Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O

[Florian Weimer]

* 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