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 d7597f59d1 ("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 3c6f33b727 ("mm/ksm: support fork/exec for prctl")
supported inheriting the MMF_VM_MERGE_ANY flag when a task calls execve().
Finally, commit 3a9e567ca4 ("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 d7597f59d1 ("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.
Link: https://lkml.kernel.org/r/20251007182504440BJgK8VXRHh8TD7IGSUIY4@zte.com.cn
Link: https://lkml.kernel.org/r/20251007182821572h_SoFqYZXEP1mvWI4n9VL@zte.com.cn
Fixes: 3c6f33b727 ("mm/ksm: support fork/exec for prctl")
Fixes: d7597f59d1 ("mm: add new api to enable ksm per process")
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
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>
Cc: Yang Yang <yang.yang29@zte.com.cn>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
590c03ca6a