[ofa-general] Re: [PATCH 0 of 9] mmu notifier #v12

[Andrea Arcangeli]

On Tue, Apr 22, 2008 at 08:36:04AM -0500, Robin Holt wrote:
> I am a little confused about the value of the seq_lock versus a simple
> atomic, but I assumed there is a reason and left it at that.

There's no value for anything but get_user_pages (get_user_pages takes
its own lock internally though). I preferred to explain it as a
seqlock because it was simpler for reading, but I totally agree in the
final implementation it shouldn't be a seqlock. My code was meant to
be pseudo-code only. It doesn't even need to be atomic ;).

> I don't know what you mean by "it'd" run slower and what you mean by
> "armed and disarmed".

1) when armed the time-window where the kvm-page-fault would be
blocked would be a bit larger without invalidate_page for no good
reason

2) if you were to remove invalidate_page when disarmed the VM could
would need two branches instead of one in various places

I don't want to waste cycles if not wasting them improves performance
both when armed and disarmed.

> For the sake of this discussion, I will assume "it'd" means the kernel in
> general and not KVM.  With the two call sites for range_begin/range_end,

I actually meant for both.

> By disarmed, I will assume you mean no notifiers registered for a
> particular mm.  In that case, the cache will make the second call
> effectively free.  So, for the disarmed case, I see no measurable
> difference.

For rmap is sure effective free, for do_wp_page it costs one branch
for no good reason.

> For the case where there is a notifier registered, I certainly can see
> a difference.  I am not certain how to quantify the difference as it

Agreed.

> When I was discussing this difference with Jack, he reminded me that
> the GRU, due to its hardware, does not have any race issues with the
> invalidate_page callout simply doing the tlb shootdown and not modifying
> any of its internal structures.  He then put a caveat on the discussion
> that _either_ method was acceptable as far as he was concerned.  The real
> issue is getting a patch in that satisfies all needs and not whether
> there is a seperate invalidate_page callout.

Sure, we have that patch now, I'll send it out in a minute, I was just
trying to explain why it makes sense to have an invalidate_page too
(which remains the only difference by now), removing it would be a
regression on all sides, even if a minor one.