[PATCH 1/2] oom: don't assume that a coredumping thread will exit soon

[Oleg Nesterov]

oom_kill.c assumes that PF_EXITING task should exit and free the memory
soon. This is wrong in many ways and one important case is the coredump.
A task can sleep in exit_mm() "forever" while the coredumping sub-thread
can need more memory.

Change the PF_EXITING checks to take SIGNAL_GROUP_COREDUMP into account,
we add the new trivial helper for that.

Note: this is only the first step, this patch doesn't try to solve other
problems. For example it doesn't try to clear the wrongly set TIF_MEMDIE
(SIGNAL_GROUP_COREDUMP check is obviously racy), fatal_signal_pending()
can be false positive, etc.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 mm/oom_kill.c |   12 +++++++++---
 1 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 5340f6b..7af33b5 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -254,6 +254,12 @@ static enum oom_constraint constrained_alloc(struct zonelist *zonelist,
 }
 #endif
 
+static inline bool task_will_free_mem(struct task_struct *task)
+{
+	return (task->flags & PF_EXITING) &&
+		!(task->signal->flags & SIGNAL_GROUP_COREDUMP);
+}
+
 enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
 		unsigned long totalpages, const nodemask_t *nodemask,
 		bool force_kill)
@@ -281,7 +287,7 @@ enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
 	if (oom_task_origin(task))
 		return OOM_SCAN_SELECT;
 
-	if (task->flags & PF_EXITING && !force_kill) {
+	if (task_will_free_mem(task) && !force_kill) {
 		/*
 		 * If this task is not being ptraced on exit, then wait for it
 		 * to finish before killing some other task unnecessarily.
@@ -443,7 +449,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	 * If the task is already exiting, don't alarm the sysadmin or kill
 	 * its children or threads, just set TIF_MEMDIE so it can die quickly
 	 */
-	if (p->flags & PF_EXITING) {
+	if (task_will_free_mem(p)) {
 		set_tsk_thread_flag(p, TIF_MEMDIE);
 		put_task_struct(p);
 		return;
@@ -649,7 +655,7 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 	 * select it.  The goal is to allow it to allocate so that it may
 	 * quickly exit and free its memory.
 	 */
-	if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
+	if (fatal_signal_pending(current) || task_will_free_mem(current)) {
 		set_thread_flag(TIF_MEMDIE);
 		return;
 	}
-- 
1.5.5.1

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