Re: KVM: guest: only batch user pte updates

Marcelo Tosatti
2009-02-10T22:42:23Z

On Tue, Feb 10, 2009 at 02:17:49PM -0800, Jeremy Fitzhardinge wrote:
> Marcelo Tosatti wrote:
>> KVM's paravirt mmu pte batching has issues with, at least, kernel
>> updates from DEBUG_PAGEALLOC.
>>
>> This has been experienced with slab allocation from irq context from
>> within lazy mmu sections:
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=480822
>>
>> DEBUG_PAGEALLOC will map/unmap the kernel pagetables to catch bad
>> accesses, with code such as:
>>
>> __change_page_attr():
>>
>> /*
>> * Do we really change anything ?
>> */
>> if (pte_val(old_pte) != pte_val(new_pte)) {
>> set_pte_atomic(kpte, new_pte);
>> cpa->flags |= CPA_FLUSHTLB;
>> }
>>
>> A present->nonpresent update can be queued, but not yet committed to
>> memory. So the set_pte_atomic will be skipped but the update flushed
>> afterwards. set_pte_ATOMIC.
>>
>
> Are you saying that there's a queued update which means that old_pte is
> a stale value which happens to equal new_pte, so new_pte is never set?
> OK, sounds like a generic problem, of the same sort we've had with
> kmap_atomic being used in interrupt routines in lazy mode.

Yes. It seems however that only set_pte_at/pte_update/_defer are
used under significatly long lazy mmu sections (long as in number of
updates). Is it worthwhile to bother (and risk) batching kernel pte updates ?

Until someone forgets about arch_flush_lazy_mmu_mode again...

> In this case, I think the proper fix is to call
> arch_flush_lazy_mmu_mode() before reading old_pte to make sure its up to
> date, and calling it again when processing CPA_FLUSHTLB.

> Could you try the patch below instead?

It should work yes.

> (BTW, set_pte_atomic doesn't mean synchronous; it just means its safe to
> use on live ptes on 32-bit PAE machines which can't otherwise atomically
> update a pte.)

Doh, of course.