Until 2.6.19, __vmalloc with GFP_ATOMIC was possible, but __get_vm_area_node
would allocate the node itself with GFP_KERNEL, causing a warning. In 2.6.19,
this was "fixed" by using the same flags that were passed to __vmalloc also
in __get_vm_area_node. However, __get_vm_area_node does
BUG_ON(in_interrupt()) now, since vmlist_lock is obtained without disabling
bottom-half's. The patch below uses bh disabled lock for vmlist_lock, so that
__vmalloc can be used in interrupt context.
In 2.6.21, __vmalloc with GFP_ATOMIC is used by arch/um/kernel/process.c;
__vmalloc is also used in ntfs, xfs, but it is not clear to me if they use it
with GFP_ATOMIC or GFP_KERNEL.