linux-kernel - Re: [PATCH v2] Introduce a version6 of autofs interface, to fix design error.

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Date:	Fri, 27 Apr 2012 16:27:17 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	David Miller <davem@...emloft.net>, mjt@....msk.ru,
	linux-kernel@...r.kernel.org, autofs@...r.kernel.org,
	raven@...maw.net, thomas@...3r.de, stable@...nel.org
Subject: Re: [PATCH v2] Introduce a version6 of autofs interface, to fix
 design error.

On Fri, Apr 27, 2012 at 3:42 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>
> The obvious way to set the flag would seem to me to also btake O_DIRECT
> on the pipe as meaning this ?

Ok, that really does seem to be pretty natural.

The attached patch implements that, and makes autofs automatically set
the O_DIRECT flag. And now that it is exposed to user-space, here's a
test-program that uses it:

  [torvalds@i5 ~]$ cat packet.c
  #define _GNU_SOURCE
  #include <unistd.h>
  #include <fcntl.h>
  #include <stdio.h>
  #include <stdlib.h>

  #define REPORT(op,fd,size) do { 			\
  	int ret = op(fd, buffer, size);			\
  	fprintf(stderr, #op "(%d)=%d\n", size, ret);	\
  } while (0)

  #define WRITE(x) REPORT(write,fd[1],x)
  #define READ(x) REPORT(read,fd[0],x)

  int main(void)
  {
  	int fd[2];
  	static char buffer[1024];

  	if (pipe2(fd, O_DIRECT)) {
  		perror("pipe2 O_DIRECT");
  		exit(1);
  	}
  	WRITE(8);
  	WRITE(12);
  	WRITE(300);
  	WRITE(5);
  	READ(64);
  	READ(64);
  	WRITE(7);
  	READ(64);
  	READ(64);
  	READ(64);
  	return 0;
  }

which results in this output:

  [torvalds@i5 ~]$ ./a.out
  write(8)=8
  write(12)=12
  write(300)=300
  write(5)=5
  read(64)=8
  read(64)=12
  write(7)=7
  read(64)=64
  read(64)=5
  read(64)=7

iow, notice how all the read() calls return individual packets (and
the 300-byte packet was truncated to 64 bytes).

HOWEVER. I haven't actually tested the autofs part. But I suspect it
"should just work".

Thomas, Michael, can you test whether this patch (again: note the "+8"
in autofs_v5_packet_size() purely for debug purposes, so this would be
interesting to test for other people too) makes your setups work?

                           Linus

Download attachment "patch.diff" of type "application/octet-stream" (5162 bytes)