From: xu xin xu.xin16@zte.com.cn
This series aim to fix exec/fork inheritance. See the detailed description of issue at the following patch.
PATCH 1: the patch to fix the issue. PATCH 2: a reproduce program or testcase.
xu xin (2): mm/ksm: fix exec/fork inheritance support for prctl selftests: update ksm inheritation tests for prctl fork/exec
include/linux/ksm.h | 4 +- mm/ksm.c | 20 ++++++- .../selftests/mm/ksm_functional_tests.c | 57 +++++++++++++++++++ 3 files changed, 76 insertions(+), 5 deletions(-)
From: xu xin xu.xin16@zte.com.cn
Background ==========
The commit d7597f59d1d33 ("mm: add new api to enable ksm per process") introduce MMF_VM_MERGE_ANY for mm->flags, and allow user to set it by prctl() so that the process's VMAs are forcely scanned by ksmd. Sequently, the commit 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") support inheritsingMMF_VM_MERGE_ANY flag when a task calls execve(). Lastly, The commit 3a9e567ca45fb ("mm/ksm: fix ksm exec support for prctl") fixed the issue that ksmd doesn't scan the mm_struct with MMF_VM_MERGE_ANY by adding the mm_slot to ksm_mm_head in __bprm_mm_init().
Problem =======
In some extreme scenarios, however, this inheritance of MMF_VM_MERGE_ANY during exec/fork can fail. For example, when the scanning frequency of ksmd is tuned extremely high, a process carrying MMF_VM_MERGE_ANY may still fail to pass it to the newly exec'd process. This happens because ksm_execve() is executed too early in the do_execve flow (prematurely adding the new mm_struct to the ksm_mm_slot list).
As a result, before do_execve completes, ksmd may have already performed a scan and found that this new mm_struct has no VM_MERGEABLE VMAs, thus clearing its MMF_VM_MERGE_ANY flag. Consequently, when the new program executes, the flag MMF_VM_MERGE_ANY inheritance missed.
Root reason ===========
The commit d7597f59d1d33 ("mm: add new api to enable ksm per process") clear the flag MMF_VM_MERGE_ANY when ksmd found no VM_MERGEABLE VMAs.
Solution ========
First, Don't clear MMF_VM_MERGE_ANY when ksmd found no VM_MERGEABLE VMAs, because perhaps their mm_struct has just been added to ksm_mm_slot list, and its process has not yet officially started running or has not yet performed mmap/brk to allocate anonymous VMAS.
Second, recheck MMF_VM_MERGEABLE again if a process takes MMF_VM_MERGE_ANY, and create a mm_slot and join it into ksm_scan_list again.
Fixes: 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") Fixes: d7597f59d1d3 ("mm: add new api to enable ksm per process") Signed-off-by: xu xin xu.xin16@zte.com.cn Cc: stable@vger.kernel.org Cc: Stefan Roesch shr@devkernel.io Cc: David Hildenbrand david@redhat.com Cc: Jinjiang Tu tujinjiang@huawei.com Cc: Wang Yaxin wang.yaxin@zte.com.cn --- include/linux/ksm.h | 4 ++-- mm/ksm.c | 20 +++++++++++++++++--- 2 files changed, 19 insertions(+), 5 deletions(-)
diff --git a/include/linux/ksm.h b/include/linux/ksm.h index 067538fc4d58..c982694c987b 100644 --- a/include/linux/ksm.h +++ b/include/linux/ksm.h @@ -17,7 +17,7 @@ #ifdef CONFIG_KSM int ksm_madvise(struct vm_area_struct *vma, unsigned long start, unsigned long end, int advice, vm_flags_t *vm_flags); -vm_flags_t ksm_vma_flags(const struct mm_struct *mm, const struct file *file, +vm_flags_t ksm_vma_flags(struct mm_struct *mm, const struct file *file, vm_flags_t vm_flags); int ksm_enable_merge_any(struct mm_struct *mm); int ksm_disable_merge_any(struct mm_struct *mm); @@ -103,7 +103,7 @@ bool ksm_process_mergeable(struct mm_struct *mm);
#else /* !CONFIG_KSM */
-static inline vm_flags_t ksm_vma_flags(const struct mm_struct *mm, +static inline vm_flags_t ksm_vma_flags(struct mm_struct *mm, const struct file *file, vm_flags_t vm_flags) { return vm_flags; diff --git a/mm/ksm.c b/mm/ksm.c index 04019a15b25d..19efe3d41c75 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -2617,8 +2617,14 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page) spin_unlock(&ksm_mmlist_lock);
mm_slot_free(mm_slot_cache, mm_slot); + /* + * Only clear MMF_VM_MERGEABLE. We must not clear + * MMF_VM_MERGE_ANY, because for those MMF_VM_MERGE_ANY process, + * perhaps their mm_struct has just been added to ksm_mm_slot + * list, and its process has not yet officially started running + * or has not yet performed mmap/brk to allocate anonymous VMAS. + */ mm_flags_clear(MMF_VM_MERGEABLE, mm); - mm_flags_clear(MMF_VM_MERGE_ANY, mm); mmap_read_unlock(mm); mmdrop(mm); } else { @@ -2736,12 +2742,20 @@ static int __ksm_del_vma(struct vm_area_struct *vma) * * Returns: @vm_flags possibly updated to mark mergeable. */ -vm_flags_t ksm_vma_flags(const struct mm_struct *mm, const struct file *file, +vm_flags_t ksm_vma_flags(struct mm_struct *mm, const struct file *file, vm_flags_t vm_flags) { if (mm_flags_test(MMF_VM_MERGE_ANY, mm) && - __ksm_should_add_vma(file, vm_flags)) + __ksm_should_add_vma(file, vm_flags)) { vm_flags |= VM_MERGEABLE; + /* + * Generally, the flags here always include MMF_VM_MERGEABLE. + * However, in rare cases, this flag may be cleared by ksmd who + * scans a cycle without finding any mergeable vma. + */ + if (unlikely(!mm_flags_test(MMF_VM_MERGEABLE, mm))) + __ksm_enter(mm); + }
return vm_flags; }
From: xu xin xu.xin16@zte.com.cn
To reproduce the issue mentioned by [1], this add a setting of pages_to_scan and sleep_millisecs at the start of test_prctl_fork_exec(). The main change is just raise the scanning frequency of ksmd.
