On Fri, Mar 27, 2015 at 09:30:46AM -0700, Andrew Morton wrote:

But from an interface perspective the behaviour you're asking for is
insane, frankly - if the kernel copied out 8k of data then pread2()
should return 8k.  Otherwise there's no way for userspace to know that
the 8k copy actually happened and we have just wasted a great pile of
CPU doing a pointless memcpy.

Why would it do the copy in the first place if we asked (for example)
for 16k, but only 8k was available ? Just return EAGAIN and have
done with it.

I expect that this situation (first part in cache, latter part not in
cache) is rare - for reasonably small requests the common cases will be
"all cached" and "nothing cached".  So perhaps the best approach here
is for samba to add special handling for the short read, to work out
the reason for its occurrence.

We can do that, but as Volker says this is a very hot code path.

I take it from your comments that nobody has actually wired up pread2()
into samba yet?  That's a bit disturbing, because if we later want to
go and change something like this short-read behaviour, we're screwed -
it's a non back-compat userspace-visible change.

It's been done as a test, so the code exists and has run (and improved
perforamance as I recall). Not much point commiting it without kernel
support :-).

And a note on cosmetics: why are we using EAGAIN here rather than
EWOULDBLOCK?  They have the same numerical value, but EWOULDBLOCK is a
better name - EAGAIN says "run it again", but that won't work.

Sounds good to me !

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