linux-ext4 - 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: <20160426081451.GA25616@infradead.org>
Date:	Tue, 26 Apr 2016 01:14:51 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Dave Chinner <david@...morbit.com>, torvalds@...ux-foundation.org
Cc:	Dmitry Monakhov <dmonlist@...il.com>,
	Theodore Ts'o <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org
Subject: O_DIRECT as a hint, was: Re: [PATCH] ext4: refuse O_DIRECT opens for
 mode where DIO doesn't work

On Tue, Apr 26, 2016 at 09:49:46AM +1000, Dave Chinner wrote:
> Why not just transparently fall back to buffered IO if direct IO
> cannot be done? Saves people from wondering why applications fail
> on one ext4 filesystem and not another....

I've been doing an audit of our direct I/O implementations, and most
of them does some form of transparent fallback, including some that
only pretend to support O_DIRECT, but do anything special for it at all,
while at the same time we go through greast efforts to check a file
system actualy supports direct I/O, leading to nasty no-op ->direct_IO
implementations as we even got that abstraction wrong.

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.
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