Re: [NUMA] Fix memory policy refcounting

[Christoph Lameter <clameter@sgi.com>]

On Tue, 30 Oct 2007, Lee Schermerhorn wrote:

> As part of my shared policy cleanup and enhancement series, I "fixed"
> numa_maps to display the sub-ranges of policies in a shm segment mapped
> by a single vma. As part of this fix, I also modified mempolicy.c so
> that it does not split vmas that support set_policy vm_ops, because
> handling both split vmas and non-split vmas for a single shm segment
> would have complicated the code more than I thought necessary.  This is
> still at prototype stage--altho' it works against 23-rc8-mm2.

I have not looked at that yet. Maybe you could post another patch?

> Re:  'ref = 3' -- One reference for the rbtree--the shm segment and it's
> policies continue to exist independent of any vma mappings--and one for
> each attached vma.  Because the vma references are protected by the
> respective task/mm_struct's  mmap_sem, we won't need to add an
> additional reference during lookup, nor release it when finished with
> the policy.  And, we won't need to mess with any other task's mm data
> structures when installing/removing shmem policies.  Of course, munmap()
> of a vma will need to decrement the ref count of all policies in a
> shared policy tree, but this is not a "fast path".  Unfortunately, we
> don't have a unmap file operation, so I'd have to add one, or otherwise
> arrange to remove the unmapping vma's ref--perhaps via a vm_op so that
> we only need to call it on vmas that support it--i.e., that support
> shared policy.

Yup that sounds like it is going to be a good solution.

> if overkill.  This involves:  1) fixing do_set_mempolicy() to hold
> mmap_sem for write over change, 2) fixing up reference counting for
> interleaving for both normal [forgot unref] and huge [unconditional
> unref should be conditional] and 3) adding ref to policy in
> shm_get_policy() to match shmem_get_policy.  All 3 of these are required
> to be correct w/o changing any of the rest of the current ref counting.

I know about 1. I'd have to look through 2 + 3. I would suggest to fix the 
refcounting by doing the refcounting using vmas as you explained above and 
simply remove the problems that exist there right now.

> Then, once the vma-protected shared policy mechanism discussed above is
> in mergable, we can back out all of the extra ref's on other task and
> vma policies and the lookup-time ref on shared policies, along with all
> of the matching unrefs.

Too complicated. Lets go there directly.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>