linux-kernel - Re: [PATCH] vmsplice: relax alignement requirements for SPLICE_F

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Message-ID: <20120420081953.GJ7505@kernel.dk>
Date:	Fri, 20 Apr 2012 10:19:54 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Changli Gao <xiaosuo@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <mszeredi@...e.cz>
Subject: Re: [PATCH] vmsplice: relax alignement requirements for
 SPLICE_F_GIFT

On Wed, Apr 04 2012, Eric Dumazet wrote:
> It seems there is no fundamental reason to limit vmsplice()
> SPLICE_F_GIFT to page aligned chunks.
> 
> All helpers are prepared to cope with offsets in page.
> 
> This limitation makes vmsplice() API very impractical in the zero-copy
> land.

Sorry for the slow reply on this one. It's been a long time since I
wrote that code... If I recall correctly, the reason it currently
requires alignment, is that a gifted page is no longer yours, by
definition. So the alignment check was to ensure that we gift full
pages. But if the caller screws that up, I guess it's the callers
problem and it's a shame to limit the API in this way.

I will apply it for 3.5.

-- 
Jens Axboe

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