On Tue, Jun 14, 2022 at 12:09 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > 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(). Ah, I misunderstood. I thought your goal was to mmap it and prevent page faults from allocating. It is indeed the case (and has been since before quite a few of us were born) that a hole in a sparse file is logically just a bunch of zeros. A way to make a file for which a hole is an actual hole seems like it would solve this problem nicely. It could also be solved more specifically for KVM by making sure that the private/shared mode that userspace programs is strict enough to prevent accidental allocations -- if a GPA is definitively private, shared, neither, or (potentially, on TDX only) both, then a page that *isn't* shared will never be accidentally allocated by KVM. If the shared backing is not mmapped, it also won't be accidentally allocated by host userspace on a stray or careless write. --Andy
