linux-kernel - [PATCH linux-next 0/2] ksm: fix exec/fork inheritance

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Message-ID: <202510012256278259zrhgATlLA2C510DMD3qI@zte.com.cn>
Date: Wed, 1 Oct 2025 22:56:27 +0800 (CST)
From: <xu.xin16@....com.cn>
To: <akpm@...ux-foundation.org>, <david@...hat.com>
Cc: <chengming.zhou@...ux.dev>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        <tujinjiang@...wei.com>, <shr@...kernel.io>
Subject: [PATCH linux-next 0/2] ksm: fix exec/fork inheritance

From: xu xin <xu.xin16@....com.cn>

This series aim to fix exec/fork inheritance and introduce ksm-utils tools
including ksm-set and ksm-get, you can see the detail in PATCH 1.

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.

xu xin (2):
  tools: add ksm-utils tools
  mm/ksm: fix exec/fork inheritance support for prctl

 mm/ksm.c                     |   8 +-
 tools/mm/Makefile            |  12 +-
 tools/mm/ksm-utils/Makefile  |  10 +
 tools/mm/ksm-utils/ksm-get.c | 397 +++++++++++++++++++++++++++++++++++
 tools/mm/ksm-utils/ksm-set.c | 144 +++++++++++++
 5 files changed, 567 insertions(+), 4 deletions(-)
 create mode 100644 tools/mm/ksm-utils/Makefile
 create mode 100644 tools/mm/ksm-utils/ksm-get.c
 create mode 100644 tools/mm/ksm-utils/ksm-set.c

-- 
2.25.1