Re: [PATCH] mremap: enforce rmap src/dst vma ordering in case of vma_merge succeeding in copy_vma

[Mel Gorman <mgorman@suse.de>]

On Sat, Nov 05, 2011 at 10:00:52AM +0800, Nai Xia wrote:
> > <SNIP>
> > The only safe way to do it is to have _two_ different vmas, with two
> > different ->vm_pgoff. Then it will work. And by always creating a new
> > vma we'll always have it queued at the end, and it'll be safe for the
> > same reasons fork is safe.
> >
> > Always allocate a new vma, and then after the whole vma copy is
> > complete, look if we can merge and free some vma. After the fact, so
> > it means we can't use vma_merge anymore. vma_merge assumes the
> > new_range is "virtual" and no vma is mapped there I think. Anyway
> > that's an implementation issue. In some unlikely case we'll allocate 1
> > more vma than before, and we'll free it once mremap is finished, but
> > that's small problem compared to solving this once and for all.
> >
> > And that will fix it without ordering games and it'll fix the *vmap=
> > new_vma case too. That case really tripped on me as I was assuming
> > *that* was correct.
> 
> Yes. "Allocating a new vma, copy first and merge later " seems
> another solution without the tricky reordering. But you know,
> I now share some of Hugh's feeling that maybe we are too
> desperate using racing in places where locks are simpler
> and guaranteed to be safe.
> 

I'm tending to agree. The number of cases that must be kept in mind
is getting too tricky. Taking the anon_vma lock may be slower but at
the risk of sounding chicken, it's easier to understand.

> But I think Mel indicated that anon_vma_locking might be
> harmful to JVM SMP performance.
> How severe you expect this to be, Mel ?
> 

I would only expect it to be a problem during garbage collection when
there is a greater likelihood that mremap is heavily used. While it
would have been nice to avoid additional overhead in mremap, I don't
think the JVM GC case on its own is sufficient justification to avoid
taking the anon_vma lock.

-- 
Mel Gorman
SUSE Labs