On Fri, 17 Jun 2011, Linus Torvalds wrote:

On Fri, Jun 17, 2011 at 11:32 AM, Peter Zijlstra [off-list ref] wrote:

quoted

something like so I guess, completely untested etc..

Having gone over it a bit more, I actually think I prefer to just
special-case the allocation instead.

We already have to drop the anon_vma lock for the "out of memory"
case, and a slight re-organization of clone_anon_vma() makes it easy
to just first try a NOIO allocation with the lock still held, and then
if that fails do the "drop lock, retry, and hard-fail" case.

IOW, something like the attached (on top of the patches already posted
except for your memory reclaim thing)

Hugh, does this fix the lockdep issue?

Yes, that fixed the lockdep issue, and ran nicely under load for an hour.

I agree that it's better to do this GFP_NOWAIT and fallback,
than trylock the anon_vma.

And I'm happy that you've still got that WARN_ON_ONCE(root) in: I do not
have a fluid mental model of the anon_vma_chains, get lost there; and
though it's obvious that we must have the same anon_vma->root going
down the same_anon_vma list, I could not put my finger on a killer
demonstration for why the same has to be true of the same_vma list.

But I've not seen your WARN_ON_ONCE fire, and it's hard to imagine
how there could be more than one root in the whole bundle of lists.

Hugh

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