mm: add merging after mremap resize

When mremap call results in expansion, it might be possible to merge the
VMA with the next VMA which might become adjacent. This patch adds
vma_merge call after the expansion is done to try and merge.

[[e-mail]: coding-style cleanups]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jakub Matěna <[e-mail]>
Reviewed-by: Vlastimil Babka <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: "Kirill A . Shutemov" <[e-mail]>
Cc: Liam Howlett <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Peter Zijlstra (Intel) <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/mremap.c

···
  */
 
 #include <linux/mm.h>
+#include <linux/mm_inline.h>
 #include <linux/hugetlb.h>
 #include <linux/shm.h>
 #include <linux/ksm.h>
···
 #include <linux/mmu_notifier.h>
 #include <linux/uaccess.h>
 #include <linux/userfaultfd_k.h>
+#include <linux/mempolicy.h>
 
 #include <asm/cacheflush.h>
 #include <asm/tlb.h>
···
 		/* can we just expand the current mapping? */
 		if (vma_expandable(vma, new_len - old_len)) {
 			long pages = (new_len - old_len) >> PAGE_SHIFT;
+			unsigned long extension_start = addr + old_len;
+			unsigned long extension_end = addr + new_len;
+			pgoff_t extension_pgoff = vma->vm_pgoff + (old_len >> PAGE_SHIFT);
 
 			if (vma->vm_flags & VM_ACCOUNT) {
 				if (security_vm_enough_memory_mm(mm, pages)) {
···
 				}
 			}
 
-			if (vma_adjust(vma, vma->vm_start, addr + new_len,
-				       vma->vm_pgoff, NULL)) {
+			/*
+			 * Function vma_merge() is called on the extension we are adding to
+			 * the already existing vma, vma_merge() will merge this extension with
+			 * the already existing vma (expand operation itself) and possibly also
+			 * with the next vma if it becomes adjacent to the expanded vma and
+			 * otherwise compatible.
+			 */
+			vma = vma_merge(mm, vma, extension_start, extension_end,
+					vma->vm_flags, vma->anon_vma, vma->vm_file,
+					extension_pgoff, vma_policy(vma),
+					vma->vm_userfaultfd_ctx, anon_vma_name(vma));
+			if (!vma) {
 				vm_unacct_memory(pages);
 				ret = -ENOMEM;
 				goto out;

tools/testing/selftests/vm/mremap_test.c

···
 }
 
 /*
+ * This test validates that merge is called when expanding a mapping.
+ * Mapping containing three pages is created, middle page is unmapped
+ * and then the mapping containing the first page is expanded so that
+ * it fills the created hole. The two parts should merge creating
+ * single mapping with three pages.
+ */
+static void mremap_expand_merge(unsigned long page_size)
+{
+	char *test_name = "mremap expand merge";
+	FILE *fp;
+	char *line = NULL;
+	size_t len = 0;
+	bool success = false;
+	char *start = mmap(NULL, 3 * page_size, PROT_READ | PROT_WRITE,
+			   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+
+	munmap(start + page_size, page_size);
+	mremap(start, page_size, 2 * page_size, 0);
+
+	fp = fopen("/proc/self/maps", "r");
+	if (fp == NULL) {
+		ksft_test_result_fail("%s\n", test_name);
+		return;
+	}
+
+	while (getline(&line, &len, fp) != -1) {
+		char *first = strtok(line, "- ");
+		void *first_val = (void *)strtol(first, NULL, 16);
+		char *second = strtok(NULL, "- ");
+		void *second_val = (void *) strtol(second, NULL, 16);
+
+		if (first_val == start && second_val == start + 3 * page_size) {
+			success = true;
+			break;
+		}
+	}
+	if (success)
+		ksft_test_result_pass("%s\n", test_name);
+	else
+		ksft_test_result_fail("%s\n", test_name);
+	fclose(fp);
+}
+
+/*
  * Returns the start address of the mapping on success, else returns
  * NULL on failure.
  */
···
 	int i, run_perf_tests;
 	unsigned int threshold_mb = VALIDATION_DEFAULT_THRESHOLD;
 	unsigned int pattern_seed;
+	int num_expand_tests = 1;
 	struct test test_cases[MAX_TEST];
 	struct test perf_test_cases[MAX_PERF_TEST];
 	int page_size;
···
 				(threshold_mb * _1MB >= _1GB);
 
 	ksft_set_plan(ARRAY_SIZE(test_cases) + (run_perf_tests ?
-		      ARRAY_SIZE(perf_test_cases) : 0));
+		      ARRAY_SIZE(perf_test_cases) : 0) + num_expand_tests);
 
 	for (i = 0; i < ARRAY_SIZE(test_cases); i++)
 		run_mremap_test_case(test_cases[i], &failures, threshold_mb,
 				     pattern_seed);
+
+	mremap_expand_merge(page_size);
 
 	if (run_perf_tests) {
 		ksft_print_msg("\n%s\n",