linux-kernel - Re: autofs: make the autofsv5 packet file descriptor use a packetized pipe

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Message-ID: <4F9DD994.70202@zytor.com>
Date:	Sun, 29 Apr 2012 17:15:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michael Tokarev <mjt@....msk.ru>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ian Kent <raven@...maw.net>, Thomas Meyer <thomas@...3r.de>,
	autofs@...r.kernel.org
Subject: Re: autofs: make the autofsv5 packet file descriptor use a packetized
 pipe

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.

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