On Thu, 13 Nov 2008, Eric Dumazet wrote:

So yes, this bit should be documented, since SLAB_DESTROY_BY_RCU is
not really used in linux kernel at this moment.

It is used for the anonymous vmas. That is the purpose that Hugh
introduced it for since he saw regression if he would use straight rcu
freeing. See mm/rmap.c. SLAB_DESTROY_BY_RCU a pretty strange way of
using RCU and slab so it should always be documented in detail.