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

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Date:	Mon, 15 Sep 2014 17:58:44 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Milosz Tanski <milosz@...in.com>
Cc:	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)

Hi, Milosz,

I CC'd Michael Kerrisk, in case he has any opinions on the matter.

Milosz Tanski <milosz@...in.com> writes:

> This patcheset introduces an ability to perform a non-blocking read from 
> regular files in buffered IO mode. This works by only for those filesystems
> that have data in the page cache.
>
> It does this by introducing new syscalls new syscalls readv2/writev2 and
> preadv2/pwritev2. These new syscalls behave like the network sendmsg, recvmsg
> syscalls that accept an extra flag argument (O_NONBLOCK).

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

More generally, do you expect the open(2) of a regular file with
O_NONBLOCK to perform the same way as a pipe, fifo, or device (namely,
that the open itself won't block)?  Should O_NONBLOCK affect writes to
regular files?  What do you think the return value from poll and friends
should be when a file is opened in this manner (probably not important,
as poll always returns data ready on regular files)?  Also consider
whether you want the O_NONBLOCK behaviour for mandatory file locks in
your use case (or any other, for that matter).  If you issue a read and
it returns -EAGAIN, should it be up to the application to kick off I/O
to ensure it makes progress?

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.

Cheers,
Jeff
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