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

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Message-ID: <x49bnqf3ciu.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 16 Sep 2014 09:44:57 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Milosz Tanski <milosz@...in.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-fsdevel\@vger.kernel.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)

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

> Jeff,
>
> This patchset creates a new read (readv2/preadv2) syscall(s) that take
> a extra flag argument (kind of like recvmsg). What it doesn't do is
> change the current behavior of of the O_NONBLOCK, if the file is
> open() with O_NONBLOCK flag. It shouldn't break any existing
> applications since you have to opt into using this by using the new
> syscall.

Hi, Milosz,

Ah, I misread one of the patches.  Now that I've applied the series, I
see that you're testing the flag argument, not the file open flags.

> I don't have a preference either way if we should create a new flag or
> re-use O_NONBLOCK the flag. Instead, I'm hoping to get some consensus
> here from senior kernel developers like yourself. Maybe a RWF_NONBLOCK
> (I'm stealing from eventfd, EFD_NONBLOCK).

I think I'd rather name the flag something other than O_NONBLOCK, if for
no other reason that to avoid confusion.

> As a side note, I noticed that EFD_NONBLOCK, SFD_NONBLOCK, etc... all
> alias to the value of O_NONBLOCK and there's a bunch of bug checks in
> the code like this:
> BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);

That's because the flag is passed on to anon_inode_getfile.  See also
this define:

#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)

A general note on your subjects -- you should make them more specific to
the subsystem you're updating.  Commit titles like "documentation
update" are a bit too broad.  ;-)

Cheers,
Jeff
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