
* We must use the _oldest_ possible anon_vma for the
* page mapping! That anon_vma is guaranteed to be
* present in all processes that could share this page.
*
* So take the last AVC chain entry in the vma, which is the
* deepest ancestor, and use the anon_vma from that.
*/
avc = list_entry(vma->anon_vma_chain.prev, struct anon_vma_chain, same_vma);
anon_vma = avc->anon_vma;
}

> Sorry if I am missing.
>

Not at all. The more people that look at this the better.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab