Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO_SIGBUS ioctl

[Mike Kravetz]

On 05/11/23 11:24, Axel Rasmussen wrote:
> The basic idea here is to "simulate" memory poisoning for VMs. A VM
> running on some host might encounter a memory error, after which some
> page(s) are poisoned (i.e., future accesses SIGBUS). They expect that
> once poisoned, pages can never become "un-poisoned". So, when we live
> migrate the VM, we need to preserve the poisoned status of these pages.
> 
> When live migrating, we try to get the guest running on its new host as
> quickly as possible. So, we start it running before all memory has been
> copied, and before we're certain which pages should be poisoned or not.
> 
> So the basic way to use this new feature is:
> 
> - On the new host, the guest's memory is registered with userfaultfd, in
>   either MISSING or MINOR mode (doesn't really matter for this purpose).
> - On any first access, we get a userfaultfd event. At this point we can
>   communicate with the old host to find out if the page was poisoned.

Just curious, what is this communication channel with the old host?
-- 
Mike Kravetz

> - If so, we can respond with a UFFDIO_SIGBUS - this places a swap marker
>   so any future accesses will SIGBUS. Because the pte is now "present",
>   future accesses won't generate more userfaultfd events, they'll just
>   SIGBUS directly.
> 
> UFFDIO_SIGBUS does not handle unmapping previously-present PTEs. This
> isn't needed, because during live migration we want to intercept
> all accesses with userfaultfd (not just writes, so WP mode isn't useful
> for this). So whether minor or missing mode is being used (or both), the
> PTE won't be present in any case, so handling that case isn't needed.
>