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 fails!

Reproduce
========
Prepare ksm-utils in the prerequisite PATCH, and simply do as follows

echo 1 > /sys/kernel/mm/ksm/run;
echo 2000 > /sys/kernel/mm/ksm/pages_to_scan;
echo 0 > /sys/kernel/mm/ksm/sleep_millisecs;
ksm-set -s on [NEW_PROGRAM_BIN] &
ksm-get -a -e

you can see like this:
Pid         Comm                Merging_pages  Ksm_zero_pages    Ksm_profit     Ksm_mergeable     Ksm_merge_any
206         NEW_PROGRAM_BIN     7680           0                 30965760       yes               no

Note:
If the first time don't reproduce the issue, pkill NEW_PROGRAM_BIN and try run it
again. Usually, we can reproduce it in 5 times.

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
========
Remove the action of clearing 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.

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>
---
 mm/ksm.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 04019a15b25d..17c7ed7df700 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 {
-- 
2.25.