Re: [PATCH v2 2/3] KVM: selftests: arm64: Skip sea_to_user without 1GB hugepages

[Like Xu <like.xu.linux@gmail.com>]

On 8/18/26 7:36 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] get_free_hugepages() overestimates available hugepages by ignoring reserved pages, which allows the test to proceed and crash with ENOMEM on systems with reserved pools.
> --
> 
> --- Patch [2]: [PATCH v2 2/3] KVM: selftests: arm64: Skip sea_to_user without 1GB hugepages ---
> commit 5fa5666824342b3dd28a5ef6a04bb5ac183257c8
> Author: Like Xu <like.xu.linux@gmail.com>
> 
> KVM: selftests: arm64: Skip sea_to_user without 1GB hugepages
> 
> This commit adds a check to skip the sea_to_user test if the host does not
> have enough 1GB hugepages available, preventing a hard failure on systems
> without an empty pool.
> 
> [ ... ]
>> diff --git a/tools/testing/selftests/kvm/lib/test_util.c b/tools/testing/selftests/kvm/lib/test_util.c
>> index bab1bd2b775b6..29f9c1d60b1b7 100644
>> --- a/tools/testing/selftests/kvm/lib/test_util.c
>> +++ b/tools/testing/selftests/kvm/lib/test_util.c
>> @@ -222,6 +222,21 @@ size_t get_def_hugetlb_pagesz(void)
>>   	TEST_FAIL("Error in reading /proc/meminfo");
>>   }
>>   
>> +size_t get_free_hugepages(size_t page_size)
>> +{
>> +	char path[128];
>> +	size_t free;
>> +
>> +	snprintf(path, sizeof(path),
>> +		 "/sys/kernel/mm/hugepages/hugepages-%zukB/free_hugepages",
>> +		 page_size >> 10);
>> +	if (!test_sysfs_path(path))
>> +		return 0;
>> +
>> +	free = get_sysfs_val(path);
>> +	return free * page_size;
> 
> [Severity: Medium]
> Does this calculation overestimate the available hugepages by ignoring the
> reserved pool?
> 
> The sysfs free_hugepages value includes pages that are already committed to
> other mappings but not yet allocated (which are tracked via resv_hugepages).
> 
> If the test runs on a host where free_hugepages meets the VM_MEM_SIZE
> requirement but some or all of those pages are reserved, the test will
> incorrectly proceed and still crash with an -ENOMEM abort instead of cleanly
> skipping.
> 
> Could we also read and subtract resv_hugepages to determine the actual
> allocatable count?

For any human to double check:

diff --git a/tools/testing/selftests/kvm/arm64/sea_to_user.c 
b/tools/testing/selftests/kvm/arm64/sea_to_user.c
index 1c2a743ca8e23..7cf95da8e594d 100644
--- a/tools/testing/selftests/kvm/arm64/sea_to_user.c
+++ b/tools/testing/selftests/kvm/arm64/sea_to_user.c
@@ -281,6 +281,12 @@ static struct kvm_vm 
*vm_create_with_sea_handler(struct kvm_vcpu **vcpu)
  	alignment = max(backing_page_size, guest_page_size);
  	num_guest_pages = VM_MEM_SIZE / guest_page_size;

+	/*
+	 * The region is backed by 1GB hugepages; skip gracefully rather than
+	 * failing with mmap() -ENOMEM if the host has none reserved.
+	 */
+	TEST_REQUIRE(get_free_hugepages(backing_page_size) >= VM_MEM_SIZE);
+
  	vm = __vm_create_with_one_vcpu(vcpu, num_guest_pages, guest_code);
  	vm_init_descriptor_tables(vm);
  	vcpu_init_descriptor_tables(*vcpu);
diff --git a/tools/testing/selftests/kvm/include/test_util.h 
b/tools/testing/selftests/kvm/include/test_util.h
index a56271c237ae9..0624922c2735d 100644
--- a/tools/testing/selftests/kvm/include/test_util.h
+++ b/tools/testing/selftests/kvm/include/test_util.h
@@ -168,6 +168,7 @@ struct vm_mem_backing_src_alias {
  bool thp_configured(void);
  size_t get_trans_hugepagesz(void);
  size_t get_def_hugetlb_pagesz(void);
+size_t get_free_hugepages(size_t page_size);
  const struct vm_mem_backing_src_alias *vm_mem_backing_src_alias(u32 i);
  size_t get_backing_src_pagesz(u32 i);
  bool is_backing_src_hugetlb(u32 i);
diff --git a/tools/testing/selftests/kvm/lib/test_util.c 
b/tools/testing/selftests/kvm/lib/test_util.c
index bab1bd2b775b6..3bff9d825e8af 100644
--- a/tools/testing/selftests/kvm/lib/test_util.c
+++ b/tools/testing/selftests/kvm/lib/test_util.c
@@ -222,6 +222,32 @@ size_t get_def_hugetlb_pagesz(void)
  	TEST_FAIL("Error in reading /proc/meminfo");
  }

+size_t get_free_hugepages(size_t page_size)
+{
+	char path[128];
+	size_t free, resv;
+
+	snprintf(path, sizeof(path),
+		 "/sys/kernel/mm/hugepages/hugepages-%zukB/free_hugepages",
+		 page_size >> 10);
+	if (!test_sysfs_path(path))
+		return 0;
+	free = get_sysfs_val(path);
+
+	/*
+	 * free_hugepages still counts pages that are reserved by existing
+	 * mappings but not yet faulted in, so only free minus resv can back a
+	 * new reservation (see available_huge_pages() in mm/hugetlb.c). Ignore
+	 * surplus/overcommit: it does not apply to gigantic pages at runtime.
+	 */
+	snprintf(path, sizeof(path),
+		 "/sys/kernel/mm/hugepages/hugepages-%zukB/resv_hugepages",
+		 page_size >> 10);
+	resv = test_sysfs_path(path) ? get_sysfs_val(path) : 0;
+
+	return free > resv ? (free - resv) * page_size : 0;
+}
+
  #define ANON_FLAGS	(MAP_PRIVATE | MAP_ANONYMOUS)
  #define ANON_HUGE_FLAGS	(ANON_FLAGS | MAP_HUGETLB)

> 
>> +}
>> +
>>   #define ANON_FLAGS	(MAP_PRIVATE | MAP_ANONYMOUS)
>>   #define ANON_HUGE_FLAGS	(ANON_FLAGS | MAP_HUGETLB)
>>
>