Michal Hocko wrote: > > But if we consider non system-wide OOM events, it is not very unlikely to > > hit > > this race. This queue is useful for situations where memcg1 and memcg2 hit > > memcg OOM at the same time and victim1 in memcg1 cannot terminate > > immediately. > > This can happen of course but the likelihood is _much_ smaller without > the global OOM because the memcg OOM killer is invoked from a lockless > context so the oom context cannot block the victim to proceed.
Suppose mem_cgroup_out_of_memory() is called from a lockless context via mem_cgroup_oom_synchronize() called from pagefault_out_of_memory(), that "lockless" is talking about only current thread, doesn't it? Since oom_kill_process() sets TIF_MEMDIE on first mm!=NULL thread of a victim process, it is possible that non-first mm!=NULL thread triggers pagefault_out_of_memory() and first mm!=NULL thread gets TIF_MEMDIE, isn't it? Then, where is the guarantee that victim1 (first mm!=NULL thread in memcg1 which got TIF_MEMDIE) is not waiting at down_read(&victim2->mm->mmap_sem) when victim2 (first mm!=NULL thread in memcg2 which got TIF_MEMDIE) is waiting at down_write(&victim2->mm->mmap_sem) or both victim1 and victim2 are waiting on a lock somewhere in memory reclaim path (e.g. mutex_lock(&inode->i_mutex))?
