Re: 4.9.30 NULL pointer dereference in __remove_shared_vm_struct

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Tommi Rantala wrote:
> I have hit this kernel bug twice with 4.9.30 while running trinity, any 
> ideas? It's not easily reproducible.

No idea. But if you can reproduce this problem, I think you can retry with
the OOM reaper disabled (like shown below), for the latter report is 10 seconds
after the OOM reaper reclaimed memory.

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index ec9f11d..7e17242 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -560,8 +560,8 @@ static void oom_reap_task(struct task_struct *tsk)
 	struct mm_struct *mm = tsk->signal->oom_mm;
 
 	/* Retry the down_read_trylock(mmap_sem) a few times */
-	while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task_mm(tsk, mm))
-		schedule_timeout_idle(HZ/10);
+	//while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task_mm(tsk, mm))
+	//	schedule_timeout_idle(HZ/10);
 
 	if (attempts <= MAX_OOM_REAP_RETRIES)
 		goto done;

Since line 137 is atomic_inc(), file->f_inode was for some reason NULL, wasn't it?

	if (vma->vm_flags & VM_DENYWRITE)
		atomic_inc(&file_inode(file)->i_writecount);

And mmput() from exit_mm() from do_exit() is called before exit_files() is
called from do_exit(). Thus, something by error made file->f_inode == NULL,
despite quite few locations set f_inode to NULL.

# grep -nFr -- '->f_inode ' *
fs/file_table.c:168:    file->f_inode = path->dentry->d_inode;
fs/file_table.c:224:    file->f_inode = NULL;
fs/open.c:711:  f->f_inode = inode;
fs/open.c:782:  f->f_inode = NULL;
fs/overlayfs/copy_up.c:36:      if (f->f_inode == d_inode(dentry))

Maybe the OOM reaper by error reclaimed and somebody zeroed the reclaimed
page containing file->f_inode.

JFYI, 4.9.30 does not have commit 235190738aba7c5c ("oom-reaper: use
madvise_dontneed() logic to decide if unmap the VMA") backported.

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