Re: [PATCH gmem FIXUP] kvm: guestmem: do not use a file system

From: Sean Christopherson
Date: Mon Oct 09 2023 - 17:26:44 EST


On Mon, Oct 09, 2023, Al Viro wrote:
> On Mon, Oct 09, 2023 at 01:20:06PM -0700, Sean Christopherson wrote:
> > On Mon, Oct 09, 2023, Al Viro wrote:
> > > On Mon, Oct 09, 2023 at 07:32:48AM -0700, Sean Christopherson wrote:
> > >
> > > > Yeah, we found that out the hard way. Is using the "secure" variant to get a
> > > > per-file inode a sane approach, or is that abuse that's going to bite us too?
> > > >
> > > > /*
> > > > * Use the so called "secure" variant, which creates a unique inode
> > > > * instead of reusing a single inode. Each guest_memfd instance needs
> > > > * its own inode to track the size, flags, etc.
> > > > */
> > > > file = anon_inode_getfile_secure(anon_name, &kvm_gmem_fops, gmem,
> > > > O_RDWR, NULL);
> > >
> > > Umm... Is there any chance that your call site will ever be in a module?
> > > If not, you are probably OK with that variant.
> >
> > Yes, this code can be compiled as a module. I assume there issues with the inode
> > outliving the module?
>
> The entire file, actually... If you are using that mechanism in a module, you
> need to initialize kvm_gmem_fops.owner to THIS_MODULE; AFAICS, you don't have
> that done.

Ah, that's handled indirectly handled by a chain of refcounted objects. Every
VM that KVM creates gets a reference to the module, and each guest_memfd instance
gets a reference to its owning VM.

Thanks much for the help!