I've at least identified two possible memory leaks in blkback, both
related to the shutdown path of a VBD:
- blkback doesn't wait for any pending purge work to finish before
cleaning the list of free_pages. The purge work will call
put_free_pages and thus we might end up with pages being added to
the free_pages list after we have emptied it. Fix this by making
sure there's no pending purge work before exiting
xen_blkif_schedule, and moving the free_page cleanup code to
xen_blkif_free.
- blkback doesn't wait for pending requests to end before cleaning
persistent grants and the list of free_pages. Again this can add
pages to the free_pages list or persistent grants to the
persistent_gnts red-black tree. Fixed by moving the persistent
grants and free_pages cleanup code to xen_blkif_free.
Also, add some checks in xen_blkif_free to make sure we are cleaning
everything.