Re: [PATCH] KVM: guest_memfd: Disable VMA merging with VM_DONTEXPAND

[David Hildenbrand (Arm)]

On 2/9/26 19:24, Ackerley Tng wrote:
> "David Hildenbrand (Arm)" <david@xxxxxxxxxx> writes:
>
> > On 2/8/26 18:34, Ackerley Tng wrote:
> > > Ackerley Tng <ackerleytng@xxxxxxxxxx> writes:
> > >
> > >
> > > I trimmed the repro to this:
> > >
> > > static void test_guest_memfd_repro(void)
> > > {
> > > 	struct kvm_vcpu *vcpu;
> > > 	uint8_t *unaligned_mem;
> > > 	struct kvm_vm *vm;
> > > 	uint8_t *mem;
> > > 	int fd;
> > >
> > > 	vm = __vm_create_shape_with_one_vcpu(VM_SHAPE_DEFAULT, &vcpu, 1, guest_code);
> > >
> > > 	fd = vm_create_guest_memfd(vm, SZ_2M * 2, GUEST_MEMFD_FLAG_MMAP |
> > > GUEST_MEMFD_FLAG_INIT_SHARED);
> > >
> > > 	unaligned_mem = mmap(NULL, SZ_2M + SZ_2M, PROT_READ | PROT_WRITE,
> > > MAP_FIXED | MAP_SHARED, fd, 0);
> > > 	mem = align_ptr_up(unaligned_mem, SZ_2M);
> > > 	TEST_ASSERT(((unsigned long)mem & (SZ_2M - 1)) == 0, "returned
> > > address must be aligned to SZ_2M");
> > >
> > > 	TEST_ASSERT_EQ(madvise(mem, SZ_2M, MADV_HUGEPAGE), 0);
> > >
> > > 	for (int i = 0; i < SZ_2M; i += SZ_4K)
> > > 		READ_ONCE(mem[i]);
> > >
> > > 	TEST_ASSERT_EQ(madvise(mem, SZ_2M, MADV_COLLAPSE), 0);
> > >
> > > 	TEST_ASSERT_EQ(madvise(mem, SZ_2M, MADV_DONTNEED), 0);
> > >
> > > 	/* This triggers the WARNing. */
> > > 	READ_ONCE(mem[0]);
> > >
> > > 	munmap(unaligned_mem, SZ_2M * 2);
> > >
> > > 	close(fd);
> > > 	kvm_vm_free(vm);
> > > }
> > >
> > > And tried to replace the fd creation the secretmem equivalent
> > >
> > > 	fd = syscall(__NR_memfd_secret, 0);
> > > 	TEST_ASSERT(fd >= 0, "Couldn't create secretmem fd.");
> > > 	TEST_ASSERT_EQ(ftruncate(fd, SZ_2M * 2), 0);
> > >
> > > Should a guest_memfd selftest be added to cover this?
> > >
> > > MADV_COLLAPSE fails with EINVAL, but it does go through to
> > > hpage_collapse_scan_file() -> collapse_file(), before failing because
> > > when collapsing the page, copy_mc_highpage() returns > 0.
> >
> > Just what I suspected. :)
> >
> > Thanks for digging into the details!
>
> Happy to help :)
>
> In general, do we want the reproducers added as selftests? Should this
> be added as part of tools/testing/selftests/kvm/guest_memfd_test.c

I guess adding it to guest_memfd_test.c and asserting that MADV_COLLAPSE 
fails as expected could be a reasonable test case. It's not a lot of 
code and easy to verify.

--
Cheers,

David