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Message-ID: <bfdd4a08-1887-421f-8ea4-d48eb0cee778@lucifer.local>
Date: Fri, 9 May 2025 16:55:03 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-kselftest@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...hat.com>,
Peter Xu <peterx@...hat.com>, Dev Jain <dev.jain@....com>
Subject: Re: [PATCH v2] selftests/mm: add simple VM_PFNMAP tests based on
mmap'ing /dev/mem
On Fri, May 09, 2025 at 05:30:32PM +0200, David Hildenbrand wrote:
> Let's test some basic functionality using /dev/mem. These tests will
> implicitly cover some PAT (Page Attribute Handling) handling on x86.
>
> These tests will only run when /dev/mem access to the first two pages
> in physical address space is possible and allowed; otherwise, the tests
> are skipped.
>
> On current x86-64 with PAT inside a VM, all tests pass:
>
> TAP version 13
> 1..6
> # Starting 6 tests from 1 test cases.
> # RUN pfnmap.madvise_disallowed ...
> # OK pfnmap.madvise_disallowed
> ok 1 pfnmap.madvise_disallowed
> # RUN pfnmap.munmap_split ...
> # OK pfnmap.munmap_split
> ok 2 pfnmap.munmap_split
> # RUN pfnmap.mremap_fixed ...
> # OK pfnmap.mremap_fixed
> ok 3 pfnmap.mremap_fixed
> # RUN pfnmap.mremap_shrink ...
> # OK pfnmap.mremap_shrink
> ok 4 pfnmap.mremap_shrink
> # RUN pfnmap.mremap_expand ...
> # OK pfnmap.mremap_expand
> ok 5 pfnmap.mremap_expand
> # RUN pfnmap.fork ...
> # OK pfnmap.fork
> ok 6 pfnmap.fork
> # PASSED: 6 / 6 tests passed.
> # Totals: pass:6 fail:0 xfail:0 xpass:0 skip:0 error:0
>
> However, we are able to trigger:
>
> [ 27.888251] x86/PAT: pfnmap:1790 freeing invalid memtype [mem 0x00000000-0x00000fff]
>
> There are probably more things worth testing in the future, such as
> MAP_PRIVATE handling. But this set of tests is sufficient to cover most of
> the things we will rework regarding PAT handling.
>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Shuah Khan <shuah@...nel.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Dev Jain <dev.jain@....com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
Nice, big improvement!
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>
> Hopefully I didn't miss any review feedback.
All good afaict! :) the only thing would be to make the siglongjmp() conditional
but it's not a big deal.
>
> v1 -> v2:
> * Rewrite using kselftest_harness, which simplifies a lot of things
> * Add to .gitignore and run_vmtests.sh
> * Register signal handler on demand
> * Use volatile trick to force a read (not factoring out FORCE_READ just yet)
> * Drop mprotect() test case
> * Add some more comments why we test certain things
> * Use NULL for mmap() first parameter instead of 0
> * Smaller fixes
>
> ---
> tools/testing/selftests/mm/.gitignore | 1 +
> tools/testing/selftests/mm/Makefile | 1 +
> tools/testing/selftests/mm/pfnmap.c | 196 ++++++++++++++++++++++
> tools/testing/selftests/mm/run_vmtests.sh | 4 +
> 4 files changed, 202 insertions(+)
> create mode 100644 tools/testing/selftests/mm/pfnmap.c
>
> diff --git a/tools/testing/selftests/mm/.gitignore b/tools/testing/selftests/mm/.gitignore
> index 91db34941a143..824266982aa36 100644
> --- a/tools/testing/selftests/mm/.gitignore
> +++ b/tools/testing/selftests/mm/.gitignore
> @@ -20,6 +20,7 @@ mremap_test
> on-fault-limit
> transhuge-stress
> pagemap_ioctl
> +pfnmap
> *.tmp*
> protection_keys
> protection_keys_32
> diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
> index ad4d6043a60f0..ae6f994d3add7 100644
> --- a/tools/testing/selftests/mm/Makefile
> +++ b/tools/testing/selftests/mm/Makefile
> @@ -84,6 +84,7 @@ TEST_GEN_FILES += mremap_test
> TEST_GEN_FILES += mseal_test
> TEST_GEN_FILES += on-fault-limit
> TEST_GEN_FILES += pagemap_ioctl
> +TEST_GEN_FILES += pfnmap
> TEST_GEN_FILES += thuge-gen
> TEST_GEN_FILES += transhuge-stress
> TEST_GEN_FILES += uffd-stress
> diff --git a/tools/testing/selftests/mm/pfnmap.c b/tools/testing/selftests/mm/pfnmap.c
> new file mode 100644
> index 0000000000000..8a9d19b6020c7
> --- /dev/null
> +++ b/tools/testing/selftests/mm/pfnmap.c
> @@ -0,0 +1,196 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Basic VM_PFNMAP tests relying on mmap() of '/dev/mem'
> + *
> + * Copyright 2025, Red Hat, Inc.
