[PATCH mm-stable v2 0/3] mm/ksm: improve PR_SET_MEMORY_MERGE=0 handling and cleanup disabling KSM

[PATCH mm-stable v2 0/3] mm/ksm: improve PR_SET_MEMORY_MERGE=0 handling and cleanup disabling KSM

[David Hildenbrand]

This is a follow-up to [1]:
	[PATCH v9 0/3] mm: process/cgroup ksm support

which is now in mm-stable. Ideally we'd get at least patch #1 into the
same kernel release as [1], so the semantics of setting
PR_SET_MEMORY_MERGE=0 are unchanged between kernel versions.

(1) Make PR_SET_MEMORY_MERGE=0 unmerge pages like setting MADV_UNMERGEABLE
does, (2) add a selftest for it and (3) factor out disabling of KSM from
s390/gmap code.

v1 -> v2:
- "mm/ksm: unmerge and clear VM_MERGEABLE when setting
   PR_SET_MEMORY_MERGE=0"
 -> Cleanup one if/else
 -> Add doc for ksm_disable_merge_any()
- Added ACKs

[1] https://lkml.kernel.org/r/20230418051342.1919757-1-shr@devkernel.io

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Stefan Roesch <shr@devkernel.io>
Cc: Rik van Riel <riel@surriel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>

David Hildenbrand (3):
  mm/ksm: unmerge and clear VM_MERGEABLE when setting
    PR_SET_MEMORY_MERGE=0
  selftests/ksm: ksm_functional_tests: add prctl unmerge test
  mm/ksm: move disabling KSM from s390/gmap code to KSM code

 arch/s390/mm/gmap.c                           | 20 +-----
 include/linux/ksm.h                           |  7 ++
 kernel/sys.c                                  | 12 +---
 mm/ksm.c                                      | 70 +++++++++++++++++++
 .../selftests/mm/ksm_functional_tests.c       | 46 ++++++++++--
 5 files changed, 121 insertions(+), 34 deletions(-)

-- 
2.40.0

[PATCH mm-stable v2 1/3] mm/ksm: unmerge and clear VM_MERGEABLE when setting PR_SET_MEMORY_MERGE=0

[David Hildenbrand]

Let's unmerge any KSM pages when setting PR_SET_MEMORY_MERGE=0, and clear
the VM_MERGEABLE flag from all VMAs -- just like KSM would. Of course,
only do that if we previously set PR_SET_MEMORY_MERGE=1.

Acked-by: Stefan Roesch <shr@devkernel.io>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/ksm.h |  1 +
 kernel/sys.c        | 12 +++------
 mm/ksm.c            | 59 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+), 9 deletions(-)

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 7a9b76fb6c3f..429efa6ff4ae 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -21,6 +21,7 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
 
 void ksm_add_vma(struct vm_area_struct *vma);
 int ksm_enable_merge_any(struct mm_struct *mm);
+int ksm_disable_merge_any(struct mm_struct *mm);
 
 int __ksm_enter(struct mm_struct *mm);
 void __ksm_exit(struct mm_struct *mm);
diff --git a/kernel/sys.c b/kernel/sys.c
index 72cdb16e2636..339fee3eff6a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2695,16 +2695,10 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		if (mmap_write_lock_killable(me->mm))
 			return -EINTR;
 
-		if (arg2) {
+		if (arg2)
 			error = ksm_enable_merge_any(me->mm);
-		} else {
-			/*
-			 * TODO: we might want disable KSM on all VMAs and
-			 * trigger unsharing to completely disable KSM.
-			 */
-			clear_bit(MMF_VM_MERGE_ANY, &me->mm->flags);
-			error = 0;
-		}
+		else
+			error = ksm_disable_merge_any(me->mm);
 		mmap_write_unlock(me->mm);
 		break;
 	case PR_GET_MEMORY_MERGE:
diff --git a/mm/ksm.c b/mm/ksm.c
index 9e48258985d2..823bb3475a68 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2520,6 +2520,22 @@ static void __ksm_add_vma(struct vm_area_struct *vma)
 		vm_flags_set(vma, VM_MERGEABLE);
 }
 
+static int __ksm_del_vma(struct vm_area_struct *vma)
+{
+	int err;
+
+	if (!(vma->vm_flags & VM_MERGEABLE))
+		return 0;
+
+	if (vma->anon_vma) {
+		err = unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end);
+		if (err)
+			return err;
+	}
+
+	vm_flags_clear(vma, VM_MERGEABLE);
+	return 0;
+}
 /**
  * ksm_add_vma - Mark vma as mergeable if compatible
  *
@@ -2542,6 +2558,20 @@ static void ksm_add_vmas(struct mm_struct *mm)
 		__ksm_add_vma(vma);
 }
 
