linux-kernel - Re: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only)

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Message-ID: <20140919112355.GB4639@infradead.org>
Date:	Fri, 19 Sep 2014 04:23:55 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Milosz Tanski <milosz@...in.com>, linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
	Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>, michael.kerrisk@...il.com
Subject: Re: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only)

On Mon, Sep 15, 2014 at 05:58:44PM -0400, Jeff Moyer wrote:
> I thought you were going to introduce a new flag instead of using
> O_NONBLOCK for this.  I dug up an old email that suggested that enabling
> O_NONBLOCK for regular files (well, a device node in this case) broke a
> cd ripping or burning application.  I also found this old bugzilla,
> which states that squid would fail to start, and that gqview was also
> broken:
>   https://bugzilla.redhat.com/show_bug.cgi?id=136057

That is why we avoid looking a the per-open O_NONBLOCK flag, and only
apply it per I/O.  As mentioned in my last mail it's not quite as
trivial but still fairly easy to also do that for writes.

> I don't think O_NONBLOCK is the right flag.  What you're really
> specifying is a flag that prevents I/O in the read path, and nowhere
> else.  As such, I'd feel much better about this if we defined a new flag
> (O_NONBLOCK_READ maybe?  No, that's too verbose.).
> 
> In summary, I like the idea, but I worry about overloading O_NONBLOCK.

There's a fair argument we could use a different namespace for the
per-I/O ops, and it seems like Miklos already implemented this for the
next version.
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