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

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Message-Id: <20090129141338.34e44a1f.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 29 Jan 2009 14:13:38 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Greg KH <greg@...ah.com>
Cc:	mtk.manpages@...il.com, linux-man@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: open(2) says O_DIRECT works on 512 byte boundries?

On Wed, 28 Jan 2009 13:33:22 -0800
Greg KH <greg@...ah.com> 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.)
> 

IIUC, it's not related to 512bytes boundary. Just a race between
direct-io v.s. copy-on-write. Copy-on-Write while reading a page via DIO
is a problem.

Maybe it's true that if buffer is aligned to page size, no copy-on-write will
happen in usual program. But assuming HugeTLB page, which does Copy-on-Write,
data corruption will happen again. HugeTLB aligned buffer is nonsense.

Thanks,
-Kame


> 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.
> 
> thanks,
> 
> greg k-h
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