On Tue, Jun 14, 2022, Andy Lutomirski wrote: > On Tue, Jun 14, 2022 at 12:32 AM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote: > > > > On Thu, Jun 09, 2022 at 08:29:06PM +0000, Sean Christopherson wrote: > > > On Wed, Jun 08, 2022, Vishal Annapurve wrote: > > > > > > One argument is that userspace can simply rely on cgroups to detect misbehaving > > > guests, but (a) those types of OOMs will be a nightmare to debug and (b) an OOM > > > kill from the host is typically considered a _host_ issue and will be treated as > > > a missed SLO. > > > > > > An idea for handling this in the kernel without too much complexity would be to > > > add F_SEAL_FAULT_ALLOCATIONS (terrible name) that would prevent page faults from > > > allocating pages, i.e. holes can only be filled by an explicit fallocate(). Minor > > > faults, e.g. due to NUMA balancing stupidity, and major faults due to swap would > > > still work, but writes to previously unreserved/unallocated memory would get a > > > SIGSEGV on something it has mapped. That would allow the userspace VMM to prevent > > > unintentional allocations without having to coordinate unmapping/remapping across > > > multiple processes. > > > > Since this is mainly for shared memory and the motivation is catching > > misbehaved access, can we use mprotect(PROT_NONE) for this? We can mark > > those range backed by private fd as PROT_NONE during the conversion so > > subsequence misbehaved accesses will be blocked instead of causing double > > allocation silently. PROT_NONE, a.k.a. mprotect(), has the same vma downsides as munmap(). > This patch series is fairly close to implementing a rather more > efficient solution. I'm not familiar enough with hypervisor userspace > to really know if this would work, but: > > What if shared guest memory could also be file-backed, either in the > same fd or with a second fd covering the shared portion of a memslot? > This would allow changes to the backing store (punching holes, etc) to > be some without mmap_lock or host-userspace TLB flushes? Depending on > what the guest is doing with its shared memory, userspace might need > the memory mapped or it might not. That's what I'm angling for with the F_SEAL_FAULT_ALLOCATIONS idea. The issue, unless I'm misreading code, is that punching a hole in the shared memory backing store doesn't prevent reallocating that hole on fault, i.e. a helper process that keeps a valid mapping of guest shared memory can silently fill the hole. What we're hoping to achieve is a way to prevent allocating memory without a very explicit action from userspace, e.g. fallocate().
