* Jiri Kosina:

On Wed, 30 Jan 2019, Florian Weimer wrote:

quoted

quoted

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.

But it wouldn't schedule asynchronous readahead before?

I'm worried that something, say PostgreSQL doing a sequential scan,
would implement a two-pass approach, first using RWF_NOWAIT to process
what's in the kernel page cache, and then read the rest without it.  If
RWF_NOWAIT is treated as a prefetch hint, there could be much more read
activity, and a lot of it would be pointless because the data might have
to be evicted before userspace can use it.

Thanks,
Florian