Re: [PATCH 0/5] Volatile Ranges (v12) & LSF-MM discussion fodder

[John Stultz]

On 04/02/2014 11:31 AM, Andrea Arcangeli wrote:
> On Tue, Apr 01, 2014 at 09:03:57PM -0700, John Stultz wrote:
>> Now... once you've chosen SIGBUS semantics, there will be folks who will
>> try to exploit the fact that we get SIGBUS on purged page access (at
>> least on the user-space side) and will try to access pages that are
>> volatile until they are purged and try to then handle the SIGBUS to fix
>> things up. Those folks exploiting that will have to be particularly
>> careful not to pass volatile data to the kernel, and if they do they'll
>> have to be smart enough to handle the EFAULT, etc. That's really all
>> their problem, because they're being clever. :)
> I'm actually working on feature that would solve the problem for the
> syscalls accessing missing volatile pages. So you'd never see a
> -EFAULT because all syscalls won't return even if they encounters a
> missing page in the volatile range dropped by the VM pressure.
>
> It's called userfaultfd. You call sys_userfaultfd(flags) and it
> connects the current mm to a pseudo filedescriptor. The filedescriptor
> works similarly to eventfd but with a different protocol.
So yea! I actually think (its been awhile now) I mentioned your work to
Taras (or maybe he mentioned it to me?), but it did seem like the
userfaltfd would be a better solution for the style of fault handling
they were thinking about. (Especially as actually handling SIGBUS and
doing something sane in a large threaded application seems very difficult).

That said, explaining volatile ranges as a concept has been difficult
enough without mixing in other new concepts :), so I'm hesitant to tie
the functionality together in until its clear the userfaultfd approach
is likely to land. But maybe I need to take a closer look at it.

thanks
-john
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