From: "H. Peter Anvin" <hpa@zytor.com>
To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Michael Tokarev <mjt@tls.msk.ru>,
Alan Cox <alan@lxorguk.ukuu.org.uk>, Ian Kent <raven@themaw.net>,
Thomas Meyer <thomas@m3y3r.de>,
autofs@vger.kernel.org
Subject: Re: autofs: make the autofsv5 packet file descriptor use a packetized pipe
Date: Sun, 29 Apr 2012 17:15:16 -0700 [thread overview]
Message-ID: <4F9DD994.70202@zytor.com> (raw)
In-Reply-To: <20120429205429.63CCD7C0064@ra.kernel.org>
On 04/29/2012 01:54 PM, Linux Kernel Mailing List wrote:
>
> With both automount and systemd doing a single read() system call, and
> verifying that they get *exactly* the size they expect but using
> different sizes, it seemed that fixing one of them inevitably seemed to
> break the other. At one point, a patch I seriously considered applying
> from Michael Tokarev did a "strcmp()" to see if it was automount that
> was doing the operation. Ugly, ugly.
>
> However, a prettier solution exists now thanks to the packetized pipe
> mode. By marking the communication pipe as being packetized (by simply
> setting the O_DIRECT flag), we can always just write the bigger packet
> size, and if user-space does a smaller read, it will just get that
> partial end result and the extra alignment padding will simply be thrown
> away.
>
> This makes both automount and systemd happy, since they now get the size
> they asked for, and the kernel side of autofs simply no longer needs to
> care - it could pad out the packet arbitrarily.
>
> Of course, if there is some *other* user of autofs (please, please,
> please tell me it ain't so - and we haven't heard of any) that tries to
> read the packets with multiple writes, that other user will now be
> broken - the whole point of the packetized mode is that one system call
> gets exactly one packet, and you cannot read a packet in pieces.
>
I just looked at am-utils; am-utils *does* use autofs v5, and *will*
loop back and read more data on a short read.
-hpa
--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.
next parent reply other threads:[~2012-04-30 0:16 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20120429205429.63CCD7C0064@ra.kernel.org>
2012-04-30 0:15 ` H. Peter Anvin [this message]
2012-04-30 0:25 ` autofs: make the autofsv5 packet file descriptor use a packetized pipe Linus Torvalds
2012-04-30 0:28 ` H. Peter Anvin
2012-04-30 0:33 ` Linus Torvalds
2012-04-30 0:35 ` H. Peter Anvin
2012-04-30 0:43 ` Linus Torvalds
2012-04-30 0:45 ` H. Peter Anvin
2012-04-30 1:29 ` Ian Kent
2012-04-30 1:56 ` Linus Torvalds
2012-04-30 5:57 ` Ian Kent
2012-04-30 0:30 ` H. Peter Anvin
2012-04-30 0:37 ` Linus Torvalds
2012-04-30 20:03 ` H. Peter Anvin
2012-04-30 20:11 ` Linus Torvalds
2012-04-30 20:14 ` H. Peter Anvin
2012-04-30 6:27 ` Michael Tokarev
2012-04-30 6:43 ` Michael Tokarev
2012-04-30 6:48 ` H. Peter Anvin
2012-04-30 6:55 ` Ian Kent
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