On Thu, Jan 17, 2019 at 9:37 AM Matthew Wilcox [off-list ref] wrote:

Your patch 3/3 just removes the test.  Am I right in thinking that it
doesn't need to be *moved* because the existing test after !PageUptodate
catches it?

That's the _hope_.

That's the simplest patch I can come up with as a potential solution.
But it's possible that there's some nasty performance regression
because somebody really relies on not even triggering read-ahead, and
we might need to do some totally different thing.

So it may be that somebody has a case that really wants something
else, and we'd need to move the RWF_NOWAIT test elsewhere and do
something slightly more complicated. As with the mincore() change,
maybe reality doesn't like the simplest fix...

Of course, there aren't any tests for RWF_NOWAIT in xfstests.  Are there
any in LTP?

RWF_NOWAIT is actually _fairly_ new.  It was introduced "only" about
18 months ago and made it into 4.13.

Which makes me hopeful there aren't a lot of people who care deeply.

And starting readahead *may* actually be what a RWF_NOWAIT read user
generally wants, so for all we know it might even improve performance
and/or allow new uses. With the "start readahead but don't wait for
it" semantics, you can have a model where you try to handle all the
requests that can be handled out of cache first (using RWF_NOWAIT) and
then when you've run out of cached cases you clear the RWF_NOWAIT
flag, but now the IO has been started early (and could overlap with
the cached request handling), so then when you actually do a blocking
version, you get much better performance.

So there is an argument that removing that one RWF_NOWAIT case might
actually be a good thing in general, outside of  the "don't allow
probing the cache without changing the state of it" issue.

But that's handwavy and optimistic. Reality is often not as accomodating ;)

                   Linus