Re: [patch] pipe: add support for shrinking and growing pipes

[Jens Axboe <axboe@kernel.dk>]

On Tue, May 25 2010, Michael Kerrisk wrote:
> On Mon, May 24, 2010 at 7:56 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
> > On Mon, May 24 2010, Michael Kerrisk wrote:
> >> On Mon, May 24, 2010 at 7:35 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
> >> > On Mon, May 24 2010, Michael Kerrisk wrote:
> >> >> > Right, that looks like a thinko.
> >> >> >
> >> >> > I'll submit a patch changing it to bytes and the agreed API and fix this
> >> >> > -Eerror. Thanks for your comments and suggestions!
> >> >>
> >> >> Thanks. And of course you are welcome. (Please CC linux-api@vger on
> >> >> this patche (and all patches that change the API/ABI.)
> >> >
> >> > The first change is this:
> >> >
> >> > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=0191f8697bbdfefcd36e7b8dc3eeddfe82893e4b
> >> >
> >> > and the one dealing with the pages vs bytes API is this:
> >> >
> >> > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=b9598db3401282bb27b4aef77e3eee12015f7f29
> >> >
> >> > Not tested yet, will do so before sending in of course.
> >>
> >> Eyeballing it quickly, these changes look right.
> >
> > Good, thanks.
> >
> >> Do you have some test programs you can make available?
> >
> > Actually I don't, I test it by modifying fio's splice engine to set/get
> > the pipe size and test the resulting transfers.
> 
> Two more questions: is the rationale for this feature written up
> somewhere? I could not find it. Is it primarily intended for
> splice/vmsplice/tee, with the effect for pipe(2) being a side effect?

Yes it's primarily for splice, where the 64kb size can sometimes become
a limiting factor because of the pipe mutex lock/unlocking.

> Also, the minuimum size of the buffer is 2 pages. Why is it not 1?
> (Notwithstanding Linus's assertion, a buffer size of 1 page did give
> us POSIX compliance in kernels before 2.6.10.)

I'll defer to Linus on that, I remember some emails on that part from
way back when. As far as I can tell, POSIX wants atomic writes of "less
than a page size", which would make more sense as "of a page size and
less". And since it should not be a page size from either side on a
uni-directional pipe, then 1 page seems enough for that guarantee at
least.

-- 
Jens Axboe