+static int ksm_del_vmas(struct mm_struct *mm)
+{
+	struct vm_area_struct *vma;
+	int err;
+
+	VMA_ITERATOR(vmi, mm, 0);
+	for_each_vma(vmi, vma) {
+		err = __ksm_del_vma(vma);
+		if (err)
+			return err;
+	}
+	return 0;
+}
+
 /**
  * ksm_enable_merge_any - Add mm to mm ksm list and enable merging on all
  *                        compatible VMA's
@@ -2569,6 +2599,35 @@ int ksm_enable_merge_any(struct mm_struct *mm)
 	return 0;
 }
 
+/**
+ * ksm_disable_merge_any - Disable merging on all compatible VMA's of the mm,
+ *			   previously enabled via ksm_enable_merge_any().
+ *
+ * Disabling merging implies unmerging any merged pages, like setting
+ * MADV_UNMERGEABLE would. If unmerging fails, the whole operation fails and
+ * merging on all compatible VMA's remains enabled.
+ *
+ * @mm: Pointer to mm
+ *
+ * Returns 0 on success, otherwise error code
+ */
+int ksm_disable_merge_any(struct mm_struct *mm)
+{
+	int err;
+
+	if (!test_bit(MMF_VM_MERGE_ANY, &mm->flags))
+		return 0;
+
+	err = ksm_del_vmas(mm);
+	if (err) {
+		ksm_add_vmas(mm);
+		return err;
+	}
+
+	clear_bit(MMF_VM_MERGE_ANY, &mm->flags);
+	return 0;
+}
+
 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
 		unsigned long end, int advice, unsigned long *vm_flags)
 {
-- 
2.40.0

[PATCH mm-stable v2 2/3] selftests/ksm: ksm_functional_tests: add prctl unmerge test

[David Hildenbrand]

Let's test whether setting PR_SET_MEMORY_MERGE to 0 after setting it to
1 will unmerge pages, similar to how setting MADV_UNMERGEABLE after setting
MADV_MERGEABLE would.

Acked-by: Stefan Roesch <shr@devkernel.io>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 .../selftests/mm/ksm_functional_tests.c       | 46 ++++++++++++++++---
 1 file changed, 40 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index 7bc9fc17c9f0..26853badae70 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -91,9 +91,10 @@ static int ksm_merge(void)
 	return 0;
 }
 
-static char *mmap_and_merge_range(char val, unsigned long size)
+static char *mmap_and_merge_range(char val, unsigned long size, bool use_prctl)
 {
 	char *map;
+	int ret;
 
 	map = mmap(NULL, size, PROT_READ|PROT_WRITE,
 		   MAP_PRIVATE|MAP_ANON, -1, 0);
@@ -110,7 +111,17 @@ static char *mmap_and_merge_range(char val, unsigned long size)
 
 	/* Make sure each page contains the same values to merge them. */
 	memset(map, val, size);
-	if (madvise(map, size, MADV_MERGEABLE)) {
+
+	if (use_prctl) {
+		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");
+			goto unmap;
+		} else if (ret) {
+			ksft_test_result_fail("PR_SET_MEMORY_MERGE=1 failed\n");
+			goto unmap;
+		}
+	} else if (madvise(map, size, MADV_MERGEABLE)) {
 		ksft_test_result_fail("MADV_MERGEABLE failed\n");
 		goto unmap;
 	}
@@ -133,7 +144,7 @@ static void test_unmerge(void)
 
 	ksft_print_msg("[RUN] %s\n", __func__);
 
-	map = mmap_and_merge_range(0xcf, size);
+	map = mmap_and_merge_range(0xcf, size, false);
 	if (map == MAP_FAILED)
 		return;
 
@@ -155,7 +166,7 @@ static void test_unmerge_discarded(void)
 
 	ksft_print_msg("[RUN] %s\n", __func__);
 
-	map = mmap_and_merge_range(0xcf, size);
+	map = mmap_and_merge_range(0xcf, size, false);
 	if (map == MAP_FAILED)
 		return;
 
@@ -187,7 +198,7 @@ static void test_unmerge_uffd_wp(void)
 
 	ksft_print_msg("[RUN] %s\n", __func__);
 
-	map = mmap_and_merge_range(0xcf, size);
+	map = mmap_and_merge_range(0xcf, size, false);
 	if (map == MAP_FAILED)
 		return;
 
@@ -323,9 +334,31 @@ static void test_prctl_fork(void)
 	ksft_test_result_pass("PR_SET_MEMORY_MERGE value is inherited\n");
 }
 
+static void test_prctl_unmerge(void)
+{
+	const unsigned int size = 2 * MiB;
+	char *map;
+
+	ksft_print_msg("[RUN] %s\n", __func__);
+
+	map = mmap_and_merge_range(0xcf, size, true);
+	if (map == MAP_FAILED)
+		return;
+
+	if (prctl(PR_SET_MEMORY_MERGE, 0, 0, 0, 0)) {
+		ksft_test_result_fail("PR_SET_MEMORY_MERGE=0 failed\n");
+		goto unmap;
+	}
+
+	ksft_test_result(!range_maps_duplicates(map, size),
+			 "Pages were unmerged\n");
+unmap:
+	munmap(map, size);
+}
+
 int main(int argc, char **argv)
 {
-	unsigned int tests = 4;
+	unsigned int tests = 5;
 	int err;
 
