[PATCH] mm, oom_reaper: Move oom_lock from __oom_reap_task_mm() to oom_reap_task().

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Since commit 862e3073b3eed13f
("mm, oom: get rid of signal_struct::oom_victims")
changed to wait until MMF_OOM_SKIP is set rather than wait while
TIF_MEMDIE is set, rationale comment for commit e2fe14564d3316d1
("oom_reaper: close race with exiting task") needs to be updated.

While holding oom_lock can make sure that other threads waiting for
oom_lock at __alloc_pages_may_oom() are given a chance to call
get_page_from_freelist() after the OOM reaper called unmap_page_range()
via __oom_reap_task_mm(), it can defer calling of __oom_reap_task_mm().

Therefore, this patch moves oom_lock from __oom_reap_task_mm() to
oom_reap_task() (without any functional change). By doing so, the OOM
killer can call __oom_reap_task_mm() if we don't want to defer calling
of __oom_reap_task_mm() (e.g. when oom_evaluate_task() aborted by
finding existing OOM victim's mm without MMF_OOM_SKIP).

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 mm/oom_kill.c | 39 +++++++++++++--------------------------
 1 file changed, 13 insertions(+), 26 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index ec9f11d..53b6e0c 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -467,28 +467,9 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
 	struct vm_area_struct *vma;
 	struct zap_details details = {.check_swap_entries = true,
 				      .ignore_dirty = true};
-	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;
-	}
+	if (!down_read_trylock(&mm->mmap_sem))
+		return false;
 
 	/*
 	 * increase mm_users only after we know we will reap something so
@@ -497,7 +478,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
 	 */
 	if (!mmget_not_zero(mm)) {
 		up_read(&mm->mmap_sem);
-		goto unlock_oom;
+		return true;
 	}
 
 	/*
@@ -548,9 +529,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
 	 * put the oom_reaper out of the way.
 	 */
 	mmput_async(mm);
-unlock_oom:
-	mutex_unlock(&oom_lock);
-	return ret;
+	return true;
 }
 
 #define MAX_OOM_REAP_RETRIES 10
@@ -560,8 +539,16 @@ static void oom_reap_task(struct task_struct *tsk)
 	struct mm_struct *mm = tsk->signal->oom_mm;
 
 	/* Retry the down_read_trylock(mmap_sem) a few times */
-	while (attempts++ < MAX_OOM_REAP_RETRIES && !__oom_reap_task_mm(tsk, mm))
+	while (attempts++ < MAX_OOM_REAP_RETRIES) {
+		bool ret;
+
+		mutex_lock(&oom_lock);
+		ret = __oom_reap_task_mm(tsk, mm);
+		mutex_unlock(&oom_lock);
+		if (ret)
+			break;
 		schedule_timeout_idle(HZ/10);
+	}
 
 	if (attempts <= MAX_OOM_REAP_RETRIES)
 		goto done;
-- 
1.8.3.1

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