> + *
> + * Author(s): David Hildenbrand <david@...hat.com>
> + */
> +#define _GNU_SOURCE
> +#include <stdlib.h>
> +#include <string.h>
> +#include <stdint.h>
> +#include <unistd.h>
> +#include <errno.h>
> +#include <fcntl.h>
> +#include <signal.h>
> +#include <setjmp.h>
> +#include <linux/mman.h>
> +#include <sys/mman.h>
> +#include <sys/wait.h>
> +
> +#include "../kselftest_harness.h"
> +#include "vm_util.h"
> +
> +static sigjmp_buf sigjmp_buf_env;
> +
> +static void signal_handler(int sig)
> +{
> + siglongjmp(sigjmp_buf_env, -EFAULT);
> +}
> +
> +static int test_read_access(char *addr, size_t size, size_t pagesize)
> +{
> + size_t offs;
> + int ret;
> +
> + if (signal(SIGSEGV, signal_handler) == SIG_ERR)
> + return -EINVAL;
> +
> + ret = sigsetjmp(sigjmp_buf_env, 1);
> + if (!ret) {
> + for (offs = 0; offs < size; offs += pagesize)
> + /* Force a read that the compiler cannot optimize out. */
> + *((volatile char *)(addr + offs));
> + }
> + if (signal(SIGSEGV, signal_handler) == SIG_ERR)
> + return -EINVAL;
> +
> + return ret;
> +}
> +
> +FIXTURE(pfnmap)
> +{
> + size_t pagesize;
> + int dev_mem_fd;
> + char *addr1;
> + size_t size1;
> + char *addr2;
> + size_t size2;
> +};
> +
> +FIXTURE_SETUP(pfnmap)
> +{
> + self->pagesize = getpagesize();
> +
> + self->dev_mem_fd = open("/dev/mem", O_RDONLY);
> + if (self->dev_mem_fd < 0)
> + SKIP(return, "Cannot open '/dev/mem'\n");
> +
> + /* We'll require the first two pages throughout our tests ... */
> + self->size1 = self->pagesize * 2;
> + self->addr1 = mmap(NULL, self->size1, PROT_READ, MAP_SHARED,
> + self->dev_mem_fd, 0);
> + if (self->addr1 == MAP_FAILED)
> + SKIP(return, "Cannot mmap '/dev/mem'\n");
> +
> + /* ... and want to be able to read from them. */
> + if (test_read_access(self->addr1, self->size1, self->pagesize))
> + SKIP(return, "Cannot read-access mmap'ed '/dev/mem'\n");
> +
> + self->size2 = 0;
> + self->addr2 = MAP_FAILED;
> +}
> +
> +FIXTURE_TEARDOWN(pfnmap)
> +{
> + if (self->addr2 != MAP_FAILED)
> + munmap(self->addr2, self->size2);
> + if (self->addr1 != MAP_FAILED)
> + munmap(self->addr1, self->size1);
> + if (self->dev_mem_fd >= 0)
> + close(self->dev_mem_fd);
> +}
> +
> +TEST_F(pfnmap, madvise_disallowed)
> +{
> + int advices[] = {
> + MADV_DONTNEED,
> + MADV_DONTNEED_LOCKED,
> + MADV_FREE,
> + MADV_WIPEONFORK,
> + MADV_COLD,
> + MADV_PAGEOUT,
> + MADV_POPULATE_READ,
> + MADV_POPULATE_WRITE,
> + };
> + int i;
> +
> + /* All these advices must be rejected. */
> + for (i = 0; i < ARRAY_SIZE(advices); i++) {
> + EXPECT_LT(madvise(self->addr1, self->pagesize, advices[i]), 0);
> + EXPECT_EQ(errno, EINVAL);
> + }
> +}
> +
> +TEST_F(pfnmap, munmap_split)
> +{
> + /*
> + * Unmap the first page. This munmap() call is not really expected to
> + * fail, but we might be able to trigger other internal issues.
