Re: [RFC PATCH] mm, oom_reaper: gather each vma to prevent leaking TLB entry
From: Wangnan (F)
Date: Mon Nov 06 2017 - 22:52:01 EST
On 2017/11/6 19:57, Michal Hocko wrote:
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.
If the complete_signal is the mechanism we rely on to ensure
all threads are exited, then I'm sure it is not enough. As
you said, we still have a small race window. In some platform,
an IPI from one core to another core takes a little bit longer
than you may expect, and the core who receive the IPI may in
a very low frequency.
In our situation, we put the reaper code in do_exit after receiving
SIGKILL, and observe TLB entry leaking. Since this is a SIGKILL,
complete_signal should have been executed. So I think oom_reaper
have similar problem.
Thank you.