On Wed, 30 Jan 2019, Florian Weimer wrote: > > 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. It still avoid synchronous I/O, due to this code still being in place: if (!PageUptodate(page)) { if (iocb->ki_flags & IOCB_NOWAIT) { put_page(page); goto would_block; } but goes the would_block path only after initiating asynchronous readahead. -- Jiri Kosina SUSE Labs