xenpaging: handle evict failures

xenpaging: handle evict failures

Evict of a nominated gfn must fail if some other process mapped the
page without checking the p2mt of that gfn first.
Add a check to cancel eviction if the page usage count is not 1.

Handle the possible eviction failure in the page-in paths.
After nominate and before evict, something may check the p2mt and call
populate. Handle this case and let the gfn enter the page-in path. The
gfn may still be connected to a mfn, so there is no need to allocate a
new page in prep.

Adjust do_mmu_update to return -ENOENT only if the gfn has entered the
page-in path and if it is not yet connected to a mfn. Otherwise
linux_privcmd_map_foreign_bulk() may loop forever.

Add MEM_EVENT_FLAG_EVICT_FAIL to inform pager that a page-in request for
a possible not-evicted page was sent. xenpaging does currently not need
that flag because failure to evict a gfn will be caught.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>