Re: [PATCH 2/3] selftests/mm: hmm-tests: don't hardcode THP size to 2MB

Sayali Patil
2026-04-02T06:32:56Z

On 31/03/26 12:04, Alistair Popple wrote:
> Several HMM tests hardcode TWOMEG as the THP size. This is wrong on
> architectures where the PMD size is not 2MB such as arm64 with 64K base
> pages where THP is 512MB. Fix this by using read_pmd_pagesize() from
> vm_util instead.
>
> While here also replace the custom file_read_ulong() helper used to
> parse the default hugetlbfs page size from /proc/meminfo with the
> existing default_huge_page_size() from vm_util.
>
> [1] https://lore.kernel.org/linux-mm/8bd0396a-8997-4d2e-a13f-5aac033083d7@linux.dev/
>
> Fixes: fee9f6d1b8df ("mm/hmm/test: add selftests for HMM")
> Fixes: 519071529d2a ("selftests/mm/hmm-tests: new tests for zone device THP migration")
> Reported-by: Zenghui Yu <zenghui.yu@linux.dev>
> Closes: https://lore.kernel.org/linux-mm/8bd0396a-8997-4d2e-a13f-5aac033083d7@linux.dev/
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
> tools/testing/selftests/mm/hmm-tests.c | 83 +++++---------------------
> 1 file changed, 16 insertions(+), 67 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
> index e8328c89d855..788689497e92 100644
> --- a/tools/testing/selftests/mm/hmm-tests.c
> +++ b/tools/testing/selftests/mm/hmm-tests.c
> @@ -34,6 +34,7 @@
> */
> #include <lib/test_hmm_uapi.h>
> #include <mm/gup_test.h>
> +#include <mm/vm_util.h>
>
> struct hmm_buffer {
> void *ptr;
> @@ -548,7 +549,7 @@ TEST_F(hmm, anon_write_child)
>
> for (migrate = 0; migrate < 2; ++migrate) {
> for (use_thp = 0; use_thp < 2; ++use_thp) {
> - npages = ALIGN(use_thp ? TWOMEG : HMM_BUFFER_SIZE,
> + npages = ALIGN(use_thp ? read_pmd_pagesize() : HMM_BUFFER_SIZE,
> self->page_size) >> self->page_shift;
> ASSERT_NE(npages, 0);
> size = npages << self->page_shift;
Can we handle the case where read_pmd_pagesize() returns 0? Currently,
it’s passed to ALIGN(), which could produce unexpected results if 0 is
returned. Perhaps we can add a fallback to 2MB in that case.

