Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap

[Andrea Arcangeli <aarcange@redhat.com>]

On Mon, Jul 24, 2017 at 09:23:32AM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> David has noticed that the oom killer might kill additional tasks while
> the exiting oom victim hasn't terminated yet because the oom_reaper marks
> the curent victim MMF_OOM_SKIP too early when mm->mm_users dropped down
> to 0. The race is as follows
> 
> oom_reap_task				do_exit
> 					  exit_mm
>   __oom_reap_task_mm
> 					    mmput
> 					      __mmput
>     mmget_not_zero # fails
>     						exit_mmap # frees memory
>   set_bit(MMF_OOM_SKIP)
> 
> The victim is still visible to the OOM killer until it is unhashed.

I think this is a very minor problem, in the worst case you get a
false positive oom kill, and it requires a race condition for it to
happen. I wouldn't add mmap_sem in exit_mmap just for this considering
the mmget_not_zero is already enough to leave exit_mmap alone.

Could you first clarify these points then I'll understand better what
the above is about:

1) if exit_mmap runs for a long time with terabytes of RAM with
   mmap_sem held for writing like your patch does, wouldn't then
   oom_reap_task_mm fail the same way after a few tries on
   down_read_trylock? Despite your patch got applied? Isn't that
   simply moving the failure that leads to set_bit(MMF_OOM_SKIP) from
   mmget_not_zero to down_read_trylock?

2) why isn't __oom_reap_task_mm returning different retvals in case
   mmget_not_zero fails? What is the point to schedule_timeout
   and retry MAX_OOM_REAP_RETRIES times if mmget_not_zero caused it to
   return null as it can't do anything about such task anymore? Why
   are we scheduling those RETRIES times if mm_users is 0?

3) if exit_mmap is freeing lots of memory already, why should there be
   another OOM immediately? I thought oom reaper only was needed when
   the task on the right column couldn't reach the final mmput to set
   mm_users to 0. Why exactly is a problem that MMF_OOM_SKIP gets set
   on the mm, if exit_mmap is already guaranteed to be running? Why
   isn't the oom reaper happy to just stop in such case and wait it to
   complete? exit_mmap doesn't even take the mmap_sem and it's running
   in R state, how would it block in a way that requires the OOM
   reaper to free memory from another process to complete?

4) how is it safe to overwrite a VM_FAULT_RETRY that returns without
   mmap_sem and then the arch code will release the mmap_sem despite
   it was already released by handle_mm_fault? Anonymous memory faults
   aren't common to return VM_FAULT_RETRY but an userfault
   can. Shouldn't there be a block that prevents overwriting if
   VM_FAULT_RETRY is set below? (not only VM_FAULT_ERROR)

	if (unlikely((current->flags & PF_KTHREAD) && !(ret & VM_FAULT_ERROR)
				&& test_bit(MMF_UNSTABLE, &vma->vm_mm->flags)))
		ret = VM_FAULT_SIGBUS;

Thanks,
Andrea