linux-kernel - Re: open(2) says O_DIRECT works on 512 byte boundries?

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Date:	Wed, 28 Jan 2009 18:41:49 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	linux-kernel@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject:  Re: open(2) says O_DIRECT works on 512 byte boundries?

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..

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