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

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Message-ID: <AANLkTikogQtGpO9c94A7YSmP8zdyM8dZBrwJf1nwlR4j@mail.gmail.com>
Date:	Mon, 24 May 2010 09:27:12 +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 9:05 AM, Jens Axboe <jens.axboe@...cle.com> wrote:
>> The "at least" idea makes sense. So, I'd change my recommendation to:
>>
>> Pass the buffer size in and out in bytes (for consistency with other
>> APIs). Round the input (F_SETPIPE_SZ) value up as required by the
>> implementation. For the output (F_GETPIPE_SZ) value do one of the
>> following:
>> a) Return the value given on input.
>> b) Return the rounded up value actually used by the kernel.
>>
>> I suspect (b) might be more useful: if an application cares enough
>> about pipe size to want to change it, then at least some such
>> applications might care to know exactly the size that the kernel used.
>> (And: I can't see any downside to (b).)
>
> b definitely, since it's the real size (plus then we don't have to track
> the passed in size).

Okay.

>> One other comment about the interface. We have
>>
>>                 if (!capable(CAP_SYS_ADMIN) && arg > pipe_max_pages)
>>                         return -EINVAL;
>>
>> The usual error on a capability denied is EPERM. Please change.
>
> 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@...r on
this patche (and all patches that change the API/ABI.)

Cheers,

Michael


-- 
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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