> + */
> + ASSERT_EQ(munmap(self->addr1, self->pagesize), 0);
> +
> + /*
> + * Remap the first page while the second page is still mapped. This
> + * makes sure that any PAT tracking on x86 will allow for mmap()'ing
> + * a page again while some parts of the first mmap() are still
> + * around.
> + */
> + self->size2 = self->pagesize;
> + self->addr2 = mmap(NULL, self->pagesize, PROT_READ, MAP_SHARED,
> + self->dev_mem_fd, 0);
> + ASSERT_NE(self->addr2, MAP_FAILED);
> +}
> +
> +TEST_F(pfnmap, mremap_fixed)
> +{
> + char *ret;
> +
> + /* Reserve a destination area. */
> + self->size2 = self->size1;
> + self->addr2 = mmap(NULL, self->size2, PROT_READ, MAP_ANON | MAP_PRIVATE,
> + -1, 0);
> + ASSERT_NE(self->addr2, MAP_FAILED);
> +
> + /* mremap() over our destination. */
> + ret = mremap(self->addr1, self->size1, self->size2,
> + MREMAP_FIXED | MREMAP_MAYMOVE, self->addr2);
> + ASSERT_NE(ret, MAP_FAILED);
> +}
> +
> +TEST_F(pfnmap, mremap_shrink)
> +{
> + char *ret;
> +
> + /* Shrinking is expected to work. */
> + ret = mremap(self->addr1, self->size1, self->size1 - self->pagesize, 0);
> + ASSERT_NE(ret, MAP_FAILED);
> +}
> +
> +TEST_F(pfnmap, mremap_expand)
> +{
> + /*
> + * Growing is not expected to work, and getting it right would
> + * be challenging. So this test primarily serves as an early warning
> + * that something that probably should never work suddenly works.
> + */
> + self->size2 = self->size1 + self->pagesize;
> + self->addr2 = mremap(self->addr1, self->size1, self->size2, MREMAP_MAYMOVE);
> + ASSERT_EQ(self->addr2, MAP_FAILED);
> +}
> +
> +TEST_F(pfnmap, fork)
> +{
> + pid_t pid;
> + int ret;
> +
> + /* fork() a child and test if the child can access the pages. */
> + pid = fork();
> + ASSERT_GE(pid, 0);
> +
> + if (!pid) {
> + EXPECT_EQ(test_read_access(self->addr1, self->size1,
> + self->pagesize), 0);
> + exit(0);
> + }
> +
> + wait(&ret);
> + if (WIFEXITED(ret))
> + ret = WEXITSTATUS(ret);
> + else
> + ret = -EINVAL;
> + ASSERT_EQ(ret, 0);
> +}
> +
> +TEST_HARNESS_MAIN
> diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
> index 188b125bf1f6b..dddd1dd8af145 100755
> --- a/tools/testing/selftests/mm/run_vmtests.sh
> +++ b/tools/testing/selftests/mm/run_vmtests.sh
> @@ -63,6 +63,8 @@ separated by spaces:
> test soft dirty page bit semantics
> - pagemap
> test pagemap_scan IOCTL
> +- pfnmap
> + tests for VM_PFNMAP handling
> - cow
> test copy-on-write semantics
> - thp
> @@ -472,6 +474,8 @@ fi
>
> CATEGORY="pagemap" run_test ./pagemap_ioctl
>
> +CATEGORY="pfnmap" run_test ./pfnmap
> +
> # COW tests
> CATEGORY="cow" run_test ./cow
>
> --
> 2.49.0
>
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