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Message-ID: <20251007182821572h_SoFqYZXEP1mvWI4n9VL@zte.com.cn>
Date: Tue, 7 Oct 2025 18:28:21 +0800 (CST)
From: <xu.xin16@....com.cn>
To: <xu.xin16@....com.cn>, <akpm@...ux-foundation.org>, <david@...hat.com>,
<shr@...kernel.io>
Cc: <akpm@...ux-foundation.org>, <david@...hat.com>, <tujinjiang@...wei.com>,
<shr@...kernel.io>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
<yang.yang29@....com.cn>, <wang.yaxin@....com.cn>
Subject: [PATCH linux-next v2 1/2] mm/ksm: fix exec/fork inheritance support for prctl
From: xu xin <xu.xin16@....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@....com.cn>
Cc: stable@...r.kernel.org
Cc: Stefan Roesch <shr@...kernel.io>
Cc: David Hildenbrand <david@...hat.com>
Cc: Jinjiang Tu <tujinjiang@...wei.com>
Cc: Wang Yaxin <wang.yaxin@....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;
}
--
2.25.1
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