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

[Mel Gorman <mel@csn.ul.ie>]

On Thu, May 06, 2010 at 07:01:54PM +0900, Minchan Kim wrote:
> On Thu, May 6, 2010 at 6:54 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> > 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).

Of course, I have to clean out my own references to "deepest" :/

> That
> > list_first entry is what gets us from
> >
> > local anon_vma -> avc for the local anon_vma -> local vma
> >
> 
> Yes. Sorry for confusing word. :)
> Let's have a question again. What I have a question is that why we
> have to use list_first_entry not list_entry for getting local_vma?
> 

Nothing other than it's easier to read and a bit more self-documenting
than;

avc = list_entry(anon_vma->head.next, struct anon_vma_chain, same_anon_vma);

> 
> >> Sorry if I am missing.
> >>
> >
> > Not at all. The more people that look at this the better.
> 
> Thanks. Mel.
> 

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