Re: [PATCH v5 11/11] selftests/mm: add PMD swap entry tests

On 2026-07-22 11:19, Usama Arif wrote:
> Exercise the PMD swap entry paths. The tests allocate a PMD-mapped
> THP, write a known pattern, swap it out via MADV_PAGEOUT, and then
> exercise different code paths:
> 
>   - swap-out / swap-in round-trip with data verification
>   - fork with read-only access from both parent and child
>   - fork with writes in both processes to verify COW isolation
>   - repeated swap cycles to catch reference counting issues
>   - write fault on a swapped PMD to verify dirty handling and PMD
>     mapping restoration
>   - userfaultfd RWP preservation across PMD-order swap-in
>   - munmap of a swapped PMD (zap_huge_pmd swap slot cleanup)
>   - mprotect on a swapped PMD (change_non_present_huge_pmd)
>   - UFFDIO_MOVE on a swapped PMD, including destination RWP preservation
>   - mremap of a swapped PMD (move_soft_dirty_pmd)
>   - pagemap reading (pagemap_pmd_range_thp softleaf_has_pfn guard)
>   - mincore on a swapped PMD without faulting it in
>   - MADV_FREE on a swapped PMD: verifies swap slots are freed via
>     pagemap and the memory reads back as zero
>   - MADV_WILLNEED on a swapped PMD
>   - swapoff with active PMD swap entries, including RWP preservation
> 
> Add a pmd_swap category and ksft_pmd_swap.sh wrapper so the default mm
> selftest invocation and CI execute the test. PMD_SWAP_DEVICE remains
> optional for the destructive swapoff case.
> 
> When zswap is enabled, PMD-order consumers may split a PMD swap entry
> and retry through the PTE path because zswap stores the range as
> per-page entries. In that configuration, the tests still verify data
> correctness and log that the PMD mapping assertion is skipped. With
> zswap disabled, the tests assert that write faults, UFFDIO_MOVE,
> MADV_WILLNEED, and swapoff restore a PMD-mapped THP where expected.
> 
> Signed-off-by: Usama Arif <[email protected]>
> ---
>   tools/testing/selftests/mm/Makefile         |   2 +
>   tools/testing/selftests/mm/ksft_pmd_swap.sh |   4 +
>   tools/testing/selftests/mm/pmd_swap.c       | 874 ++++++++++++++++++++
>   tools/testing/selftests/mm/run_vmtests.sh   |   4 +
>   4 files changed, 884 insertions(+)
>   create mode 100755 tools/testing/selftests/mm/ksft_pmd_swap.sh
>   create mode 100644 tools/testing/selftests/mm/pmd_swap.c
> 
> diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
> index eae504bd94c8..25af4d7b633e 100644
> --- a/tools/testing/selftests/mm/Makefile
> +++ b/tools/testing/selftests/mm/Makefile
> @@ -104,6 +104,7 @@ TEST_GEN_FILES += guard-regions
>   TEST_GEN_FILES += merge
>   TEST_GEN_FILES += rmap
>   TEST_GEN_FILES += folio_split_race_test
> +TEST_GEN_FILES += pmd_swap
>   
>   ifneq ($(ARCH),arm64)
>   TEST_GEN_FILES += soft-dirty
> @@ -165,6 +166,7 @@ TEST_PROGS += ksft_mremap.sh
>   TEST_PROGS += ksft_pagemap.sh
>   TEST_PROGS += ksft_pfnmap.sh
>   TEST_PROGS += ksft_pkey.sh
> +TEST_PROGS += ksft_pmd_swap.sh
>   TEST_PROGS += ksft_process_madv.sh
>   TEST_PROGS += ksft_process_mrelease.sh
>   TEST_PROGS += ksft_rmap.sh
> diff --git a/tools/testing/selftests/mm/ksft_pmd_swap.sh b/tools/testing/selftests/mm/ksft_pmd_swap.sh
> new file mode 100755
> index 000000000000..0f070b4729a8
> --- /dev/null
> +++ b/tools/testing/selftests/mm/ksft_pmd_swap.sh
> @@ -0,0 +1,4 @@
> +#!/bin/sh -e
> +# SPDX-License-Identifier: GPL-2.0
> +
> +./run_vmtests.sh -t pmd_swap
> diff --git a/tools/testing/selftests/mm/pmd_swap.c b/tools/testing/selftests/mm/pmd_swap.c
> new file mode 100644
> index 000000000000..7cf9ac074e96
> --- /dev/null
> +++ b/tools/testing/selftests/mm/pmd_swap.c
> @@ -0,0 +1,874 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Test PMD-level swap entries.
> + *
> + * Verifies that when a PMD-mapped THP is swapped out the kernel installs
> + * a single PMD-level swap entry (instead of splitting into 512 PTE-level
> + * entries), and that operations on the swapped region behave correctly:
> + *   basic         - swap out + swap in preserves data
> + *   fork          - parent and child both see the data
> + *   fork_cow      - COW after fork keeps parent's data isolated
> + *   cycles        - repeated swap out/in does not corrupt data
> + *   write         - faulting in via a write restores a PMD-mapped THP
> + *   rwp_swapin    - userfaultfd RWP survives PMD-order swap-in
> + *   munmap        - munmap on a PMD swap entry frees swap slots cleanly
> + *   mprotect      - mprotect on a PMD swap entry preserves data
> + *   mremap        - mremap on a PMD swap entry preserves data
> + *   pagemap        - pagemap reports the entries as swapped
> + *   mincore        - mincore walks a PMD swap entry without faulting it in
> + *   madvise_free   - MADV_FREE on a PMD swap entry does not crash
> + *   madvise_willneed - MADV_WILLNEED handles a PMD swap entry
> + *   uffdio_move    - UFFDIO_MOVE moves a PMD swap entry
> + *   swapoff        - swapoff handles PMD swap entries (needs PMD_SWAP_DEVICE)
> + */
> +#define _GNU_SOURCE
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include <sys/mman.h>
> +#include <sys/wait.h>
> +#include <fcntl.h>
> +#include <errno.h>
> +#include <stdint.h>
> +#include <sys/random.h>
> +#include <sys/swap.h>
> +#include <sys/syscall.h>
> +#include <sys/ioctl.h>
> +#include <poll.h>
> +#include <pthread.h>
> +#include <linux/userfaultfd.h>
> +#include <time.h>
> +
> +#include "kselftest_harness.h"
> +#include "vm_util.h"
> +
> +#define ZSWAP_ENABLED_PATH "/sys/module/zswap/parameters/enabled"
> +
> +static bool check_swapped(int pagemap_fd, char *addr, unsigned long size)
> +{
> +	unsigned long off;
> +
> +	for (off = 0; off < size; off += getpagesize())
> +		if (!pagemap_is_swapped(pagemap_fd, addr + off))
> +			return false;
> +	return true;
> +}
> +
> +static bool zswap_enabled(void)
> +{
> +	char enabled = 0;
> +	FILE *f;
> +
> +	f = fopen(ZSWAP_ENABLED_PATH, "r");
> +	if (!f)
> +		return false;
> +
> +	if (fscanf(f, " %c", &enabled) != 1)
> +		enabled = 0;
> +	fclose(f);
> +
> +	return enabled == 'Y' || enabled == 'y' || enabled == '1';
> +}
> +
> +static bool swap_available(int pagemap_fd)
> +{
> +	char *p;
> +	bool ret;
> +
> +	p = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
> +		 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +	if (p == MAP_FAILED)
> +		return false;
> +
> +	memset(p, 0xab, getpagesize());
> +	madvise(p, getpagesize(), MADV_PAGEOUT);
> +	ret = pagemap_is_swapped(pagemap_fd, p);
> +	munmap(p, getpagesize());
> +	return ret;
> +}

