Re: race with page_referenced_one->ptep_test_and_clear_young and pagetable setup/pulldown

Jeremy Fitzhardinge
2007-10-05T09:05:44Z

Keir Fraser wrote:
>
> Hang on! How is the access unlocked? By my reading
> page_referenced_one()->page_check_address()->spin_lock(pte_lockptr()).
>

Ah, OK. I'd overlooked that.

> The problem here is most likely insufficient locking in the pin/unpin
> table-walking code, in light of the fact that you are probably running
> with
> per-page spinlocks (SPLIT_PTLOCK_CPUS). Because we nobble that option
> in our
> own kernel ports it suffices to take the page_table_lock when doing the
> walk-[un]pin-remap routine. This is *not* true with SPLIT_PTLOCK_CPUS.
>

Hm, I see.

> Fortuitously, Jan Beulich has a patch to fix this. It's not going to be
> directly applicable to 2.6.23-rc series, but should be easily ported:
> <http://lists.xensource.com/archives/html/xen-devel/2007-03/msg01200.html>.
>

OK, I can use that.

Andi says:
> Do I misread that patch or does it really walk the complete address
> space and try to take all possible locks? Isn't that very slow?
>

That's pretty much what it has to do. Pinning/unpinning walks the whole
pagetable anyway, so it shouldn't be much more expensive. And they're
relatively rare operations (fork, exec, exit).

J