redis bgsave failed because fork Cannot allocate memory

all: here is my server memory info with 'free -m'

              total       used       free     shared    buffers     cached
 Mem:         64433       49259      15174          0          3         31
 -/+ buffers/cache:      49224      15209
 Swap:         8197        184       8012

my redis-server has used 46G memory, there is almost 15G memory left free

As my knowledge,fork is copy on write, it should not failed when there has 15G free memory,which is enough to malloc necessary kernel structures .

besides, when redis-server used 42G memory, bgsave is ok and fork is ok too.

Is there any vm parameter I can tune to make fork return success ?