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Subject: Re: [PATCH] KVM: selftests: Fix mapping length truncation in
 m{,un}map()
From:   Christian Borntraeger <borntraeger@de.ibm.com>
To:     Zenghui Yu <yuzenghui@huawei.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, pbonzini@redhat.com,
        vkuznets@redhat.com
Cc:     wanghaibin.wang@huawei.com
References: <20210624070931.565-1-yuzenghui@huawei.com>
 <78f94832-4a87-91fa-77fc-6b32252664de@de.ibm.com>
Message-ID: <83857466-6f9b-b45b-284c-708183800e56@de.ibm.com>
Date:   Wed, 30 Jun 2021 10:23:20 +0200
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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@huawei.com>
> 
> 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));