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

[Minchan Kim <minchan.kim@gmail.com>]

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). 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?


>> 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
>



-- 
Kind regards,
Minchan Kim

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