From: Michal Hocko <mho...@suse.com>
David has noticed that the oom killer might kill additional tasks while
the existing 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)
Currently we are try to reduce a risk of this race by taking oom_lock
and wait for out_of_memory sleep while holding the lock to give the
victim some time to exit. This is quite suboptimal approach because
there is no guarantee the victim (especially a large one) will manage
to unmap its address space and free enough memory to the particular oom
domain which needs a memory (e.g. a specific NUMA node).
Fix this problem by allowing __oom_reap_task_mm and __mmput path to
race. __oom_reap_task_mm is basically MADV_DONTNEED and that is allowed
to run in parallel with other unmappers (hence the mmap_sem for read).
The only tricky part is we have to exclude page tables tear down and all
operations which modify the address space in the __mmput path. exit_mmap
doesn't expect any other users so it doesn't use any locking. Nothing
really forbids us to use mmap_sem for write, though. In fact we are
already relying on this lock earlier in the __mmput path to synchronize
with ksm and khugepaged.
Take the exclusive mmap_sem when calling free_pgtables and destroying
vmas to sync with __oom_reap_task_mm which take the lock for read. All
other operations can safely race with the parallel unmap.
Reported-by: David Rientjes <rient...@google.com>
Fixes: 26db62f179d1 ("oom: keep mm of the killed task available")
Signed-off-by: Michal Hocko <mho...@suse.com>
---
Hi,
I am sending this as an RFC because I am not yet sure I haven't missed
something subtle here but the appoach should work in principle. I have
run it through some of my OOM stress tests to see if anything blows up
and it all went smoothly.
The issue has been brought up by David [1]. There were some attempts to
address it in oom proper [2][3] but the first one would cause problems
on their own [4] while the later is just too hairy.
Thoughts, objections, alternatives?
[1]
http://lkml.kernel.org/r/alpine.deb.2.10.1706141632100.93...@chino.kir.corp.google.com
[2]
http://lkml.kernel.org/r/201706171417.jhg48401.joqlhmfsvoo...@i-love.sakura.ne.jp
[3] http://lkml.kernel.org/r/201706220053.v5m0rmou078...@www262.sakura.ne.jp
[4] http://lkml.kernel.org/r/201706210217.v5l2hazc081...@www262.sakura.ne.jp
mm/mmap.c | 7 +++++++
mm/oom_kill.c | 40 ++--------------------------------------
2 files changed, 9 insertions(+), 38 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index 3bd5ecd20d4d..253808e716dc 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2962,6 +2962,11 @@ void exit_mmap(struct mm_struct *mm)
/* Use -1 here to ensure all VMAs in the mm are unmapped */
unmap_vmas(&tlb, vma, 0, -1);
+ /*
+ * oom reaper might race with exit_mmap so make sure we won't free
+ * page tables or unmap VMAs under its feet
+ */
+ down_write(&mm->mmap_sem);
free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
tlb_finish_mmu(&tlb, 0, -1);
@@ -2974,7 +2979,9 @@ void exit_mmap(struct mm_struct *mm)
nr_accounted += vma_pages(vma);
vma = remove_vma(vma);
}
+ mm->mmap = NULL;
vm_unacct_memory(nr_accounted);
+ up_write(&mm->mmap_sem);
}
/* Insert vm structure into process list sorted by address
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 0e2c925e7826..5dc0ff22d567 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -472,36 +472,8 @@ static bool __oom_reap_task_mm(struct task_struct *tsk,
struct mm_struct *mm)
struct vm_area_struct *vma;
bool ret = true;
- /*
- * We have to make sure to not race with the victim exit path
- * and cause premature new oom victim selection:
- * __oom_reap_task_mm exit_mm
- * mmget_not_zero
- * mmput
- * atomic_dec_and_test
- * exit_oom_victim
- * [...]
- * out_of_memory
- * select_bad_process
- * # no TIF_MEMDIE task selects new
victim
- * unmap_page_range # frees some memory
- */
- mutex_lock(&oom_lock);
-
- if (!down_read_trylock(&mm->mmap_sem)) {
- ret = false;
- goto unlock_oom;
- }
-
- /*
- * increase mm_users only after we know we will reap something so
- * that the mmput_async is called only when we have reaped something
- * and delayed __mmput doesn't matter that much
- */
- if (!mmget_not_zero(mm)) {
- up_read(&mm->mmap_sem);
- goto unlock_oom;
- }
+ if (!down_read_trylock(&mm->mmap_sem))
+ return false;
/*
* Tell all users of get_user/copy_from_user etc... that the content
@@ -538,14 +510,6 @@ static bool __oom_reap_task_mm(struct task_struct *tsk,
struct mm_struct *mm)
K(get_mm_counter(mm, MM_SHMEMPAGES)));
up_read(&mm->mmap_sem);
- /*
- * Drop our reference but make sure the mmput slow path is called from a
- * different context because we shouldn't risk we get stuck there and
- * put the oom_reaper out of the way.
- */
- mmput_async(mm);
-unlock_oom:
- mutex_unlock(&oom_lock);
return ret;
}
--
2.11.0
