FAILED: patch "[PATCH] mm/ksm: fix exec/fork inheritance support for prctl" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 590c03ca6a3fbb114396673314e2aa483839608b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025122925-designed-overture-2e7d@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 590c03ca6a3fbb114396673314e2aa483839608b Mon Sep 17 00:00:00 2001
From: xu xin <[email protected]>
Date: Tue, 7 Oct 2025 18:28:21 +0800
Subject: [PATCH] mm/ksm: fix exec/fork inheritance support for prctl

Patch series "ksm: fix exec/fork inheritance", v2.

This series fixes exec/fork inheritance. See the detailed description of the issue below.

This patch (of 2):

Background
==========

commit d7597f59d1d33 ("mm: add new api to enable ksm per process") introduced MMF_VM_MERGE_ANY for mm->flags, and allowed user to set it by prctl() so that the process's VMAs are forcibly scanned by ksmd.

Subsequently, the 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") supported inheriting the MMF_VM_MERGE_ANY flag when a task calls execve().

Finally, 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
===========

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
========

Firstly, 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.

Secondly, 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.

index 067538fc4d58..c982694c987b 100644
@@ -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;

index cdefba633856..4f672f4f2140 100644
@@ -2712,8 +2712,14 @@ 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 {

@@ -2831,12 +2837,20 @@ 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 <[email protected]>

[ Upstream commit 590c03ca6a3fbb114396673314e2aa483839608b ]

Patch series "ksm: fix exec/fork inheritance", v2.

This series fixes exec/fork inheritance. See the detailed description of the issue below.

This patch (of 2):

Background
==========

commit d7597f59d1d33 ("mm: add new api to enable ksm per process") introduced MMF_VM_MERGE_ANY for mm->flags, and allowed user to set it by prctl() so that the process's VMAs are forcibly scanned by ksmd.

Subsequently, the 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") supported inheriting the MMF_VM_MERGE_ANY flag when a task calls execve().

Finally, 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
===========

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
========

Firstly, 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.

Secondly, 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.

---
 mm/ksm.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

index 2e4cd681622d..96a0feb19c09 100644
@@ -2451,8 +2451,14 @@ 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.
+		 */
 		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
-		clear_bit(MMF_VM_MERGE_ANY, &mm->flags);
 		mmap_read_unlock(mm);
 		mmdrop(mm);
 	} else {

@@ -2567,8 +2573,16 @@ void ksm_add_vma(struct vm_area_struct *vma)
 {
 	struct mm_struct *mm = vma->vm_mm;
 
-	if (test_bit(MMF_VM_MERGE_ANY, &mm->flags))
+	if (test_bit(MMF_VM_MERGE_ANY, &mm->flags)) {
 		__ksm_add_vma(vma);
+		/*
+		 * 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(!test_bit(MMF_VM_MERGEABLE, &mm->flags)))
+			__ksm_enter(mm);
+	}
 }
 
 static void ksm_add_vmas(struct mm_struct *mm)

2.51.0