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Subject[PATCH] mm, oom: allow oom reaper to race with exit_mmap
Date
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.

Currently we 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 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.

Changes
- bail on null mm->mmap early as per David Rientjes

Reported-by: David Rientjes <rientjes@google.com>
Fixes: 26db62f179d1 ("oom: keep mm of the killed task available")
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
Hi,
I've sent this as an RFC [1] previously and it seems that the original
issue has been resolved [2], although an explicit tested-by would be
appreciated of course. Hugh has pointed out [3] that using mmap_sem
in exit_mmap will allow to drop the tricky
down_write(mmap_sem);
up_write(mmap_sem);
in both paths. I hope I will get to that in a forseeable future.

I am not yet sure this is important enough to merge to stable trees,
I would rather wait for a report to show up.

[1] http://lkml.kernel.org/r/20170626130346.26314-1-mhocko@kernel.org
[2] http://lkml.kernel.org/r/alpine.DEB.2.10.1707111336250.60183@chino.kir.corp.google.com
[3] http://lkml.kernel.org/r/alpine.LSU.2.11.1707191716030.2055@eggly.anvils

mm/mmap.c | 7 +++++++
mm/oom_kill.c | 45 +++++++--------------------------------------
2 files changed, 14 insertions(+), 38 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 24e9261bdcc0..0eeb658caa30 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2993,6 +2993,11 @@ 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);

@@ -3005,7 +3010,9 @@ 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 9e8b4f030c1c..a6dabe3691c1 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -470,40 +470,15 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
{
struct mmu_gather tlb;
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;
trace_skip_task_reaping(tsk->pid);
- goto unlock_oom;
+ return false;
}

- /*
- * 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);
- trace_skip_task_reaping(tsk->pid);
- goto unlock_oom;
- }
+ /* There is nothing to reap so bail out without signs in the log */
+ if (!mm->mmap)
+ goto unlock;

trace_start_task_reaping(tsk->pid);

@@ -540,17 +515,11 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
K(get_mm_counter(mm, MM_ANONPAGES)),
K(get_mm_counter(mm, MM_FILEPAGES)),
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);
trace_finish_task_reaping(tsk->pid);
-unlock_oom:
- mutex_unlock(&oom_lock);
+unlock:
+ up_read(&mm->mmap_sem);
+
return ret;
}

--
2.13.2
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