Re: [RFC PATCH] asynchronous page fault.

[KAMEZAWA Hiroyuki]

Peter Zijlstra wrote:
> On Mon, 2009-12-28 at 18:58 +0900, KAMEZAWA Hiroyuki wrote:
>> Peter Zijlstra wrote:
>> > On Mon, 2009-12-28 at 09:36 +0900, KAMEZAWA Hiroyuki wrote:
>> >>
>> >> > The idea is to let the RCU lock span whatever length you need the
>> vma
>> >> > for, the easy way is to simply use PREEMPT_RCU=y for now,
>> >>
>> >> I tried to remove his kind of reference count trick but I can't do
>> that
>> >> without synchronize_rcu() somewhere in unmap code. I don't like that
>> and
>> >> use this refcnt.
>> >
>> > Why, because otherwise we can access page tables for an already
>> unmapped
>> > vma? Yeah that is the interesting bit ;-)
>> >
>> Without that
>>   vma->a_ops->fault()
>> and
>>   vma->a_ops->unmap()
>> can be called at the same time. and vma->vm_file can be dropped while
>> vma->a_ops->fault() is called. etc...
>
> Right, so acquiring the PTE lock will either instantiate page tables for
> a non-existing vma, leaving you with an interesting mess to clean up, or
> you can also RCU free the page tables (in the same RCU domain as the
> vma) which will mostly[*] avoid that issue.
>
> [ To make live really really interesting you could even re-use the
>   page-tables and abort the RCU free when the region gets re-mapped
>   before the RCU callbacks happen, this will avoid a free/alloc cycle
>   for fast remapping workloads. ]
>
> Once you hold the PTE lock, you can validate the vma you looked up,
> since ->unmap() syncs against it. If at that time you find the
> speculative vma is dead, you fail and re-try the fault.
>
My previous one did similar but still used vma->refcnt. I'll consider again.

> [*] there still is the case of faulting on an address that didn't
> previously have page-tables hence the unmap page table scan will have
> skipped it -- my hacks simply leaked page tables here, but the idea was
> to acquire the mmap_sem for reading and cleanup properly.
>
Hmm, thank you for hints.

But this current version implementation has some reasons.
  - because pmd has some trobles because of quicklists..I don't wanted to
    touch free routine of them.
  - pmd can be removed asynchronously while page fault is going on.
  - I'd like to avoid modification to free_pte_range etc...

I feel pmd/page-table-lock is a hard to handle object than expected.

I'll consider some about per-thread approach or split vma approach
or scalable range lock or some synchronization without heavy atomic op.

Anyway, I think I show something can be done without mmap_sem modification.
See you next year.

Thanks,
-Kame

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