sched: Clean up active_mm reference counting
mainline inclusion from mainline-5.4-rc1 commit 139d025cda1da5484e7287b35c019fe1dcf9b650 category: bugfix bugzilla: 28332 [preparation] CVE: NA ------------------------------------------------- The current active_mm reference counting is confusing and sub-optimal. Rewrite the code to explicitly consider the 4 separate cases: user -> user When switching between two user tasks, all we need to consider is switch_mm(). user -> kernel When switching from a user task to a kernel task (which doesn't have an associated mm) we retain the last mm in our active_mm. Increment a reference count on active_mm. kernel -> kernel When switching between kernel threads, all we need to do is pass along the active_mm reference. kernel -> user When switching between a kernel and user task, we must switch from the last active_mm to the next mm, hoping of course that these are the same. Decrement a reference on the active_mm. The code keeps a different order, because as you'll note, both 'to user' cases require switch_mm(). And where the old code would increment/decrement for the 'kernel -> kernel' case, the new code observes this is a neutral operation and avoids touching the reference count. Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: NRik van Riel <riel@surriel.com> Reviewed-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: luto@kernel.org Signed-off-by: NCheng Jian <cj.chengjian@huawei.com> Reviewed-By: NXie XiuQi <xiexiuqi@huawei.com> Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
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