To: linux-kernel@vger.kernel.org
From: Robert Hancock <hancockr@shaw.ca>
Subject: Re: open(2) says O_DIRECT works on 512 byte boundries?
Date: Wed, 28 Jan 2009 18:41:49 -0600
Message-ID: <4980FB4D.9090009@shaw.ca>
References: <20090128213322.GA15789@kroah.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Cc: linux-man@vger.kernel.org
User-Agent: Thunderbird 2.0.0.19 (Windows/20081209)
In-Reply-To: <20090128213322.GA15789@kroah.com>
Cc: linux-kernel@vger.kernel.org
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1105
Lines: 20

Greg KH wrote:
> In looking at open(2), it says that O_DIRECT works on 512 byte boundries
> with the 2.6 kernel release:
> 	Under Linux 2.4, transfer sizes, and the alignment of the user
> 	buffer and  the file offset must all be multiples of the logical
> 	block size of the file system.  Under Linux 2.6, alignment  to
> 	512-byte  boundaries suffices.
> 
> However if you try to access an O_DIRECT opened file with a buffer that
> is PAGE_SIZE aligned + 512 bytes, it fails in a bad way (wrong data is
> read.)
> 
> Is this just a mistake in the documentation?  Or am I reading it
> incorrectly?
> 
> I have a test program that shows this if anyone wants it.

Well, it sounds like a bug to me.. even if it's not supported, if you do 
such an access, surely the kernel should detect that and return EINVAL 
or something rather than reading corrupted data..

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/