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

[mawupeng <mawupeng1@huawei.com>]

Since page_remove_rmap in wp_page_copy only clear this mlocked page iff
page's mapcount is -1. which can be seen as follow.

wp_page_copy
  page_remove_rmap
    if (!atomic_add_negative(-1, &page->_mapcount))
      goto out;
    clear_page_mlock(page);	// clear mlocked flag

During out test, we can test this mapcount before mlock the kpage, this
can close this race.

diff --git a/mm/ksm.c b/mm/ksm.c
index 62feb478a367..347f4c0339c2 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1295,7 +1295,8 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
                if (!PageMlocked(kpage)) {
                        unlock_page(page);
                        lock_page(kpage);
-                       mlock_vma_page(kpage);
+                       if (page_mapcount(kpage) > 0)
+                               mlock_vma_page(kpage);
                        page = kpage;           /* for final unlock */
                }
        }


On 2023/8/15 15:07, mawupeng wrote:
> 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