On Fri 22-07-22 19:45:39, Tetsuo Handa wrote: > 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. OK, I have misunderstood your previous wording and now I have realized that there are 2 issues here. One of them is a (less likely) dead lock on the oom_lock not making a fwd progress (that would require global OOM racing with memcg OOM) and the other is the GFP_NOFS->GFP_KERNEL dependency which can deadlock even without global the above race. Correct? Sorry I could have realized that sooner. > 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 -- Michal Hocko SUSE Labs
