question: why use vzalloc() and vzfree() in mem_cgroup_alloc() and mem_cgroup_free()

[Laurent Chavey <chavey@xxxxxxxxxx>]

context:

While testing patches from Glauber Costa, "adding support

for tcp memory allocation in kmem cgroup", we hit a

BUG_ON(in_interrupt()) in vfree(). The code path in question

is taken because the izeof(struct mem_cgroup) is
>= PAGE_SIZE in the call to mem_cgroup_free(), 

Since socket may get free in an interrupt context,

the combination of vzalloc(), vfree() should not be used

when accounting for socket mem (unless the code is modified).

question:

Is there reasons why vzalloc() is used in mem_cgroup_alloc() ?

    . are we seeing mem fragmentations to level that fail

      kzalloc() or kmalloc(). 

    . do we have empirical data that shows the allocation failure

      rate for kmalloc(), kzalloc() per alloc size (num pages)