Since r212650 and before this change sendfile(2) could produce
a partially valid page for a trailing portion of a ZFS vnode.
vm_fault() always wants to see a fully valid page even if it's the last
page that partially extends beyond vnode's end. Otherwise it calls
vop_getpages() to bring in the page. In the case of ZFS this means
that the data is read from the page into the same page and this breaks
checks in ZFS mappedread() - a thread that set VPO_BUSY on the page in
vm_fault() will get blocked forever waiting for it to be cleared.
Many thanks to Kai and Jeremy for reproducing the issue and providing
important debugging information and help.
Reported by: Kai Gallasch <gallasch@free.de>,
Jeremy Chadwick <freebsd@jdc.parodius.com>
Tested by: Kai Gallasch <gallasch@free.de>,
Jeremy Chadwick <freebsd@jdc.parodius.com>
Reviewed by: kib
MFC after: 3 days
To-Do: apply the same treatment to tmpfs + sendfile
f6bb41924c