Why not check /proc/swaps instead? We're performing the first test-case
here, if this fails because of a bug the test would be skipped.

> +
> +static unsigned long read_vm_event(const char *name)
> +{
> +	char line[256];
> +	size_t name_len = strlen(name);
> +	unsigned long val = 0;
> +	FILE *f;
> +
> +	f = fopen("/proc/vmstat", "r");
> +	if (!f)
> +		return 0;
> +	while (fgets(line, sizeof(line), f)) {
> +		if (!strncmp(line, name, name_len) && line[name_len] == ' ') {
> +			val = strtoul(line + name_len + 1, NULL, 10);
> +			break;
> +		}
> +	}
> +	fclose(f);
> +	return val;
> +}
> +
> +static unsigned int random_seed(void)
> +{
> +	unsigned int seed;
> +
> +	if (getrandom(&seed, sizeof(seed), 0) != sizeof(seed))
> +		seed = (unsigned int)time(NULL);
> +	return seed;
> +}
> +
> +static unsigned char pattern_byte(unsigned int seed, unsigned long off)
> +{
> +	return (unsigned char)(seed + off);
> +}
> +
> +static void fill_pattern(char *buf, unsigned long size, unsigned int seed)
> +{
> +	unsigned long i;
> +
> +	for (i = 0; i < size; i++)
> +		buf[i] = (char)pattern_byte(seed, i);
> +}
> +
> +static bool verify_pattern(char *buf, unsigned long size, unsigned int seed)
> +{
> +	unsigned long i;
> +
> +	for (i = 0; i < size; i++)
> +		if ((unsigned char)buf[i] != pattern_byte(seed, i))
> +			return false;
> +	return true;
> +}
> +
> +/*
> + * mmap an anonymous PMD-aligned region of pmd_size bytes. Over-allocates
> + * by one PMD and trims the unaligned head/tail so the returned address is
> + * PMD-aligned (required for whole-PMD UFFDIO_MOVE).
> + */
> +static char *mmap_pmd_aligned(unsigned long pmd_size)
> +{
> +	unsigned long pad = pmd_size;
> +	char *raw, *aligned;
> +
> +	raw = mmap(NULL, pmd_size + pad, PROT_READ | PROT_WRITE,
> +		   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +	if (raw == MAP_FAILED)
> +		return MAP_FAILED;
> +
> +	aligned = (char *)(((uintptr_t)raw + pmd_size - 1) & ~(pmd_size - 1));
> +	if (aligned != raw)
> +		munmap(raw, aligned - raw);
> +	if (aligned + pmd_size != raw + pmd_size + pad)
> +		munmap(aligned + pmd_size,
> +		       (raw + pmd_size + pad) - (aligned + pmd_size));
> +	return aligned;
> +}
> +
> +/*
> + * mmap a PMD-aligned PMD-sized region, request THP, fill with a pattern,
> + * and swap it out. Verifies via the thp_swpout_pmd vmstat counter that
> + * the swap-out installed a PMD swap entry rather than splitting to PTEs.
> + */
> +static char *alloc_fill_swap_thp(unsigned long pmd_size, int pagemap_fd,
> +				 unsigned int seed)
> +{
> +	unsigned long pmd_before, pmd_after;
> +	char *mem;
> +
> +	mem = mmap_pmd_aligned(pmd_size);
> +	if (mem == MAP_FAILED)
> +		return MAP_FAILED;
> +
> +	madvise(mem, pmd_size, MADV_HUGEPAGE);

This is minor, but I'd check madvise() return.

> +	fill_pattern(mem, pmd_size, seed);
> +
> +	pmd_before = read_vm_event("thp_swpout_pmd");
> +
> +	if (madvise(mem, pmd_size, MADV_PAGEOUT) ||
> +	    !check_swapped(pagemap_fd, mem, pmd_size)) {
> +		munmap(mem, pmd_size);
> +		return MAP_FAILED;
> +	}
> +
> +	pmd_after = read_vm_event("thp_swpout_pmd");
> +	printf("# thp_swpout_pmd: %lu -> %lu\n", pmd_before, pmd_after);

Should we use ksft_print_msg() instead?

