Re: [PATCH] KVM: selftests: Fix mapping length truncation in m{,un}map()

[Christian Borntraeger]

On 30.06.21 09:37, Christian Borntraeger wrote:
> On 24.06.21 09:09, Zenghui Yu wrote:
> > max_mem_slots is now declared as uint32_t. The result of (0x200000 * 32767)
> > is unexpectedly truncated to be 0xffe00000, whilst we actually need to
> > allocate about, 63GB. Cast max_mem_slots to size_t in both mmap() and
> > munmap() to fix the length truncation.
> >
> > We'll otherwise see the failure on arm64 thanks to the access_ok() checking
> > in __kvm_set_memory_region(), as the unmapped VA happen to go beyond the
> > task's allowed address space.
> >
> >   # ./set_memory_region_test
> > Allowed number of memory slots: 32767
> > Adding slots 0..32766, each memory region with 2048K size
> > ==== Test Assertion Failure ====
> >    set_memory_region_test.c:391: ret == 0
> >    pid=94861 tid=94861 errno=22 - Invalid argument
> >       1    0x00000000004015a7: test_add_max_memory_regions at set_memory_region_test.c:389
> >       2     (inlined by) main at set_memory_region_test.c:426
> >       3    0x0000ffffb8e67bdf: ?? ??:0
> >       4    0x00000000004016db: _start at :?
> >    KVM_SET_USER_MEMORY_REGION IOCTL failed,
> >    rc: -1 errno: 22 slot: 2615
> >
> > Fixes: 3bf0fcd75434 ("KVM: selftests: Speed up set_memory_region_test")
> > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
>
> While likely correct, this now breaks on many test systems in our CI that have less memory than 64GB.
> (We do get ENOMEM). I have not seen the ENOMEM failures in earlier versions. Strange

As we do not need the memory, the following seems to do the trick (MAP_NORESERVE)


diff --git a/tools/testing/selftests/kvm/set_memory_region_test.c b/tools/testing/selftests/kvm/set_memory_region_test.c
index d8812f27648c..0fc68371fb1b 100644
--- a/tools/testing/selftests/kvm/set_memory_region_test.c
+++ b/tools/testing/selftests/kvm/set_memory_region_test.c
@@ -377,7 +377,7 @@ static void test_add_max_memory_regions(void)
                (max_mem_slots - 1), MEM_REGION_SIZE >> 10);
 
        mem = mmap(NULL, (size_t)max_mem_slots * MEM_REGION_SIZE + alignment,
-                  PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+                  PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, -1, 0);
        TEST_ASSERT(mem != MAP_FAILED, "Failed to mmap() host");
        mem_aligned = (void *)(((size_t) mem + alignment - 1) & ~(alignment - 1));