in_atomic() check in page_cache_get_speculative()

[Scott Wood]

I'm seeing the in_atomic() check in page_cache_get_speculative()
(linux/pagemap.h:138) fail when running e500 KVM with .  It's coming
from get_user_pages_fast(), from KVM's hva_to_pfn().  This is on kvm.git
plus a few local patches that should be completely unrelated, but it
looks like this code hasn't changed much in a couple years.

Interrupts are disabled by get_user_pages_fast(), but apparently
preemption was not separately disabled.  The comment in
page_cache_get_speculative() says that preemption disabling is done by
rcu_read_lock(), and that "this function must be called inside the same
rcu_read_lock() section as has been used to lookup the page in the
pagecache radix-tree (or page table)".

Where is this RCU lock supposed to be acquired?  I don't see any RCU in
arch/powerpc/mm/gup.c.  Is it buried in some macro or function call?

-Scott