> +	if (pmd_after - pmd_before < 1) {
> +		munmap(mem, pmd_size);
> +		return MAP_FAILED;
> +	}
> +	return mem;
> +}
> +
> +struct rwp_access_args {
> +	unsigned char *addr;
> +	unsigned char expected;
> +	bool write;
> +	bool ok;
> +};
> +
> +static void *rwp_access_thread(void *data)
> +{
> +	struct rwp_access_args *args = data;
> +
> +	if (args->write)
> +		*args->addr = args->expected;
> +	args->ok = *args->addr == args->expected;
> +	return NULL;
> +}
> +
> +static int register_rwp(char *addr, unsigned long size, bool protect)
> +{
> +	struct uffdio_register reg = {};
> +	struct uffdio_rwprotect rwp = {};
> +	struct uffdio_api api = {};
> +	int uffd;
> +
> +	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
> +	if (uffd < 0)
> +		return -1;
> +
> +	api.api = UFFD_API;
> +	api.features = UFFD_FEATURE_RWP;
> +	if (ioctl(uffd, UFFDIO_API, &api) ||
> +	    !(api.features & UFFD_FEATURE_RWP))
> +		goto error;
> +
> +	reg.range.start = (unsigned long)addr;
> +	reg.range.len = size;
> +	reg.mode = UFFDIO_REGISTER_MODE_RWP;
> +	if (ioctl(uffd, UFFDIO_REGISTER, &reg))
> +		goto error;
> +
> +	if (!protect)
> +		return uffd;
> +
> +	rwp.range.start = (unsigned long)addr;
> +	rwp.range.len = size;
> +	rwp.mode = UFFDIO_RWPROTECT_MODE_RWP;
> +	if (!ioctl(uffd, UFFDIO_RWPROTECT, &rwp))
> +		return uffd;
> +
> +error:
> +	close(uffd);
> +	return -1;
> +}
> +
> +static bool expect_rwp_fault(int uffd, char *addr, unsigned long size,
> +			     unsigned char expected, bool write)
> +{
> +	struct rwp_access_args args = {
> +		.addr = (unsigned char *)addr,
> +		.expected = expected,
> +		.write = write,
> +	};
> +	struct uffdio_rwprotect rwp = {
> +		.range = {
> +			.start = (unsigned long)addr,
> +			.len = size,
> +		},
> +	};
> +	struct pollfd pollfd = {
> +		.fd = uffd,
> +		.events = POLLIN,
> +	};
> +	struct uffd_msg msg = {};
> +	pthread_t thread;
> +	bool saw_rwp = false;
> +	int ret;
> +
> +	if (pthread_create(&thread, NULL, rwp_access_thread, &args))
> +		return false;
> +
> +	ret = poll(&pollfd, 1, 5000);
> +	if (ret == 1 && (pollfd.revents & POLLIN) &&
> +	    read(uffd, &msg, sizeof(msg)) == (ssize_t)sizeof(msg)) {
> +		saw_rwp = msg.event == UFFD_EVENT_PAGEFAULT &&
> +			  (msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_RWP);
> +	}
> +
> +	/* Resolve the access even on failure so the worker cannot remain blocked. */
> +	ioctl(uffd, UFFDIO_RWPROTECT, &rwp);
> +	if (pthread_join(thread, NULL))
> +		return false;
> +	return saw_rwp && args.ok;
> +}
> +
> +FIXTURE(pmd_swap)
> +{
> +	unsigned long pmd_size;
> +	int pagemap_fd;
> +	unsigned int seed;
> +	bool zswap_enabled;
> +};
> +
> +FIXTURE_SETUP(pmd_swap)
> +{
> +	self->pagemap_fd = -1;
> +
> +	self->pmd_size = read_pmd_pagesize();
> +	if (!self->pmd_size)
> +		SKIP(return, "Cannot determine PMD size\n");
> +
> +	self->pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
> +	if (self->pagemap_fd < 0)
> +		SKIP(return, "Cannot open /proc/self/pagemap\n");
> +
> +	if (!swap_available(self->pagemap_fd))
> +		SKIP(return, "Swap not available or not working\n");
> +
> +	self->seed = random_seed();
> +	self->zswap_enabled = zswap_enabled();
> +}
> +
> +FIXTURE_TEARDOWN(pmd_swap)
> +{
> +	if (self->pagemap_fd >= 0)
> +		close(self->pagemap_fd);
> +}
> +
> +/*
> + * Allocate a PMD-sized THP, write a pattern, swap it out, read it back,
> + * verify the pattern.
> + */
> +TEST_F(pmd_swap, basic)
> +{
> +	char *mem;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, self->seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");

I think that skipping on failure may hide potential regressions. If the
intent is to skip when you don't get a PMD THP, then you could return
different error codes but do fail the test if swapping fails. Note that
this applies to all the tests calling alloc_fill_swap_thp().

