[PATCH -mm] mmoom-fix-potentially-killing-unrelated-process-fix

[Oleg Nesterov <oleg@redhat.com>]

oom_kill_process() does atomic_inc(&mm->mm_users) to ensure that
this ->mm can't go away and this is wrong, change it to rely on
->mm_count and mmdrop().

Firstly, we do not want to delay exit_mmap/etc if the victim exits
before we do mmput(), but this is minor.

More importantly, we simply can not do mmput() in oom_kill_process(),
this can deadlock if (for example) the caller holds i_mmap_rwsem and
mmput() actually leads to exit_mmap(); the victim can have this file
mmaped and in this case unmap_vmas/free_pgtables paths will take the
same lock for writing. And at least huge_pmd_share() does pmd_alloc()
under i_mmap_rwsem because VM_HUGETLB memory is not reclaimable.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 mm/oom_kill.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 034d219..52abb78 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -571,7 +571,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 
 	/* Get a reference to safely compare mm after task_unlock(victim) */
 	mm = victim->mm;
-	atomic_inc(&mm->mm_users);
+	atomic_inc(&mm->mm_count);
 	/*
 	 * We should send SIGKILL before setting TIF_MEMDIE in order to prevent
 	 * the OOM victim from depleting the memory reserves from the user
@@ -609,7 +609,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 	}
 	rcu_read_unlock();
 
-	mmput(mm);
+	mmdrop(mm);
 	put_task_struct(victim);
 }
 #undef K
-- 
2.4.3