On Sat, Jan 5, 2019 at 6:27 PM Jiri Kosina [off-list ref] wrote:

There are possibilities [1] how mincore() could be used as a converyor of
a sidechannel information about pagecache metadata.

Provide vm.mincore_privileged sysctl, which makes it possible to mincore()
start returning -EPERM in case it's invoked by a process lacking
CAP_SYS_ADMIN.

The default behavior stays "mincore() can be used by anybody" in order to
be conservative with respect to userspace behavior.

[1] https://www.theregister.co.uk/2019/01/05/boffins_beat_page_cache/

Just checking: I guess /proc/$pid/pagemap (iow, the pagemap_read()
handler) is less problematic because it only returns data about the
state of page tables, and doesn't query the address_space? In other
words, it permits monitoring evictions, but non-intrusively detecting
that something has been loaded into memory by another process is
harder?