> +
> +	ASSERT_TRUE(verify_pattern(mem, self->pmd_size, self->seed));
> +
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * Allocate a THP, swap it out, fork, verify both parent and child see
> + * the correct data.
> + */
> +TEST_F(pmd_swap, fork)
> +{
> +	char *mem;
> +	pid_t pid;
> +	int status;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, self->seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (pid == 0)
> +		_exit(verify_pattern(mem, self->pmd_size, self->seed) ? 0 : 1);
> +
> +	ASSERT_TRUE(verify_pattern(mem, self->pmd_size, self->seed));
> +
> +	ASSERT_EQ(waitpid(pid, &status, 0), pid);
> +	ASSERT_TRUE(WIFEXITED(status));
> +	ASSERT_EQ(WEXITSTATUS(status), 0);
> +
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * Swap out, fork, then have parent and child write different patterns.
> + * Exercises COW on shared PMD swap entries: writes after fork must
> + * trigger copy-on-write so the parent's data stays isolated from the
> + * child's.  Both processes write and then verify their own pattern so
> + * the parent-side COW path is exercised too (a parent-only-read variant
> + * only proves the swap entry survived fork).
> + */
> +TEST_F(pmd_swap, fork_cow)
> +{
> +	unsigned int parent_seed = self->seed;
> +	unsigned int child_seed = ~self->seed;
> +	unsigned int post_seed = self->seed ^ 0xa5a5a5a5;
> +	char *mem;
> +	pid_t pid;
> +	int status;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, parent_seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	pid = fork();
> +	ASSERT_GE(pid, 0);
> +
> +	if (pid == 0) {
> +		fill_pattern(mem, self->pmd_size, child_seed);
> +		_exit(verify_pattern(mem, self->pmd_size, child_seed) ? 0 : 1);
> +	}
> +
> +	ASSERT_EQ(waitpid(pid, &status, 0), pid);
> +
> +	/* Child's writes must not leak back into the parent. */
> +	ASSERT_TRUE(verify_pattern(mem, self->pmd_size, parent_seed));
> +	ASSERT_TRUE(WIFEXITED(status));
> +	ASSERT_EQ(WEXITSTATUS(status), 0);
> +
> +	/* Now trigger the parent-side COW and confirm the write sticks. */
> +	fill_pattern(mem, self->pmd_size, post_seed);
> +	ASSERT_TRUE(verify_pattern(mem, self->pmd_size, post_seed));
> +
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * Swap a THP out and in repeatedly without data corruption.
> + */
> +TEST_F(pmd_swap, cycles)
> +{
> +	const int num_cycles = 5;
> +	char *mem;
> +	int cycle;
> +
> +	for (cycle = 0; cycle < num_cycles; cycle++) {
> +		unsigned int seed = self->seed + cycle;
> +
> +		mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, seed);
> +		if (mem == MAP_FAILED)
> +			SKIP(return, "Could not create swapped THP at cycle %d\n",
> +			     cycle);
> +
> +		ASSERT_TRUE(verify_pattern(mem, self->pmd_size, seed));
> +
> +		munmap(mem, self->pmd_size);
> +	}
> +}

Since all the tests already swap-out a single PMD page, I think we could improve
coverage by swapping out multiple PMD pages or maybe a PMD page and a
base page.

> +
> +/*
> + * Swap out, fault in via a write to the first page, verify the write
> + * reinstates a THP mapping and the rest of the THP is preserved.
> + */
> +TEST_F(pmd_swap, write)
> +{
> +	unsigned int seed = self->seed;
> +	char *mem;
> +	unsigned long i;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	mem[0] = 0xbb;
> +	ASSERT_EQ(mem[0], (char)0xbb);
> +
> +	if (self->zswap_enabled) {
> +		TH_LOG("zswap is enabled, so PMD mapping is not checked");
> +	} else {
> +		ASSERT_TRUE(check_huge_anon(mem, 1, self->pmd_size));
> +	}
> +
> +	for (i = 1; i < self->pmd_size; i++)
> +		ASSERT_EQ((unsigned char)mem[i], pattern_byte(seed, i));
> +
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/* RWP protection on a swap PMD must survive PMD-order swap-in. */
> +TEST_F(pmd_swap, rwp_swapin)
> +{
> +	unsigned int seed = self->seed;
> +	char *mem;
> +	int uffd;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	uffd = register_rwp(mem, self->pmd_size, true);
> +	if (uffd < 0) {
> +		munmap(mem, self->pmd_size);
> +		SKIP(return, "Userfaultfd RWP unsupported\n");
> +	}
> +
> +	ASSERT_TRUE(expect_rwp_fault(uffd, mem, self->pmd_size,
> +				     pattern_byte(seed, 0), false)) {
> +		close(uffd);
> +		munmap(mem, self->pmd_size);
> +	}
> +	ASSERT_TRUE(verify_pattern(mem, self->pmd_size, seed));
> +
> +	close(uffd);
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * munmap while the folio is swapped out. Exercises zap_huge_pmd() on a
> + * PMD swap entry — must free the swap slots without trying to look up
> + * a folio.
> + */
> +TEST_F(pmd_swap, munmap)
> +{
> +	char *mem;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, self->seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	munmap(mem, self->pmd_size);
> +}

Should we check munmap() return since that's the purpose of the test?

