* Vlastimil Babka:

preadv2(RWF_NOWAIT) can be used to open a side-channel to pagecache
contents, as it reveals metadata about residency of pages in
pagecache.

If preadv2(RWF_NOWAIT) returns immediately, it provides a clear "page
not resident" information, and vice versa.

Close that sidechannel by always initiating readahead on the cache if
we encounter a cache miss for preadv2(RWF_NOWAIT); with that in place,
probing the pagecache residency itself will actually populate the
cache, making the sidechannel useless.

I think this needs to use a different flag because the semantics are so
much different.  If I understand this change correctly, previously,
RWF_NOWAIT essentially avoided any I/O, and now it does not.

Thanks,
Florian