 #ifdef __NR_userfaultfd
@@ -355,6 +388,7 @@ int main(int argc, char **argv)
 
 	test_prctl();
 	test_prctl_fork();
+	test_prctl_unmerge();
 
 	err = ksft_get_fail_cnt();
 	if (err)
-- 
2.40.0

[PATCH mm-stable v2 3/3] mm/ksm: move disabling KSM from s390/gmap code to KSM code

[David Hildenbrand]

Let's factor out actual disabling of KSM. The existing
"mm->def_flags &= ~VM_MERGEABLE;" was essentially a NOP and can be dropped,
because def_flags should never include VM_MERGEABLE. Note that we don't
currently prevent re-enabling KSM.

This should now be faster in case KSM was never enabled, because we only
conditionally iterate all VMAs. Further, it certainly looks cleaner.

Acked-by: Janosch Frank <frankja@linux.ibm.com>
Acked-by: Stefan Roesch <shr@devkernel.io>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 arch/s390/mm/gmap.c | 20 +-------------------
 include/linux/ksm.h |  6 ++++++
 mm/ksm.c            | 11 +++++++++++
 3 files changed, 18 insertions(+), 19 deletions(-)

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index 0949811761e6..dfe905c7bd8e 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -2585,30 +2585,12 @@ EXPORT_SYMBOL_GPL(s390_enable_sie);
 
 int gmap_mark_unmergeable(void)
 {
-	struct mm_struct *mm = current->mm;
-	struct vm_area_struct *vma;
-	unsigned long vm_flags;
-	int ret;
-	VMA_ITERATOR(vmi, mm, 0);
-
 	/*
 	 * Make sure to disable KSM (if enabled for the whole process or
 	 * individual VMAs). Note that nothing currently hinders user space
 	 * from re-enabling it.
 	 */
-	clear_bit(MMF_VM_MERGE_ANY, &mm->flags);
-
-	for_each_vma(vmi, vma) {
-		/* Copy vm_flags to avoid partial modifications in ksm_madvise */
-		vm_flags = vma->vm_flags;
-		ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
-				  MADV_UNMERGEABLE, &vm_flags);
-		if (ret)
-			return ret;
-		vm_flags_reset(vma, vm_flags);
-	}
-	mm->def_flags &= ~VM_MERGEABLE;
-	return 0;
+	return ksm_disable(current->mm);
 }
 EXPORT_SYMBOL_GPL(gmap_mark_unmergeable);
 
diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 429efa6ff4ae..899a314bc487 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -22,6 +22,7 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
 void ksm_add_vma(struct vm_area_struct *vma);
 int ksm_enable_merge_any(struct mm_struct *mm);
 int ksm_disable_merge_any(struct mm_struct *mm);
+int ksm_disable(struct mm_struct *mm);
 
 int __ksm_enter(struct mm_struct *mm);
 void __ksm_exit(struct mm_struct *mm);
@@ -80,6 +81,11 @@ static inline void ksm_add_vma(struct vm_area_struct *vma)
 {
 }
 
+static inline int ksm_disable(struct mm_struct *mm)
+{
+	return 0;
+}
+
 static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
 {
 	return 0;
diff --git a/mm/ksm.c b/mm/ksm.c
index 823bb3475a68..0156bded3a66 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2628,6 +2628,17 @@ int ksm_disable_merge_any(struct mm_struct *mm)
 	return 0;
 }
 
+int ksm_disable(struct mm_struct *mm)
+{
+	mmap_assert_write_locked(mm);
+
+	if (!test_bit(MMF_VM_MERGEABLE, &mm->flags))
+		return 0;
+	if (test_bit(MMF_VM_MERGE_ANY, &mm->flags))
+		return ksm_disable_merge_any(mm);
+	return ksm_del_vmas(mm);
+}
+
 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
 		unsigned long end, int advice, unsigned long *vm_flags)
 {
-- 
2.40.0

Re: [PATCH mm-stable v2 0/3] mm/ksm: improve PR_SET_MEMORY_MERGE=0 handling and cleanup disabling KSM

[Andrew Morton]

On Sat, 22 Apr 2023 22:54:17 +0200 David Hildenbrand <david@redhat.com> wrote:

> This is a follow-up to [1]:
> 	[PATCH v9 0/3] mm: process/cgroup ksm support
> 
> which is now in mm-stable. Ideally we'd get at least patch #1 into the
> same kernel release as [1], so the semantics of setting
> PR_SET_MEMORY_MERGE=0 are unchanged between kernel versions.

Ack.  I'll prepare a second followon MM batch for Linus latish in the
merge window for material such as this.