Yes, it should cause oops?
> > Plonking the big lock around every access to VM is _not_ a solution
I never did that, I'll never do that, I only notice that the current
code is filled with races.
> >. If
> > swapper doesn't use mmap_sem - _swapper_ should be fixed. How the hell
> > does lock_kernel() have smaller deadlock potential than
> > down(&mm->mmap_sem)?
lock_kernel() is dropped on thread switch, the semaphore is not dropped.
>
> OK, folks. Code in swapper (unuse_process(), right?) is called only from
> sys_swapoff(). It's a syscall. Andrea, could you show a scenario for
> deadlock here? OK, some process (but not the process doing swapoff()) may
> have the map locked So? it is not going to release the thing - we are
> seriously screwed anyway (read: we already are in deadlock). We don't hold
> the semaphore ourselves.
AFAIK the problem is OOM:
* a process accesses a not-present, ie page fault:
...
handle_mm_fault(): this process own mm->mmap_sem.
->handle_pte_fault().
-> (eg.) do_wp_page().
-> get_free_page().
now get_free_page() notices that there is no free memory.
--> wakeup kswapd.
* the swapper runs, and it tries to swap out data from that process.
mm->mmap_sem is already acquired --> lock-up.
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