> +
> +/*
> + * Change protection on a swapped PMD entry, then fault back in and
> + * verify data. Exercises change_non_present_huge_pmd().
> + */
> +TEST_F(pmd_swap, mprotect)
> +{
> +	unsigned int seed = self->seed;
> +	char *mem;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	ASSERT_EQ(mprotect(mem, self->pmd_size, PROT_READ), 0);
> +	ASSERT_EQ(mprotect(mem, self->pmd_size, PROT_READ | PROT_WRITE), 0);
> +
> +	ASSERT_TRUE(verify_pattern(mem, self->pmd_size, seed));
> +
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * UFFDIO_MOVE a PMD swap entry from src to a registered dst. Exercises
> + * move_pages_huge_pmd() handling of pmd_is_swap_entry: the whole PMD swap
> + * entry must move to dst without splitting, and the destination must
> + * read back the original pattern after a swap-in fault.
> + */
> +TEST_F(pmd_swap, uffdio_move)
> +{
> +	unsigned int seed = self->seed;
> +	struct uffdio_register reg = {};
> +	struct uffdio_move move = {};
> +	struct uffdio_api api = {};
> +	char *src, *dst;
> +	bool rwp;
> +	int uffd;
> +
> +	dst = mmap_pmd_aligned(self->pmd_size);
> +	if (dst == MAP_FAILED)
> +		SKIP(return, "Could not mmap aligned dst\n");
> +
> +	src = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, seed);
> +	if (src == MAP_FAILED) {
> +		munmap(dst, self->pmd_size);
> +		SKIP(return, "Could not create swapped THP\n");
> +	}
> +	if ((uintptr_t)src & (self->pmd_size - 1)) {
> +		munmap(src, self->pmd_size);
> +		munmap(dst, self->pmd_size);
> +		SKIP(return, "src not PMD-aligned\n");
> +	}
> +
> +	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
> +	if (uffd < 0) {
> +		munmap(src, self->pmd_size);
> +		munmap(dst, self->pmd_size);
> +		SKIP(return, "userfaultfd unavailable\n");
> +	}
> +
> +	api.api = UFFD_API;
> +	api.features = UFFD_FEATURE_MOVE | UFFD_FEATURE_RWP;
> +	if (ioctl(uffd, UFFDIO_API, &api) ||
> +	    !(api.features & UFFD_FEATURE_MOVE)) {
> +		close(uffd);
> +		munmap(src, self->pmd_size);
> +		munmap(dst, self->pmd_size);
> +		SKIP(return, "UFFD_FEATURE_MOVE unsupported\n");
> +	}
> +	rwp = api.features & UFFD_FEATURE_RWP;
> +
> +	reg.range.start = (unsigned long)dst;
> +	reg.range.len = self->pmd_size;
> +	reg.mode = UFFDIO_REGISTER_MODE_MISSING |
> +		   (rwp ? UFFDIO_REGISTER_MODE_RWP : 0);
> +	if (ioctl(uffd, UFFDIO_REGISTER, &reg)) {
> +		close(uffd);
> +		munmap(src, self->pmd_size);
> +		munmap(dst, self->pmd_size);
> +		SKIP(return, "UFFDIO_REGISTER failed\n");
> +	}
> +
> +	move.dst = (unsigned long)dst;
> +	move.src = (unsigned long)src;
> +	move.len = self->pmd_size;
> +	if (ioctl(uffd, UFFDIO_MOVE, &move)) {
> +		int saved_errno = errno;
> +
> +		close(uffd);
> +		munmap(src, self->pmd_size);
> +		munmap(dst, self->pmd_size);
> +		ASSERT_EQ(saved_errno, 0);
> +	}
> +	ASSERT_EQ(move.move, self->pmd_size);
> +
> +	/* dst inherits the PMD swap entry; reading it must restore the data. */
> +	ASSERT_TRUE(check_swapped(self->pagemap_fd, dst, self->pmd_size));
> +	if (rwp) {
> +		ASSERT_TRUE(expect_rwp_fault(uffd, dst, self->pmd_size,
> +					     pattern_byte(seed, 0), false)) {
> +			close(uffd);
> +			munmap(src, self->pmd_size);
> +			munmap(dst, self->pmd_size);
> +		}
