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

[Jiri Kosina <jikos@kernel.org>]

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