Re: [RFC PATCH] mm, oom_reaper: gather each vma to prevent leaking TLB entry

[Michal Hocko]

On Mon 06-11-17 19:03:34, Wangnan (F) wrote:
> 
> 
> On 2017/11/6 18:40, Michal Hocko wrote:
> > On Mon 06-11-17 17:59:54, Wangnan (F) wrote:
> > > 
> > > On 2017/11/6 16:52, Michal Hocko wrote:
> > > > On Mon 06-11-17 15:04:40, Bob Liu wrote:
> > > > > On Mon, Nov 6, 2017 at 11:36 AM, Wang Nan <wangnan0@xxxxxxxxxx> wrote:
> > > > > > tlb_gather_mmu(&tlb, mm, 0, -1) means gathering all virtual memory space.
> > > > > > In this case, tlb->fullmm is true. Some archs like arm64 doesn't flush
> > > > > > TLB when tlb->fullmm is true:
> > > > > > 
> > > > > >     commit 5a7862e83000 ("arm64: tlbflush: avoid flushing when fullmm == 1").
> > > > > > 
> > > > > CC'ed Will Deacon.
> > > > > 
> > > > > > Which makes leaking of tlb entries. For example, when oom_reaper
> > > > > > selects a task and reaps its virtual memory space, another thread
> > > > > > in this task group may still running on another core and access
> > > > > > these already freed memory through tlb entries.
> > > > No threads should be running in userspace by the time the reaper gets to
> > > > unmap their address space. So the only potential case is they are
> > > > accessing the user memory from the kernel when we should fault and we
> > > > have MMF_UNSTABLE to cause a SIGBUS. So is the race you are describing
> > > > real?
> > > > 
> > > > > > This patch gather each vma instead of gathering full vm space,
> > > > > > tlb->fullmm is not true. The behavior of oom reaper become similar
> > > > > > to munmapping before do_exit, which should be safe for all archs.
> > > > I do not have any objections to do per vma tlb flushing because it would
> > > > free gathered pages sooner but I am not sure I see any real problem
> > > > here. Have you seen any real issues or this is more of a review driven
> > > > fix?
> > > We saw the problem when we try to reuse oom reaper's code in
> > > another situation. In our situation, we allow reaping a task
> > > before all other tasks in its task group finish their exiting
> > > procedure.
> > > 
> > > I'd like to know what ensures "No threads should be running in
> > > userspace by the time the reaper"?
> > All tasks are killed by the time. So they should be taken out to the
> > kernel.
> 
> Sorry. I read oom_kill_process() but still unable to understand
> why all tasks are killed.
> 
> oom_kill_process() kill victim by sending SIGKILL. It will be
> broadcast to all tasks in its task group, but it is asynchronized.
> In the following case, race can happen (Thread1 in Task1's task group):
> 
> core 1                core 2
> Thread1 running       oom_kill_process() selects Task1 as victim
>                       oom_kill_process() sends SIGKILL to Task1
>                       oom_kill_process() sends SIGKILL to Thread1
>                       oom_kill_process() wakes up oom reaper
>                       switch to oom_reaper
>                       __oom_reap_task_mm
>                       tlb_gather_mmu
>                       unmap_page_range, reap Task1
>                       tlb_finish_mmu
> Write page
> be kicked off from core
> Receives SIGKILL
> 
> So what makes Thread1 being kicked off from core 1 before core 2
> starting unmapping?

complete_signal should call signal_wake_up on all threads because this
is a group fatal signal and that should send an IPI to all of the cpus
they run on to. Even if we do not wait for IPI to complete the race
window should be few instructions only while it takes quite some time to
hand over to the oom reaper.

-- 
Michal Hocko
SUSE Labs