> +	}
> +	ASSERT_TRUE(verify_pattern(dst, self->pmd_size, seed));
> +	if (self->zswap_enabled) {
> +		TH_LOG("zswap is enabled, so PMD mapping is not checked");
> +	} else {
> +		/* The whole-PMD path must reinstate a THP, not 512 PTE folios. */
> +		ASSERT_TRUE(check_huge_anon(dst, 1, self->pmd_size));
> +	}
> +
> +	close(uffd);
> +	munmap(src, self->pmd_size);
> +	munmap(dst, self->pmd_size);
> +}
> +
> +/*
> + * Move a swapped PMD entry to a new address, fault in, verify data.
> + * Exercises move_huge_pmd() and move_soft_dirty_pmd().
> + */
> +TEST_F(pmd_swap, mremap)
> +{
> +	unsigned int seed = self->seed;
> +	char *mem, *new_mem;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	new_mem = mremap(mem, self->pmd_size, self->pmd_size, MREMAP_MAYMOVE);
> +	if (new_mem == MAP_FAILED) {
> +		munmap(mem, self->pmd_size);
> +		ASSERT_NE(new_mem, MAP_FAILED);
> +	}
> +
> +	ASSERT_TRUE(verify_pattern(new_mem, self->pmd_size, seed));
> +
> +	munmap(new_mem, self->pmd_size);
> +}
> +
> +/*
> + * Read /proc/self/pagemap on a PMD swap entry. Exercises the pagemap
> + * PMD walker which must handle PMD swap entries without trying to
> + * convert them to a page via softleaf_to_page().
> + */
> +TEST_F(pmd_swap, pagemap)
> +{
> +	char *mem;
> +	uint64_t entry;
> +	unsigned long off;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, self->seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	for (off = 0; off < self->pmd_size; off += getpagesize()) {
> +		entry = pagemap_get_entry(self->pagemap_fd, mem + off);
> +		/* Bit 62 = swapped */
> +		ASSERT_TRUE(entry & (1ULL << 62));
> +	}
> +
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * mincore() on a swapped-out PMD-mapped THP must handle the non-present PMD
> + * entry in place. The call must not fault the PMD back in or split the entry.
> + */
> +TEST_F(pmd_swap, mincore)
> +{
> +	unsigned long pages = self->pmd_size / getpagesize();
> +	unsigned char *vec;
> +	char *mem;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, self->seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	vec = calloc(pages, sizeof(*vec));
> +	ASSERT_NE(vec, NULL) {
> +		munmap(mem, self->pmd_size);
> +	}
> +
> +	ASSERT_EQ(mincore(mem, self->pmd_size, vec), 0) {
> +		free(vec);
> +		munmap(mem, self->pmd_size);
> +	}
> +	ASSERT_TRUE(check_swapped(self->pagemap_fd, mem, self->pmd_size)) {
> +		free(vec);
> +		munmap(mem, self->pmd_size);
> +	}
> +
> +	free(vec);
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * MADV_FREE on a swapped-out PMD must free the swap slots and clear the
> + * entry. After the call, pagemap must no longer report the pages as
> + * swapped, and accessing the region must yield zero pages.
> + */
> +TEST_F(pmd_swap, madvise_free)
> +{
> +	char *mem;
> +	unsigned long i;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, self->seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	ASSERT_TRUE(check_swapped(self->pagemap_fd, mem, self->pmd_size));
> +	ASSERT_EQ(madvise(mem, self->pmd_size, MADV_FREE), 0);
> +	ASSERT_FALSE(check_swapped(self->pagemap_fd, mem, self->pmd_size));
> +
> +	for (i = 0; i < self->pmd_size; i += getpagesize())
> +		ASSERT_EQ(mem[i], 0);

