Michal Hocko wrote:
> On Fri 08-12-17 00:44:11, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > David, could you test with this patch please?
> >
> > Even if this patch solved David's case, you need to update
> >
> > * tsk_is_oom_victim() cannot be set from under us
> > * either because current->mm is already set to NULL
> > * under task_lock before calling mmput and oom_mm is
> > * set not NULL by the OOM killer only if current->mm
> > * is found not NULL while holding the task_lock.
> >
> > part as well, for it is the explanation of why
> > tsk_is_oom_victim() test was expected to work.
>
> Yes, the same applies for mm_is_oom_victim. I will fixup s@tsk_@mm_@
> here.
>
If you try to "s@tsk_@mm_@", I suggest doing
----------
mm/mmap.c | 6 ------
1 file changed, 6 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c
index 68fdd14..b2cb4e5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3025,12 +3025,6 @@ void exit_mmap(struct mm_struct *mm)
* mm. Because MMF_OOM_SKIP is already set before
* calling down_read(), oom_reap_task() will not run
* on this "mm" post up_write().
- *
- * mm_is_oom_victim() cannot be set from under us
- * either because current->mm is already set to NULL
- * under task_lock before calling mmput and oom_mm is
- * set not NULL by the OOM killer only if current->mm
- * is found not NULL while holding the task_lock.
*/
set_bit(MMF_OOM_SKIP, &mm->flags);
down_write(&mm->mmap_sem);
----------
because current->mm and current->signal->oom_mm are irrelevant for
the reason to set MMF_OOM_SKIP here. What matters here is that this
mm's ->mmap must not be accessed by the OOM reaper afterwards. (And
I think we can do mm->mmap = NULL instead of setting MMF_OOM_SKIP.
Then, we can defer setting MMF_OOM_SKIP until "the OOM reaper gave up
waiting for __mmput()" or "__mmput() became ready to call mmdrop()"
whichever occurred first.)
I think we can try changes shown below for
(1) Allowing __oom_reap_task_mm() to run without oom_lock held.
(Because start OOM reaping without waiting for oom_lock can mitigate
unexpected stalls due to schedule_timeout_killable(1) with oom_lock
held. Though, root cause of the stall is that we can't guarantee that
enough CPU resource is given to a thread holding oom_lock.)
Also, we could offload __oom_reap_task_mm() to some WQ_MEM_RECLAIM
workqueue in order to allow reclaiming memory in parallel when many
victims are waiting for OOM reaping.
(2) Guarantee that last second allocation at __alloc_pages_may_oom()
is attempted after confirming that there is no victim's mm without
MMF_OOM_SKIP set. (Because moving last second allocation to after
select_bad_process() was rejected. Though, doing last second
allocation attempt after select_bad_process() can allow us to
eliminate oom_lock taken in the changes shown below...)
(3) Guarantee that MMF_OOM_SKIP is set at __mmput(). (Because this was
an unexpected change for CONFIG_MMU=n.)
----------
kernel/fork.c | 6 ++++++
mm/mmap.c | 9 ++-------
mm/oom_kill.c | 49 ++++++++++---------------------------------------
3 files changed, 18 insertions(+), 46 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 432eadf..dd1d69e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -931,6 +931,12 @@ static inline void __mmput(struct mm_struct *mm)
}
if (mm->binfmt)
module_put(mm->binfmt->module);
+ if (unlikely(mm_is_oom_victim(mm))) {
+ /* Tell select_bad_process() to start selecting next mm. */
+ mutex_lock(&oom_lock);
+ set_bit(MMF_OOM_SKIP, &mm->flags);
+ mutex_unlock(&oom_lock);
+ }
mmdrop(mm);
}
diff --git a/mm/mmap.c b/mm/mmap.c
index b2cb4e5..641b5c1 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3020,14 +3020,9 @@ void exit_mmap(struct mm_struct *mm)
unmap_vmas(&tlb, vma, 0, -1);
if (unlikely(mm_is_oom_victim(mm))) {
- /*
- * Wait for oom_reap_task() to stop working on this
- * mm. Because MMF_OOM_SKIP is already set before
- * calling down_read(), oom_reap_task() will not run
- * on this "mm" post up_write().
- */
- set_bit(MMF_OOM_SKIP, &mm->flags);
+ /* Tell oom_reap_task() to stop working on this mm. */
down_write(&mm->mmap_sem);
+ mm->mmap = NULL;
up_write(&mm->mmap_sem);
}
free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 29f8555..ec5303d 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -489,28 +489,10 @@ 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;
}
/*
@@ -525,19 +507,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
if (mm_has_notifiers(mm)) {
up_read(&mm->mmap_sem);
schedule_timeout_idle(HZ);
- goto unlock_oom;
- }
-
- /*
- * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't
- * work on the mm anymore. The check for MMF_OOM_SKIP must run
- * under mmap_sem for reading because it serializes against the
- * down_write();up_write() cycle in exit_mmap().
- */
- if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
- up_read(&mm->mmap_sem);
- trace_skip_task_reaping(tsk->pid);
- goto unlock_oom;
+ return true;
}
trace_start_task_reaping(tsk->pid);
@@ -550,6 +520,10 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
*/
set_bit(MMF_UNSTABLE, &mm->flags);
+ /*
+ * exit_mmap() sets mm->mmap to NULL with mm->mmap_sem held for write
+ * if I need to stop working on this mm.
+ */
for (vma = mm->mmap ; vma; vma = vma->vm_next) {
if (!can_madv_dontneed_vma(vma))
continue;
@@ -579,9 +553,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
up_read(&mm->mmap_sem);
trace_finish_task_reaping(tsk->pid);
-unlock_oom:
- mutex_unlock(&oom_lock);
- return ret;
+ return true;
}
#define MAX_OOM_REAP_RETRIES 10
@@ -605,11 +577,10 @@ static void oom_reap_task(struct task_struct *tsk)
done:
tsk->oom_reaper_list = NULL;
- /*
- * Hide this mm from OOM killer because it has been either reaped or
- * somebody can't call up_write(mmap_sem).
- */
+ /* Tell select_bad_process() to start selecting next mm. */
+ mutex_lock(&oom_lock);
set_bit(MMF_OOM_SKIP, &mm->flags);
+ mutex_unlock(&oom_lock);
/* Drop a reference taken by wake_oom_reaper */
put_task_struct(tsk);
----------
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