[1] https://lore.kernel.org/all/202510012256278259zrhgATlLA2C510DMD3qI@zte.com.c...
Signed-off-by: xu xin xu.xin16@zte.com.cn --- .../selftests/mm/ksm_functional_tests.c | 57 +++++++++++++++++++ 1 file changed, 57 insertions(+)
diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c index ac136f04b8d6..95afa5cfc062 100644 --- a/tools/testing/selftests/mm/ksm_functional_tests.c +++ b/tools/testing/selftests/mm/ksm_functional_tests.c @@ -38,6 +38,8 @@ enum ksm_merge_mode { };
static int mem_fd; +static int pages_to_scan_fd; +static int sleep_millisecs_fd; static int pagemap_fd; static size_t pagesize;
@@ -493,6 +495,46 @@ static void test_prctl_fork(void) ksft_test_result_pass("PR_SET_MEMORY_MERGE value is inherited\n"); }
+static int start_ksmd_and_set_frequency(char *pages_to_scan, char *sleep_ms) +{ + int ksm_fd; + + ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR); + if (ksm_fd < 0) + return -errno; + + if (write(ksm_fd, "1", 1) != 1) + return -errno; + + if (write(pages_to_scan_fd, pages_to_scan, strlen(pages_to_scan)) <= 0) + return -errno; + + if (write(sleep_millisecs_fd, sleep_ms, strlen(sleep_ms)) <= 0) + return -errno; + + return 0; +} + +static int stop_ksmd_and_restore_frequency(void) +{ + int ksm_fd; + + ksm_fd = open("/sys/kernel/mm/ksm/run", O_RDWR); + if (ksm_fd < 0) + return -errno; + + if (write(ksm_fd, "2", 1) != 1) + return -errno; + + if (write(pages_to_scan_fd, "100", 3) <= 0) + return -errno; + + if (write(sleep_millisecs_fd, "20", 2) <= 0) + return -errno; + + return 0; +} + static void test_prctl_fork_exec(void) { int ret, status; @@ -500,6 +542,9 @@ static void test_prctl_fork_exec(void)
ksft_print_msg("[RUN] %s\n", __func__);
+ if (start_ksmd_and_set_frequency("2000", "0")) + ksft_test_result_fail("set ksmd's scanning frequency failed\n"); + ret = prctl(PR_SET_MEMORY_MERGE, 1, 0, 0, 0); if (ret < 0 && errno == EINVAL) { ksft_test_result_skip("PR_SET_MEMORY_MERGE not supported\n"); @@ -542,6 +587,11 @@ static void test_prctl_fork_exec(void) return; }
+ if (stop_ksmd_and_restore_frequency()) { + ksft_test_result_fail("restore ksmd frequency failed\n"); + return; + } + ksft_test_result_pass("PR_SET_MEMORY_MERGE value is inherited\n"); }
@@ -656,6 +706,13 @@ static void init_global_file_handles(void) ksft_exit_skip("open("/proc/self/pagemap") failed\n"); if (ksm_get_self_merging_pages() < 0) ksft_exit_skip("accessing "/proc/self/ksm_merging_pages") failed\n"); + + pages_to_scan_fd = open("/sys/kernel/mm/ksm/pages_to_scan", O_RDWR); + if (pages_to_scan_fd < 0) + ksft_exit_fail_msg("opening /sys/kernel/mm/ksm/pages_to_scan failed\n"); + sleep_millisecs_fd = open("/sys/kernel/mm/ksm/sleep_millisecs", O_RDWR); + if (sleep_millisecs_fd < 0) + ksft_exit_fail_msg("opening /sys/kernel/mm/ksm/sleep_millisecs failed\n"); }
int main(int argc, char **argv)
Hi,
Thanks for your patch.
FYI: kernel test robot notices the stable kernel rule is not satisfied.
The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#opti...
Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree. Subject: [PATCH linux-next v2 2/2] selftests: update ksm inheritation tests for prctl fork/exec Link: https://lore.kernel.org/stable/20251007182935207jm31wCIgLpZg5XbXQY64S%40zte....
linux-stable-mirror@lists.linaro.org
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