On Fri 22-07-22 09:46:27, Tetsuo Handa wrote: > syzbot is reporting GFP_KERNEL allocation with oom_lock held [1]. We > must make sure that such allocation won't hit __alloc_pages_may_oom() > path which will retry forever if oom_lock is already held. Use static > buffer when oom_lock is already held. The changelog is rather cryptic. Your previous one was more readable. I would go with: " syzbot is reporting GFP_KERNEL allocation with oom_lock held [1] when reporting memcg oom. This is problematic because this creates a dependency between GFP_NOFS and GFP_KERNEL over oom_lock which could dead lock the system. Fix the problem by removing the allocation from memory_stat_format completely. Use a statically preallocated buffer instead for this path. OOM dumping is synchronized by the oom_lock so there is no exclusion required here. memory_stat_show can use GFP_KERNEL allocation. " > 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> Thanks! > --- > 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
