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

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Date:	Mon, 24 May 2010 09:28:54 +0200
From:	Michael Kerrisk <mtk.manpages@...glemail.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	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 at 9:04 AM, Jens Axboe <jens.axboe@...cle.com> wrote:
> 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?

Actually, SO_*BUF is pretty weird. It returns double what was
supplied. It's not simply a matter of rounding up: it always doubles
what was supplied.



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface" http://blog.man7.org/
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