> @@ -728,7 +729,7 @@ TEST_F(hmm, anon_write_huge)
> int *ptr;
> int ret;
>
> - size = 2 * TWOMEG;
> + size = 2 * read_pmd_pagesize();
>
> buffer = malloc(sizeof(*buffer));
> ASSERT_NE(buffer, NULL);
> @@ -744,7 +745,7 @@ TEST_F(hmm, anon_write_huge)
> buffer->fd, 0);
> ASSERT_NE(buffer->ptr, MAP_FAILED);
>
> - size = TWOMEG;
> + size /= 2;
> npages = size >> self->page_shift;
> map = (void *)ALIGN((uintptr_t)buffer->ptr, size);
> ret = madvise(map, size, MADV_HUGEPAGE);
> @@ -770,54 +771,6 @@ TEST_F(hmm, anon_write_huge)
> hmm_buffer_free(buffer);
> }
>
> -/*
> - * Read numeric data from raw and tagged kernel status files. Used to read
> - * /proc and /sys data (without a tag) and from /proc/meminfo (with a tag).
> - */
> -static long file_read_ulong(char *file, const char *tag)
> -{
> - int fd;
> - char buf[2048];
> - int len;
> - char *p, *q;
> - long val;
> -
> - fd = open(file, O_RDONLY);
> - if (fd < 0) {
> - /* Error opening the file */
> - return -1;
> - }
> -
> - len = read(fd, buf, sizeof(buf));
> - close(fd);
> - if (len < 0) {
> - /* Error in reading the file */
> - return -1;
> - }
> - if (len == sizeof(buf)) {
> - /* Error file is too large */
> - return -1;
> - }
> - buf[len] = '\0';
> -
> - /* Search for a tag if provided */
> - if (tag) {
> - p = strstr(buf, tag);
> - if (!p)
> - return -1; /* looks like the line we want isn't there */
> - p += strlen(tag);
> - } else
> - p = buf;
> -
> - val = strtol(p, &q, 0);
> - if (*q != ' ') {
> - /* Error parsing the file */
> - return -1;
> - }
> -
> - return val;
> -}
> -
> /*
> * Write huge TLBFS page.
> */
> @@ -826,15 +779,13 @@ TEST_F(hmm, anon_write_hugetlbfs)
> struct hmm_buffer *buffer;
> unsigned long npages;
> unsigned long size;
> - unsigned long default_hsize;
> + unsigned long default_hsize = default_huge_page_size();
> unsigned long i;
> int *ptr;
> int ret;
>
> - default_hsize = file_read_ulong("/proc/meminfo", "Hugepagesize:");
> - if (default_hsize < 0 || default_hsize*1024 < default_hsize)
> + if (!default_hsize)
> SKIP(return, "Huge page size could not be determined");
> - default_hsize = default_hsize*1024; /* KB to B */
>
> size = ALIGN(TWOMEG, default_hsize);
> npages = size >> self->page_shift;
> @@ -1606,7 +1557,7 @@ TEST_F(hmm, compound)
> struct hmm_buffer *buffer;
> unsigned long npages;
> unsigned long size;
> - unsigned long default_hsize;
> + unsigned long default_hsize = default_huge_page_size();
> int *ptr;
> unsigned char *m;
> int ret;
> @@ -1614,10 +1565,8 @@ TEST_F(hmm, compound)
>
> /* Skip test if we can't allocate a hugetlbfs page. */
>
> - default_hsize = file_read_ulong("/proc/meminfo", "Hugepagesize:");
> - if (default_hsize < 0 || default_hsize*1024 < default_hsize)
> + if (!default_hsize)
> SKIP(return, "Huge page size could not be determined");
> - default_hsize = default_hsize*1024; /* KB to B */
>
> size = ALIGN(TWOMEG, default_hsize);
> npages = size >> self->page_shift;
> @@ -2106,7 +2055,7 @@ TEST_F(hmm, migrate_anon_huge_empty)
> int *ptr;
> int ret;
>
> - size = TWOMEG;
> + size = read_pmd_pagesize();
>
> buffer = malloc(sizeof(*buffer));
> ASSERT_NE(buffer, NULL);
> @@ -2158,7 +2107,7 @@ TEST_F(hmm, migrate_anon_huge_zero)
> int ret;
> int val;
>
> - size = TWOMEG;
> + size = read_pmd_pagesize();
>
> buffer = malloc(sizeof(*buffer));
> ASSERT_NE(buffer, NULL);
> @@ -2221,7 +2170,7 @@ TEST_F(hmm, migrate_anon_huge_free)
> int *ptr;
> int ret;
>
> - size = TWOMEG;
> + size = read_pmd_pagesize();
>
> buffer = malloc(sizeof(*buffer));
> ASSERT_NE(buffer, NULL);
> @@ -2280,7 +2229,7 @@ TEST_F(hmm, migrate_anon_huge_fault)
> int *ptr;
> int ret;
>
> - size = TWOMEG;
> + size = read_pmd_pagesize();
>
> buffer = malloc(sizeof(*buffer));
> ASSERT_NE(buffer, NULL);
> @@ -2332,7 +2281,7 @@ TEST_F(hmm, migrate_partial_unmap_fault)
> {
> struct hmm_buffer *buffer;
> unsigned long npages;
> - unsigned long size = TWOMEG;
> + unsigned long size = read_pmd_pagesize();
> unsigned long i;
> void *old_ptr;
> void *map;
> @@ -2398,7 +2347,7 @@ TEST_F(hmm, migrate_remap_fault)
> {
> struct hmm_buffer *buffer;
> unsigned long npages;
> - unsigned long size = TWOMEG;
> + unsigned long size = read_pmd_pagesize();
> unsigned long i;
> void *old_ptr, *new_ptr = NULL;
> void *map;

Can we please handle the case where read_pmd_pagesize() returns 0,
wherever its used?
Also, looking at the code, since we are changing size, shouldn’t the
offsets[] values in migrate_remap_fault and migrate_partial_unmap_fault
be adjusted accordingly? Right now they are fixed at {0, 512KB, 1MB}.

