Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem mappings

[Lorenzo Stoakes]

On Tue, Feb 18, 2025 at 06:25:35PM +0100, David Hildenbrand wrote:
> > >
> > > QEMU, for example, will issue an mlockall(MCL_CURRENT | MCL_FUTURE); when
> > > requested to then exit(); if it fails.
> >
> > Hm under what circumstances? I use qemu extensively to test this stuff with
> > no issues. Unless you mean it's using it in the 'host' code somehow.
>
>
> -overcommit mem-lock=on
>
> or (legacy)
>
> -realtime mlock=on
>
> I think.
>
> [...]

Thanks

>
> > > >
> > > > It fails because it tries to 'touch' the memory, but 'touching' guard
> > > > region memory causes a segfault. This kind of breaks the idea of
> > > > mlock()'ing guard regions.
> > > >
> > > > I think adding workarounds to make this possible in any way is not really
> > > > worth it (and would probably be pretty gross).
> > > >
> > > > We already document that 'mlock()ing lightweight guard regions will fail'
> > > > as per man page so this is all in line with that.
> > >
> > > Right, and I claim that supporting VM_LOCKONFAULT might likely be as easy as
> > > allowing install/remove of guard regions when that flag is set.
> >
> > We already allow this flag! VM_LOCKED and VM_HUGETLB are the only flags we
> > disallow.
>
>
> See mlock2();
>
> SYSCALL_DEFINE3(mlock2, unsigned long, start, size_t, len, int, flags)
> {
> 	vm_flags_t vm_flags = VM_LOCKED;
>
> 	if (flags & ~MLOCK_ONFAULT)
> 		return -EINVAL;
>
> 	if (flags & MLOCK_ONFAULT)
> 		vm_flags |= VM_LOCKONFAULT;
>
> 	return do_mlock(start, len, vm_flags);
> }
>
>
> VM_LOCKONFAULT always as VM_LOCKED set as well.

OK cool, that makes sense.

As with much kernel stuff, I knew this in the past. Then I forgot. Then I knew
again, then... :P if only somebody would write it down in a book...

Yeah then that makes sense to check explicitly for (VM_LOCKED | VM_LOCKONFAULT)
in any MADV_GUARD_INSTALL_LOCKED variant as obviously this would be passively
excluded right now.

>
> --
> Cheers,
>
> David / dhildenb
>