Re: [PATCH v5 3/5] selftests/mm: reimplement is_backed_by_thp() with more precise check

[Zi Yan]

On 19 Aug 2025, at 12:50, Zi Yan wrote:

> On 19 Aug 2025, at 4:54, David Hildenbrand wrote:
>
>> On 18.08.25 20:46, Zi Yan wrote:
>>> and rename it to is_backed_by_folio().
>>>
>>> is_backed_by_folio() checks if the given vaddr is backed a folio with
>>> a given order. It does so by:
>>> 1. getting the pfn of the vaddr;
>>> 2. checking kpageflags of the pfn;
>>>
>>> if order is greater than 0:
>>> 3. checking kpageflags of the head pfn;
>>> 4. checking kpageflags of all tail pfns.
>>>
>>> pmd_order is added to split_huge_page_test.c and replaces max_order.
>>>
>>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>>> Reviewed-by: Wei Yang <richard.weiyang@xxxxxxxxx>
>>> Reviewed-by: wang lian <lianux.mm@xxxxxxxxx>
>>> ---
>>>   .../selftests/mm/split_huge_page_test.c       | 88 ++++++++++++++-----
>>>   tools/testing/selftests/mm/vm_util.c          | 13 +++
>>>   tools/testing/selftests/mm/vm_util.h          |  4 +
>>>   3 files changed, 81 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
>>> index 089e146efeab..56d1eaf9a860 100644
>>> --- a/tools/testing/selftests/mm/split_huge_page_test.c
>>> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
>>> @@ -25,6 +25,7 @@
>>>   uint64_t pagesize;
>>>   unsigned int pageshift;
>>>   uint64_t pmd_pagesize;
>>> +unsigned int pmd_order;
>>>    #define SPLIT_DEBUGFS "/sys/kernel/debug/split_huge_pages"
>>>   #define SMAP_PATH "/proc/self/smaps"
>>> @@ -34,26 +35,66 @@ uint64_t pmd_pagesize;
>>>   #define PID_FMT_OFFSET "%d,0x%lx,0x%lx,%d,%d"
>>>   #define PATH_FMT "%s,0x%lx,0x%lx,%d"
>>>  -#define PFN_MASK     ((1UL<<55)-1)
>>> -#define KPF_THP      (1UL<<22)
>>> -
>>> -static int is_backed_by_thp(char *vaddr, int pagemap_file, int kpageflags_file)
>>> +static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
>>> +		int kpageflags_fd)
>>>   {
>>> -	uint64_t paddr;
>>> -	uint64_t page_flags;
>>> +	unsigned long pfn_head;
>>> +	uint64_t pfn_flags;
>>> +	unsigned long pfn;
>>> +	unsigned long i;
>>
>> Final nit (sorry!) :)
>>
>> const unsigned long nr_pages = 1ul << order;
>>
>> so you can make some of the code below easier to read.
>
> Sure. Will send a fixup.
>
>>
>> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>>
> Thanks.

Hi Andrew,

The fixup below replaced 1 << order with nr_pages based on David's
suggestion. Do you mind folding it in?

Thanks.


From c07eb882465b938a70acb84e4cdc5b6598536322 Mon Sep 17 00:00:00 2001
From: Zi Yan <ziy@xxxxxxxxxx>
Date: Tue, 19 Aug 2025 12:52:13 -0400
Subject: [PATCH] fixup: selftests/mm: use nr_pages instead of 1UL << order.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 tools/testing/selftests/mm/split_huge_page_test.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index 56d1eaf9a860..f2882cfab568 100644
--- a/tools/testing/selftests/mm/split_huge_page_test.c
+++ b/tools/testing/selftests/mm/split_huge_page_test.c
@@ -38,6 +38,7 @@ unsigned int pmd_order;
 static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 		int kpageflags_fd)
 {
+	const unsigned long nr_pages = 1UL << order;
 	unsigned long pfn_head;
 	uint64_t pfn_flags;
 	unsigned long pfn;
@@ -63,7 +64,7 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 	if (!(pfn_flags & KPF_THP))
 		return false;

-	pfn_head = pfn & ~((1 << order) - 1);
+	pfn_head = pfn & ~(nr_pages - 1);

 	if (pageflags_get(pfn_head, kpageflags_fd, &pfn_flags))
 		goto fail;
@@ -73,7 +74,7 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 		return false;

 	/* check all tail PFN flags */
-	for (i = 1; i < 1UL << order; i++) {
+	for (i = 1; i < nr_pages; i++) {
 		if (pageflags_get(pfn_head + i, kpageflags_fd, &pfn_flags))
 			goto fail;
 		if (!(pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL)))
@@ -84,7 +85,7 @@ static bool is_backed_by_folio(char *vaddr, int order, int pagemap_fd,
 	 * check the PFN after this folio, but if its flags cannot be obtained,
 	 * assume this folio has the expected order
 	 */
-	if (pageflags_get(pfn_head + (1UL << order), kpageflags_fd, &pfn_flags))
+	if (pageflags_get(pfn_head + nr_pages, kpageflags_fd, &pfn_flags))
 		return true;

 	/* this folio is bigger than the given order */
-- 
2.50.1




--
Best Regards,
Yan, Zi