[5.10/5.15 LTS] Question on mlock race between ksm and cow

[mawupeng <mawupeng1@huawei.com>]

Our syzbot reports a warning on bad page state. The mlocked flag is not
cleared during page free.

During try_to_merge_one_page in ksm, kpage will be remlocked if vma
contains flag VM_LOCKED, however this flag is just cleared in wp_page_copy.
Since the mapcount of this kpage is -1, no one can remove its mlocked flag
before free, this lead to the bad page report.

Since mlock changes a lot in v5.18-rc1[1], the latest linux do not have
this problem. The 5.10/5.15 LTS do have this issue.

Here is the simplified calltrace:
try_to_merge_one_page				wp_page_copy

try_to_merge_one_page
// clear page mlocked during rmap removal
replace_page
page_remove_rmap
if (unlikely(PageMlocked(page)))
clear_page_mlock(compound_head(page));

if ((vma->vm_flags & VM_LOCKED)
									lock_page(old_page);
									if (vma->vm_flags & VM_LOCKED)
									if (PageMlocked(old_page))
									munlock_vma_page(old_page);
if (!PageMlocked(kpage))
lock_page(kpage);
mlock_vma_page(kpage);
unlock_page(kpage);
-------------------------------------------------

This problem can be easily reproduced with the following modifies:
1. enable the following CONFIG
  a) CONFIG_DEBUG_VM
  b) CONFIG_KSM
  c) CONFIG_MEMORY_FAIALURE

2. add delay in try_to_merge_one_page
diff --git a/mm/ksm.c b/mm/ksm.c
index a5716fdec1aa..f9ee2ec615ac 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1248,8 +1248,10 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
 
        if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
                munlock_vma_page(page);
+               mdelay(10);
                if (!PageMlocked(kpage)) {
                        unlock_page(page);
+                       mdelay(100);
                        lock_page(kpage);
                        mlock_vma_page(kpage);
                        page = kpage;           /* for final unlock */

3. run syzbot with the following content:

madvise(&(0x7f0000ff3000/0xc000)=nil, 0xc000, 0xc)
mlockall(0x1)
mlockall(0x5)
madvise(&(0x7f0000ff3000/0xc000)=nil, 0xc04c, 0x65)

madvise(&(0x7f0000ff5000/0x4000)=nil, 0x4000, 0xc)
mlockall(0x1)
mlockall(0xa5)
mlockall(0x0)
munlock(&(0x7f0000ff7000/0x4000)=nil, 0x4000)

-------------------------------------------------
The detail bug report can be seen as follow:

BUG: Bad page state in process rs:main Q:Reg  pfn:11406a
page:fffff7b004501a80 refcount:0 mapcount:0 mapping:0000000000000000 index:0x20ff4 pfn:0x11406a
flags: 0x30000000028000e(referenced|uptodate|dirty|swapbacked|mlocked|node=0|zone=3)
raw: 030000000028000e fffff7b00456aec8 fffff7b011439908 0000000000000000
Soft offlining pfn 0x455e8f at process virtual address 0x20ff6000
raw: 0000000000020ff4 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
Modules linked in:
CPU: 1 PID: 239 Comm: rs:main Q:Reg Not tainted 5.15.126+ #580
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
Call Trace:
 <TASK>
 dump_stack_lvl+0x33/0x46
 bad_page+0x9e/0xe0
 free_pcp_prepare+0x14b/0x1f0
 free_unref_page_list+0x7c/0x210
 release_pages+0x2fe/0x3c0
 __pagevec_lru_add+0x21a/0x360
 lru_cache_add+0x80/0xe0
 add_to_page_cache_lru+0x71/0xd0
 pagecache_get_page+0x245/0x460
 grab_cache_page_write_begin+0x1a/0x40
 ext4_da_write_begin+0xb7/0x280
 generic_perform_write+0xb4/0x1e0
 ext4_buffered_write_iter+0x9c/0x140
 ext4_file_write_iter+0x5b/0x840
 ? do_futex+0x1af/0xb60
 ? check_preempt_curr+0x21/0x60
 ? ttwu_do_wakeup.isra.140+0xd/0xf0
 new_sync_write+0x117/0x1b0
 vfs_write+0x1ff/0x260
 ksys_write+0xa0/0xe0
 do_syscall_64+0x37/0x90
 entry_SYSCALL_64_after_hwframe+0x67/0xd1
RIP: 0033:0x7fb815cef32f
Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 29 fd ff ff 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 05 <48> 3d 00 f0 ff ff 77 2d 44 89 c7 48 89 44 24 08 e8 5c fd ff ff 48
RSP: 002b:00007fb814b2b860 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fb808004f20 RCX: 00007fb815cef32f
RDX: 000000000000006e RSI: 00007fb808004f20 RDI: 0000000000000007
RBP: 00007fb808004c40 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000293 R12: 00007fb808009550
R13: 000000000000006e R14: 0000000000000000 R15: 0000000000000000
 </TASK>

[1]: https://lore.kernel.org/linux-mm/e7fbbdca-6590-7e45-3efd-279fba7f8376@suse.cz/T/#m0cb6e42b2a5ad634e1ec16e59f0f98f2e9382460