KVM: selftests: Extract guts of THP accessor to standalone sysfs helpers

Extract the guts of thp_configured() and get_trans_hugepagesz() to
standalone helpers so that the core logic can be reused for other sysfs
files, e.g. to query numa_balancing.

Opportunistically assert that the initial fscanf() read at least one byte,
and add a comment explaining the second call to fscanf().

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250508184649.2576210-2-jthoughton@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/lib/test_util.c

@@ -132,37 +132,50 @@ void print_skip(const char *fmt, ...)
 	puts(", skipping test");
 }
 
-bool thp_configured(void)
+static bool test_sysfs_path(const char *path)
 {
-	int ret;
 	struct stat statbuf;
+	int ret;
 
-	ret = stat("/sys/kernel/mm/transparent_hugepage", &statbuf);
+	ret = stat(path, &statbuf);
 	TEST_ASSERT(ret == 0 || (ret == -1 && errno == ENOENT),
-		    "Error in stating /sys/kernel/mm/transparent_hugepage");
+		    "Error in stat()ing '%s'", path);
 
 	return ret == 0;
 }
 
-size_t get_trans_hugepagesz(void)
+bool thp_configured(void)
+{
+	return test_sysfs_path("/sys/kernel/mm/transparent_hugepage");
+}
+
+static size_t get_sysfs_val(const char *path)
 {
 	size_t size;
 	FILE *f;
 	int ret;
 
-	TEST_ASSERT(thp_configured(), "THP is not configured in host kernel");
-
-	f = fopen("/sys/kernel/mm/transparent_hugepage/hpage_pmd_size", "r");
-	TEST_ASSERT(f != NULL, "Error in opening transparent_hugepage/hpage_pmd_size");
+	f = fopen(path, "r");
+	TEST_ASSERT(f, "Error opening '%s'", path);
 
 	ret = fscanf(f, "%ld", &size);
+	TEST_ASSERT(ret > 0, "Error reading '%s'", path);
+
+	/* Re-scan the input stream to verify the entire file was read. */
 	ret = fscanf(f, "%ld", &size);
-	TEST_ASSERT(ret < 1, "Error reading transparent_hugepage/hpage_pmd_size");
-	fclose(f);
+	TEST_ASSERT(ret < 1, "Error reading '%s'", path);
 
+	fclose(f);
 	return size;
 }
 
+size_t get_trans_hugepagesz(void)
+{
+	TEST_ASSERT(thp_configured(), "THP is not configured in host kernel");
+
+	return get_sysfs_val("/sys/kernel/mm/transparent_hugepage/hpage_pmd_size");
+}
+
 size_t get_def_hugetlb_pagesz(void)
 {
 	char buf[64];