Re: [PATCH v4] selftests/mm: khugepaged: initialize file contents via mmap

[David Hildenbrand (Arm)]

On 4/28/26 22:23, Zi Yan wrote:
> On 28 Apr 2026, at 16:22, Zi Yan wrote:
> 
>> On 28 Apr 2026, at 16:19, David Hildenbrand (Arm) wrote:
>>
>>>
>>> But why doesn't the write() make the folios dirty? Or are we running into 
>>> some
>>> race condition where the old code just worked by chance?
>>>
>>
>> The fd is opened with O_DSYNC, so the data from write() is transferred to
>> disk when write() returns.
> 
> And the folio is clean.

Ah, I missed the O_DSYNC. That makes sense, thanks.

v2 actually contained the msync. So I assume we could then drop the O_DSYNC.

Note: there is no need to close the file or munmap before the exit on the error
path.

The following works for me


diff --git a/tools/testing/selftests/mm/khugepaged.c
b/tools/testing/selftests/mm/khugepaged.c
index 3fe7ef04ac62..5862a3213840 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");


-- 
Cheers,

David