[PATCH v5] selftests/mm: khugepaged: initialize file contents via mmap

[Vineet Agarwal <agarwal.vineet2006@gmail.com>]

file_setup_area() currently allocates anonymous memory, fills it,
and writes it into the backing file used for collapse testing.

Instead of copying data through write(), resize the file with
ftruncate(), map it directly with MAP_SHARED, and initialize the
mapped area in place.

This simplifies the setup path and avoids the need for explicit
partial write handling.

Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>

v4 -> v5:
  - Restore msync() to keep folios clean for READ_ONLY_THP_FOR_FS
  - Remove O_DSYNC since msync() handles synchronization
  - Simplify mmap() check and drop unnecessary cleanup in error paths
---
 tools/testing/selftests/mm/khugepaged.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 3fe7ef04ac62..c8393ca52cab 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -373,7 +373,7 @@ static void *file_setup_area(int nr_hpages)
 	unlink(finfo.path);  /* Cleanup from previous failed tests */
 	printf("Creating %s for collapse%s...", finfo.path,
 	       finfo.type == VMA_SHMEM ? " (tmpfs)" : "");
-	fd = open(finfo.path, O_DSYNC | O_CREAT | O_RDWR | O_TRUNC | O_EXCL,
+	fd = open(finfo.path, O_CREAT | O_RDWR | O_TRUNC | O_EXCL,
 		  777);
 	if (fd < 0) {
 		perror("open()");
@@ -381,9 +381,21 @@ static void *file_setup_area(int nr_hpages)
 	}
 
 	size = nr_hpages * hpage_pmd_size;
-	p = alloc_mapping(nr_hpages);
+	if (ftruncate(fd, size)) {
+		perror("ftruncate()");
+		exit(EXIT_FAILURE);
+	}
+	p = mmap(BASE_ADDR, size, PROT_READ | PROT_WRITE,
+		MAP_SHARED, fd, 0);
+	if (p != BASE_ADDR) {
+		perror("mmap()");
+		exit(EXIT_FAILURE);
+	}
 	fill_memory(p, 0, size);
-	write(fd, p, size);
+	if (msync(p, size, MS_SYNC)) {
+		perror("msync()");
+		exit(EXIT_FAILURE);
+	}
 	close(fd);
 	munmap(p, size);
 	success("OK");
-- 
2.54.0