From 31a78f23bac0069004e69f98808b6988baccb6b6 Mon Sep 17 00:00:00 2001 From: Balbir Singh Date: Sun, 28 Sep 2008 23:09:31 +0100 Subject: mm owner: fix race between swapoff and exit There's a race between mm->owner assignment and swapoff, more easily seen when task slab poisoning is turned on. The condition occurs when try_to_unuse() runs in parallel with an exiting task. A similar race can occur with callers of get_task_mm(), such as /proc// or ptrace or page migration. CPU0 CPU1 try_to_unuse looks at mm = task0->mm increments mm->mm_users task 0 exits mm->owner needs to be updated, but no new owner is found (mm_users > 1, but no other task has task->mm = task0->mm) mm_update_next_owner() leaves mmput(mm) decrements mm->mm_users task0 freed dereferencing mm->owner fails The fix is to notify the subsystem via mm_owner_changed callback(), if no new owner is found, by specifying the new task as NULL. Jiri Slaby: mm->owner was set to NULL prior to calling cgroup_mm_owner_callbacks(), but must be set after that, so as not to pass NULL as old owner causing oops. Daisuke Nishimura: mm_update_next_owner() may set mm->owner to NULL, but mem_cgroup_from_task() and its callers need to take account of this situation to avoid oops. Hugh Dickins: Lockdep warning and hang below exec_mmap() when testing these patches. exit_mm() up_reads mmap_sem before calling mm_update_next_owner(), so exec_mmap() now needs to do the same. And with that repositioning, there's now no point in mm_need_new_owner() allowing for NULL mm. Reported-by: Hugh Dickins Signed-off-by: Balbir Singh Signed-off-by: Jiri Slaby Signed-off-by: Daisuke Nishimura Signed-off-by: Hugh Dickins Cc: KAMEZAWA Hiroyuki Cc: Paul Menage Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/exec.c | 2 +- kernel/cgroup.c | 5 +++-- kernel/exit.c | 12 ++++++++++-- mm/memcontrol.c | 17 +++++++++++++++++ 4 files changed, 31 insertions(+), 5 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index 32993beecbe..cecee501ce7 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -752,11 +752,11 @@ static int exec_mmap(struct mm_struct *mm) tsk->active_mm = mm; activate_mm(active_mm, mm); task_unlock(tsk); - mm_update_next_owner(old_mm); arch_pick_mmap_layout(mm); if (old_mm) { up_read(&old_mm->mmap_sem); BUG_ON(active_mm != old_mm); + mm_update_next_owner(old_mm); mmput(old_mm); return 0; } diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 13932abde15..a0123d75ec9 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -2738,14 +2738,15 @@ void cgroup_fork_callbacks(struct task_struct *child) */ void cgroup_mm_owner_callbacks(struct task_struct *old, struct task_struct *new) { - struct cgroup *oldcgrp, *newcgrp; + struct cgroup *oldcgrp, *newcgrp = NULL; if (need_mm_owner_callback) { int i; for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) { struct cgroup_subsys *ss = subsys[i]; oldcgrp = task_cgroup(old, ss->subsys_id); - newcgrp = task_cgroup(new, ss->subsys_id); + if (new) + newcgrp = task_cgroup(new, ss->subsys_id); if (oldcgrp == newcgrp) continue; if (ss->mm_owner_changed) diff --git a/kernel/exit.c b/kernel/exit.c index 16395644a98..85a83c83185 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -583,8 +583,6 @@ mm_need_new_owner(struct mm_struct *mm, struct task_struct *p) * If there are other users of the mm and the owner (us) is exiting * we need to find a new owner to take on the responsibility. */ - if (!mm) - return 0; if (atomic_read(&mm->mm_users) <= 1) return 0; if (mm->owner != p) @@ -627,6 +625,16 @@ retry: } while_each_thread(g, c); read_unlock(&tasklist_lock); + /* + * We found no owner yet mm_users > 1: this implies that we are + * most likely racing with swapoff (try_to_unuse()) or /proc or + * ptrace or page migration (get_task_mm()). Mark owner as NULL, + * so that subsystems can understand the callback and take action. + */ + down_write(&mm->mmap_sem); + cgroup_mm_owner_callbacks(mm->owner, NULL); + mm->owner = NULL; + up_write(&mm->mmap_sem); return; assign_new_owner: diff --git a/mm/memcontrol.c b/mm/memcontrol.c index c0500e4d3a2..36896f3eb7f 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -250,6 +250,14 @@ static struct mem_cgroup *mem_cgroup_from_cont(struct cgroup *cont) struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p) { + /* + * mm_update_next_owner() may clear mm->owner to NULL + * if it races with swapoff, page migration, etc. + * So this can be called with p == NULL. + */ + if (unlikely(!p)) + return NULL; + return container_of(task_subsys_state(p, mem_cgroup_subsys_id), struct mem_cgroup, css); } @@ -549,6 +557,11 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm, if (likely(!memcg)) { rcu_read_lock(); mem = mem_cgroup_from_task(rcu_dereference(mm->owner)); + if (unlikely(!mem)) { + rcu_read_unlock(); + kmem_cache_free(page_cgroup_cache, pc); + return 0; + } /* * For every charge from the cgroup, increment reference count */ @@ -801,6 +814,10 @@ int mem_cgroup_shrink_usage(struct mm_struct *mm, gfp_t gfp_mask) rcu_read_lock(); mem = mem_cgroup_from_task(rcu_dereference(mm->owner)); + if (unlikely(!mem)) { + rcu_read_unlock(); + return 0; + } css_get(&mem->css); rcu_read_unlock(); -- cgit v1.2.3