This is checking only the first byte, no? Is this intentional?

> +
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * MADV_WILLNEED on a swapped-out PMD-mapped THP may schedule PMD-order
> + * swapin I/O, find the PMD-sized folio already resident in the swap cache,
> + * or split to the PTE path when zswap has per-page state for the range.
> + */
> +TEST_F(pmd_swap, madvise_willneed)
> +{
> +	char *mem;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, self->seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +
> +	ASSERT_EQ(madvise(mem, self->pmd_size, MADV_WILLNEED), 0);
> +	ASSERT_TRUE(check_swapped(self->pagemap_fd, mem, self->pmd_size));
> +
> +	/* First touch faults the data back in. */
> +	ASSERT_TRUE(verify_pattern(mem, self->pmd_size, self->seed));
> +
> +	if (self->zswap_enabled)
> +		TH_LOG("zswap is enabled, so PMD mapping is not checked");
> +	else
> +		ASSERT_TRUE(check_huge_anon(mem, 1, self->pmd_size));
> +
> +	munmap(mem, self->pmd_size);
> +}
> +
> +/*
> + * swapoff requires a dedicated swap device path. Use a separate fixture
> + * that picks the device up from the PMD_SWAP_DEVICE environment variable
> + * and skips when unset.
> + */
> +FIXTURE(pmd_swap_swapoff)
> +{
> +	unsigned long pmd_size;
> +	int pagemap_fd;
> +	const char *swap_dev;
> +	unsigned int seed;
> +	bool zswap_enabled;
> +};
> +
> +FIXTURE_SETUP(pmd_swap_swapoff)
> +{
> +	self->pagemap_fd = -1;
> +	self->swap_dev = getenv("PMD_SWAP_DEVICE");
> +	if (!self->swap_dev)
> +		SKIP(return, "PMD_SWAP_DEVICE env var not set\n");
> +
> +	self->pmd_size = read_pmd_pagesize();
> +	if (!self->pmd_size)
> +		SKIP(return, "Cannot determine PMD size\n");
> +
> +	self->pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
> +	if (self->pagemap_fd < 0)
> +		SKIP(return, "Cannot open /proc/self/pagemap\n");
> +
> +	if (!swap_available(self->pagemap_fd))
> +		SKIP(return, "Swap not available or not working\n");
> +
> +	self->seed = random_seed();
> +	self->zswap_enabled = zswap_enabled();
> +}
> +
> +FIXTURE_TEARDOWN(pmd_swap_swapoff)
> +{
> +	if (self->pagemap_fd >= 0)
> +		close(self->pagemap_fd);
> +}