> @@ -2498,7 +2447,7 @@ TEST_F(hmm, migrate_anon_huge_err)
> int *ptr;
> int ret;
>
> - size = TWOMEG;
> + size = read_pmd_pagesize();
>
> buffer = malloc(sizeof(*buffer));
> ASSERT_NE(buffer, NULL);
> @@ -2593,7 +2542,7 @@ TEST_F(hmm, migrate_anon_huge_zero_err)
> int *ptr;
> int ret;
>
> - size = TWOMEG;
> + size = read_pmd_pagesize();
>
> buffer = malloc(sizeof(*buffer));
> ASSERT_NE(buffer, NULL);

While reviewing the code, I noticed that the benchmark_thp_migration
testcase also hardcodes the size to 2MB. Can we handle this as well?

I’ve updated it to use read_pmd_pagesize(), with a fallback to 2MB if it
returns 0.
Additionally, I am capping the maximum number of THPs in benchmark tests
to avoid integer overflow (e.g., when thp_size * 128 > INT_MAX on 16MB
PMD-sized THP systems). Compute the maximum number of THPs at runtime as
INT_MAX / thp_size and use the smaller of this value and 128 for the
largest benchmark buffer.

diff --git a/tools/testing/selftests/mm/hmm-tests.c
b/tools/testing/selftests/mm/hmm-tests.c
index 788689497e92..c1c9204dc667 100644
--- a/tools/testing/selftests/mm/hmm-tests.c
+++ b/tools/testing/selftests/mm/hmm-tests.c
@@ -2756,8 +2756,15 @@ static inline int run_migration_benchmark(int fd,
int use_thp, size_t buffer_siz
TEST_F_TIMEOUT(hmm, benchmark_thp_migration, 120)
{
struct benchmark_results thp_results, regular_results;
- size_t thp_size = 2 * 1024 * 1024; /* 2MB - typical THP size */
+ size_t thp_size = read_pmd_pagesize();
int iterations = 5;
+ size_t max_thps, num_thps;
+
+ if (!thp_size)
+ thp_size = TWOMEG;
+
+ max_thps = INT_MAX / thp_size;
+ num_thps = (max_thps > 128) ? 128 : max_thps;

printf("\nHMM THP Migration Benchmark\n");
printf("---------------------------\n");
@@ -2765,23 +2772,23 @@ TEST_F_TIMEOUT(hmm, benchmark_thp_migration, 120)

/* Test different buffer sizes */
size_t test_sizes[] = {
- thp_size / 4, /* 512KB - smaller than THP */
- thp_size / 2, /* 1MB - half THP */
- thp_size, /* 2MB - single THP */
- thp_size * 2, /* 4MB - two THPs */
- thp_size * 4, /* 8MB - four THPs */
- thp_size * 8, /* 16MB - eight THPs */
- thp_size * 128, /* 256MB - one twenty eight THPs */
+ thp_size / 4, /* Quarter THP */
+ thp_size / 2, /* half THP */
+ thp_size, /* single THP */
+ thp_size * 2, /* two THPs */
+ thp_size * 4, /* four THPs */
+ thp_size * 8, /* eight THPs */
+ thp_size * num_thps, /* max THPs without overflow */
};

static const char *const test_names[] = {
- "Small Buffer (512KB)",
- "Half THP Size (1MB)",
- "Single THP Size (2MB)",
- "Two THP Size (4MB)",
- "Four THP Size (8MB)",
- "Eight THP Size (16MB)",
- "One twenty eight THP Size (256MB)"
+ "Small Buffer",
+ "Half THP Size",
+ "Single THP Size",
+ "Two THP Size",
+ "Four THP Size",
+ "Eight THP Size",
+ "Large Buffer"
};

int num_tests = ARRAY_SIZE(test_sizes);
--
Thanks,
Sayali