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

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Message-ID: <20100524070552.GR23411@kernel.dk>
Date:	Mon, 24 May 2010 09:05:52 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	mtk.manpages@...il.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

> 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).

> 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!

-- 
Jens Axboe

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