Should swapoff() be moved here? I think it would avoid the repeated
swapoff() calls in the test.

> +
> +/*
> + * Swap out a THP, then turn off swap. Verify data is intact. When zswap is
> + * not active, the PMD-order swapoff path should preserve the huge mapping.
> + */
> +TEST_F(pmd_swap_swapoff, basic)
> +{
> +	unsigned int seed = self->seed;
> +	char *mem;
> +	int uffd, ret, err;
> +
> +	mem = alloc_fill_swap_thp(self->pmd_size, self->pagemap_fd, seed);
> +	if (mem == MAP_FAILED)
> +		SKIP(return, "Could not create swapped THP\n");
> +	uffd = register_rwp(mem, self->pmd_size, true);
> +
> +	ret = swapoff(self->swap_dev);
> +	err = errno;
> +	ASSERT_EQ(ret, 0) {
> +		TH_LOG("swapoff(%s) failed: %s", self->swap_dev, strerror(err));
> +		if (uffd >= 0)
> +			close(uffd);
> +		munmap(mem, self->pmd_size);
> +	}
> +
> +	/*
> +	 * Check the PMD residency before touching the memory.  If we read
> +	 * first, a bug that left a PMD swap entry in place after swapoff
> +	 * would silently trigger do_huge_pmd_swap_page() and reinstall a
> +	 * PMD mapping, masking the regression.
> +	 */
> +	if (self->zswap_enabled) {
> +		TH_LOG("zswap is enabled, so PMD mapping is not checked");
> +	} else {
> +		ASSERT_TRUE(check_huge_anon(mem, 1, self->pmd_size)) {
> +			swapon(self->swap_dev, 0);
> +			if (uffd >= 0)
> +				close(uffd);
> +			munmap(mem, self->pmd_size);
> +		}
> +	}
> +	if (uffd >= 0) {
> +		ASSERT_TRUE(expect_rwp_fault(uffd, mem, self->pmd_size,
> +					     pattern_byte(seed, 0), false)) {
> +			swapon(self->swap_dev, 0);
> +			close(uffd);
> +			munmap(mem, self->pmd_size);
> +		}
> +	}
> +
> +	ASSERT_TRUE(verify_pattern(mem, self->pmd_size, seed)) {
> +		swapon(self->swap_dev, 0);
> +		if (uffd >= 0)
> +			close(uffd);
> +		munmap(mem, self->pmd_size);
> +	}
> +
> +	ret = swapon(self->swap_dev, 0);
> +	err = errno;
> +	ASSERT_EQ(ret, 0) {
> +		TH_LOG("swapon(%s) failed: %s", self->swap_dev, strerror(err));
> +		if (uffd >= 0)
> +			close(uffd);
> +		munmap(mem, self->pmd_size);
> +	}
> +
> +	if (uffd >= 0)
> +		close(uffd);
> +	munmap(mem, self->pmd_size);
> +}
> +
> +TEST_HARNESS_MAIN
> diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
> index 687d115e3bd8..0a860c4852f2 100755
> --- a/tools/testing/selftests/mm/run_vmtests.sh
> +++ b/tools/testing/selftests/mm/run_vmtests.sh
> @@ -69,6 +69,8 @@ separated by spaces:
>   	test pagemap_scan IOCTL
>   - pfnmap
>   	tests for VM_PFNMAP handling
> +- pmd_swap
> +	tests for PMD-level swap entries
>   - process_madv
>   	test for process_madv
>   - cow
> @@ -399,6 +401,8 @@ CATEGORY="pagemap" run_test ./pagemap_ioctl
>   
>   CATEGORY="pfnmap" run_test ./pfnmap
>   
> +CATEGORY="pmd_swap" run_test ./pmd_swap
> +
>   # COW tests
>   CATEGORY="cow" run_test ./cow
>