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

[Zi Yan <ziy@nvidia.com>]

On 28 Apr 2026, at 16:07, David Hildenbrand (Arm) wrote:

> On 4/28/26 21:56, Zi Yan wrote:
>> On 28 Apr 2026, at 9:31, Vineet Agarwal wrote:
>>
>>> 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>
>>>
>>> v3 -> v4:
>>>   - Restore unrelated blank line removal
>>>   - Restore original close()/munmap() ordering
>>> ---
>>>  tools/testing/selftests/mm/khugepaged.c | 14 ++++++++++++--
>>>  1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> This patch breaks the khugepaged test for READ_ONLY_THP_FOR_FS.
>>
>> When I ran sudo ./khugepaged all:file ~/ on ext4,
>>
>> collapse_max_ptes_none (khugepaged:file) fails.
>
> madvise() still works? Is it maybe because of dirty folios? (but that should
> also be the case on write ...)

I asked codex instead.

Basically, mmap(MMAP_SHARED) makes the written folio dirty and the folio
survives the later drop_caches. Adding msync() fixes the issue.

The patch below fixes the issue.

From fdf796e52a94b7e21614c8cfb6cc947146221d28 Mon Sep 17 00:00:00 2001
From: Zi Yan <ziy@nvidia.com>
Date: Tue, 28 Apr 2026 16:12:08 -0400
Subject: [PATCH] flush dirty folio so that drop_caches can work

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 tools/testing/selftests/mm/khugepaged.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 87ca031cb7225..ac645d96285da 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -399,6 +399,7 @@ static void *file_setup_area_common(int nr_hpages, bool read_only)
 		exit(EXIT_FAILURE);
 	}
 	fill_memory(p, 0, size);
+	msync(p, size, MS_SYNC);
 	close(fd);
 	munmap(p, size);
 	success("OK");
-- 
2.53.0




Best Regards,
Yan, Zi