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

Linus Torvalds
2010-05-05T14:38:09Z

On Wed, 5 May 2010, Mel Gorman wrote:
>
> With the recent anon_vma changes, there can be more than one anon_vma->lock
> to take in a anon_vma_chain but a second lock cannot be spinned upon in case
> of deadlock. The rmap walker tries to take locks of different anon_vma's
> but if the attempt fails, locks are released and the operation is restarted.

Btw, is this really needed?

Nobody else takes two anon_vma locks at the same time, so in order to
avoid ABBA deadlocks all we need to guarantee is that rmap_walk_ksm() and
rmap_walk_anon() always lock the anon_vma's in the same order.

And they do, as far as I can tell. How could we ever get a deadlock when
we have both cases doing the locking by walking the same_anon_vma list?

list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {

So I think the "retry" logic looks unnecessary, and actually opens us up
to a possible livelock bug (imagine a long chain, and heavy page fault
activity elsewhere that ends up locking some anon_vma in the chain, and
just the right behavior that gets us into a lockstep situation), rather
than fixing an ABBA deadlock.

Now, if it's true that somebody else _does_ do nested anon_vma locking,
I'm obviously wrong. But I don't see such usage.

Comments?

Linus