syzbot is reporting GFP_KERNEL allocation with oom_lock held when reporting memcg OOM [1]. Such allocation request might deadlock the system, for __alloc_pages_may_oom() cannot invoke global OOM killer due to oom_lock being already held by the caller. Fix this problem by removing the allocation from memory_stat_format() completely, and pass static buffer when calling from memcg OOM path. Link: https://syzkaller.appspot.com/bug?extid=2d2aeadc6ce1e1f11d45 [1] Reported-by: syzbot <syzbot+2d2aeadc6ce1e1f11d45@xxxxxxxxxxxxxxxxxxxxxxxxx> Suggested-by: Michal Hocko <mhocko@xxxxxxxx> Fixes: c8713d0b23123759 ("mm: memcontrol: dump memory.stat during cgroup OOM") Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> --- Changes in v3: Update patch description. Changes in v2: Use static buffer for OOM reporting, suggested by Michal Hocko <mhocko@xxxxxxxx>. mm/memcontrol.c | 22 +++++++++------------- 1 file changed, 9 insertions(+), 13 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 618c366a2f07..8092be2fbb7c 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1460,14 +1460,12 @@ static inline unsigned long memcg_page_state_output(struct mem_cgroup *memcg, return memcg_page_state(memcg, item) * memcg_page_state_unit(item); } -static char *memory_stat_format(struct mem_cgroup *memcg) +static void memory_stat_format(struct mem_cgroup *memcg, char *buf, int bufsize) { struct seq_buf s; int i; - seq_buf_init(&s, kmalloc(PAGE_SIZE, GFP_KERNEL), PAGE_SIZE); - if (!s.buffer) - return NULL; + seq_buf_init(&s, buf, bufsize); /* * Provide statistics on the state of the memory subsystem as @@ -1533,8 +1531,6 @@ static char *memory_stat_format(struct mem_cgroup *memcg) /* The above should easily fit into one page */ WARN_ON_ONCE(seq_buf_has_overflowed(&s)); - - return s.buffer; } #define K(x) ((x) << (PAGE_SHIFT-10)) @@ -1570,7 +1566,10 @@ void mem_cgroup_print_oom_context(struct mem_cgroup *memcg, struct task_struct * */ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg) { - char *buf; + /* Use static buffer, for the caller is holding oom_lock. */ + static char buf[PAGE_SIZE]; + + lockdep_assert_held(&oom_lock); pr_info("memory: usage %llukB, limit %llukB, failcnt %lu\n", K((u64)page_counter_read(&memcg->memory)), @@ -1591,11 +1590,8 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg) pr_info("Memory cgroup stats for "); pr_cont_cgroup_path(memcg->css.cgroup); pr_cont(":"); - buf = memory_stat_format(memcg); - if (!buf) - return; + memory_stat_format(memcg, buf, sizeof(buf)); pr_info("%s", buf); - kfree(buf); } /* @@ -6335,11 +6331,11 @@ static int memory_events_local_show(struct seq_file *m, void *v) static int memory_stat_show(struct seq_file *m, void *v) { struct mem_cgroup *memcg = mem_cgroup_from_seq(m); - char *buf; + char *buf = kmalloc(PAGE_SIZE, GFP_KERNEL); - buf = memory_stat_format(memcg); if (!buf) return -ENOMEM; + memory_stat_format(memcg, buf, PAGE_SIZE); seq_puts(m, buf); kfree(buf); return 0; -- 2.18.4
