Re: [RFC PATCH] mm/migration: Don't lock anon vmas in rmap_walk_anon()

[Linus Torvalds <torvalds@linux-foundation.org>]

On Sat, Dec 1, 2012 at 1:49 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> I *think* you are right that for this type of migration that we
> are using here we indeed don't need to take an exclusive vma
> lock - in fact I think we don't need to take it at all:

I'm pretty sure we do need at least a read-side reference.

Even if no other MM can contain that particular pte, the vma lock
protects the chain that is created by fork and exit and vma splitting
etc.

So it's enough that another thread does a fork() at the same time. Or
a partial unmap of the vma (that splits it in two), for the rmap chain
to be modified.

Besides, there's absolutely nothing that protects that vma to be part
of the same vma chain in entirely unrelated processes. The vma chain
can get quite long over multiple forks (it's even a performance
problem under some extreme loads).

And we do walk the rmap chain - so we need the lock.

But we walk it read-only afaik, which is why I think the semaphore
could be an rwsem.

Now, there *are* likely cases where we could avoid anon_vma locking
entirely, but they are very specialized. They'd be along the lines of

 - we hold the page table lock
 - we see that vma->anon_vma == vma->anon_vma->root
 - we see that vma->anon_vma->refcount == 1

or similar, because then we can guarantee that the anon-vma chain has
a length of one without even locking, and holding the page table lock
means that any concurrent fork or mmap/munmap from another thread will
block on this particular pte.

So I suspect that in the above kind of special case (which might be a
somewhat common case for normal page faults, for example) we could
make a "we have exclusive pte access to this page" argument. But quite
frankly, I completely made the above rules up in my head, they may be
bogus too.

For the general migration case, it's definitely not possible to just
drop the anon_vma lock.

                  Linus

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