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

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Message-ID: <20100524070416.GQ23411@kernel.dk>
Date:	Mon, 24 May 2010 09:04:16 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	mtk.manpages@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] pipe: add support for shrinking and growing pipes

On Mon, May 24 2010, OGAWA Hirofumi wrote:
> Jens Axboe <jens.axboe@...cle.com> writes:
> 
> >> > We can easily make F_GETPIPE_SZ return bytes, but I don't think passing
> >> > in bytes to F_SETPIPE_SZ makes a lot of sense. The pipe array must be a
> >> > power of 2 in pages. So the question is if that makes the API cleaner,
> >> > passing in number of pages but returning bytes? Or pass in bytes all
> >> > around, but have F_SETPIPE_SZ round to the nearest multiple of pow2 in
> >> > pages if need be. Then it would return a size at least what was passed
> >> > in, or error.
> 
> I really think "power of 2 in pages" is simply current implementation
> detail, not detail of pipe API.

Completely agree, one more reason more to make that dependency exposed
in the API.

> >> I'd recommend this: Pass it in and out in bytes. Don't round to a
> >> power of 2. Require the user to know what they are doing. Give an
> >> error if the user doesn't supply a power-of-2 * page-size for
> >> F_SETPIPE_SZ. (Again, consider the case of architectures  with
> >> switchable page sizes.)
> >
> > But is there much point in erroring on an incorrect size? If the
> > application says "I need at least 120kb of space in there", kernel
> > returns "OK, you got 128kb". Would returning -1/EINVAL for that case
> > really make a better API? Doesn't seem like it to me.
> 
> FWIW, my first impression of this was setsockopt(SO_RCV/SNDBUF) of unix
> socket. Well, API itself wouldn't say "at least this size" or "exactly
> this size", so, in here, important thing is consistency of interfaces, I
> think. (And the both is sane API at least for me if those had
> consistency in the system.)
> 
> Well, so how about set/get in bytes, and kernel will set "at least
> specified size" actually like setsockopt(SO_RCV/SNDBUF)?

Isn't that pretty much what I described?

-- 
Jens Axboe

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