linux-ext4 - Re: O_DIRECT as a hint, was: Re: [PATCH] ext4: refuse O_DIRECT opens for mode where DIO doesn't work

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Message-ID: <20160427032526.GC30021@thunk.org>
Date:	Tue, 26 Apr 2016 23:25:26 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Dave Chinner <david@...morbit.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	torvalds@...ux-foundation.org,
	Dmitry Monakhov <dmonlist@...il.com>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: O_DIRECT as a hint, was: Re: [PATCH] ext4: refuse O_DIRECT opens
 for mode where DIO doesn't work

On Wed, Apr 27, 2016 at 12:27:46PM +1000, Dave Chinner wrote:
> > At this point I wonder if we should simply treat O_DIRECT as a hint
> > and always allow it, and just let the file system optimize for it
> > (skip buffering, require alignment, relaxed Posix atomicy requirements)
> > if it is set.
> 
> I thought that's how most filesystems treated it, anyway. i.e.
> anything they can't do via direct IO, they fell back to buffered IO
> to complete (e.g. for allocation or append writes, etc). Hence why I
> suggested the fallback rather than erroring out....

No, some file systems return EINVAL on the open.  In fact that's what
the _require_odirect test in xfstests relies upon....

    		     	     	      	     - Ted
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