2.4 bug: fifo-write causes diskwrites to read-only fs !

2.4 bug: fifo-write causes diskwrites to read-only fs !

[Rob van Nieuwkerk]

Hi all,

It turns out that Linux is updating inode timestamps of fifos (named
pipes) that are written to while residing on a read-only filesystem.
It is not only updating in-ram info, but it will issue *physical*
writes to the read-only fs on the disk !

I use a CompactFlash in an embedded application with a read-only root-fs
on it.  There are several processes that communicate with each other
via fifos.  This bug in Linux causes frequent writes to my CF and will
shorten it's lifetime enormously ..

I've posted a report on the "mysterious writes" before:
( http://www.ussg.iu.edu/hypermail/linux/kernel/0303.2/1753.html )
(incorrectly) linking it to a possible bug in O_SYNC.  Nothing came out
of it.

But now I've completely tracked down the bug (logging all diskaccesses
and seeing it undoubtly write in disksectors containing time-stamp
info of fifo's).  Looking back it would have been easier to prove that
something is wrong: the modified time-stamps survive power-cycles.
This is not supposed to happen on a read-only fs.

I've tried reading the kernel source to find where the bug lives,
But I'm not too familiar with it.  Anyone out there who can
pin it down ?

	greetings,
	Rob van Nieuwkerk


Sysinfo:
--------
- various 2.4 kernels including RH-2.4.20-13.9,
  but also straight 2.4(ac) ones.
- CompactFlash (= IDE disk)
- Geode GX1 CPU (i586 compatible)

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Rob van Nieuwkerk]

I wrote:
> It turns out that Linux is updating inode timestamps of fifos (named
> pipes) that are written to while residing on a read-only filesystem.
> It is not only updating in-ram info, but it will issue *physical*
> writes to the read-only fs on the disk !
	.
	.
	.
> Sysinfo:
> --------
> - various 2.4 kernels including RH-2.4.20-13.9,
>   but also straight 2.4(ac) ones.
> - CompactFlash (= IDE disk)
> - Geode GX1 CPU (i586 compatible)

Forgot to mention: I use an ext2 fs, but maybe it's a generic,
fs-independant problem.

	greetings,
	Rob van Nieuwkerk

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Richard B. Johnson]

On Wed, 28 May 2003, Rob van Nieuwkerk wrote:

>
> I wrote:
> > It turns out that Linux is updating inode timestamps of fifos (named
> > pipes) that are written to while residing on a read-only filesystem.
> > It is not only updating in-ram info, but it will issue *physical*
> > writes to the read-only fs on the disk !
> 	.
> 	.
> 	.
> > Sysinfo:
> > --------
> > - various 2.4 kernels including RH-2.4.20-13.9,
> >   but also straight 2.4(ac) ones.
> > - CompactFlash (= IDE disk)
> > - Geode GX1 CPU (i586 compatible)
>
> Forgot to mention: I use an ext2 fs, but maybe it's a generic,
> fs-independant problem.
>
> 	greetings,
> 	Rob van Nieuwkerk

How does it 'know' it's a R/O file-system? Have you mounted it
R/O, mounted it noatime, or just taken whatever you get when
you boot from a ramdisk?

FYI, I created a FIFO with mkfifo, remounted the file-system
R/O, executed `cat` with it's input coming from the FIFO, and
then waited for a few minutes. I then wrote to the FIFO.
The atime did not change with 2.4.20.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.20 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Rob van Nieuwkerk]

> On Wed, 28 May 2003, Rob van Nieuwkerk wrote:
> 
> >
> > I wrote:
> > > It turns out that Linux is updating inode timestamps of fifos (named
> > > pipes) that are written to while residing on a read-only filesystem.
> > > It is not only updating in-ram info, but it will issue *physical*
> > > writes to the read-only fs on the disk !
> > 	.
> > 	.
> > 	.
> > > Sysinfo:
> > > --------
> > > - various 2.4 kernels including RH-2.4.20-13.9,
> > >   but also straight 2.4(ac) ones.
> > > - CompactFlash (= IDE disk)
> > > - Geode GX1 CPU (i586 compatible)
> >
> > Forgot to mention: I use an ext2 fs, but maybe it's a generic,
> > fs-independant problem.
> >
> > 	greetings,
> > 	Rob van Nieuwkerk
> 
> How does it 'know' it's a R/O file-system? Have you mounted it
> R/O, mounted it noatime, or just taken whatever you get when
> you boot from a ramdisk?

Hi Richard,

The kernel has the "ro" commandline-parameter.
There is no remount after the system boots.
"touch /bla" gives a read-only fs error.

> FYI, I created a FIFO with mkfifo, remounted the file-system
> R/O, executed `cat` with it's input coming from the FIFO, and
> then waited for a few minutes. I then wrote to the FIFO.
> The atime did not change with 2.4.20.

Just did the same here (on my workstation).  And the times *did* change ..
More precisely: the "modification" & "change" were updated, the "access"
time remained unchanged.

RH9, kernel-2.4.20-13.9

	greetings,
	Rob van Nieuwkerk

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Richard B. Johnson]

On Wed, 28 May 2003, Rob van Nieuwkerk wrote:

> > On Wed, 28 May 2003, Rob van Nieuwkerk wrote:
> >
> > >
> > > I wrote:
> > > > It turns out that Linux is updating inode timestamps of fifos (named
> > > > pipes) that are written to while residing on a read-only filesystem.
> > > > It is not only updating in-ram info, but it will issue *physical*
> > > > writes to the read-only fs on the disk !
> > > 	.
> > > 	.
> > > 	.
> > > > Sysinfo:
> > > > --------
> > > > - various 2.4 kernels including RH-2.4.20-13.9,
> > > >   but also straight 2.4(ac) ones.
> > > > - CompactFlash (= IDE disk)
> > > > - Geode GX1 CPU (i586 compatible)
> > >
> > > Forgot to mention: I use an ext2 fs, but maybe it's a generic,
> > > fs-independant problem.
> > >
> > > 	greetings,
> > > 	Rob van Nieuwkerk
> >
> > How does it 'know' it's a R/O file-system? Have you mounted it
> > R/O, mounted it noatime, or just taken whatever you get when
> > you boot from a ramdisk?
>
> Hi Richard,
>
> The kernel has the "ro" commandline-parameter.
> There is no remount after the system boots.
> "touch /bla" gives a read-only fs error.
>
> > FYI, I created a FIFO with mkfifo, remounted the file-system
> > R/O, executed `cat` with it's input coming from the FIFO, and
> > then waited for a few minutes. I then wrote to the FIFO.
> > The atime did not change with 2.4.20.
>
> Just did the same here (on my workstation).  And the times *did* change ..
> More precisely: the "modification" & "change" were updated, the "access"
> time remained unchanged.
>

Okay. I can now verify the problem. There are two problems as this
script will show:

Script started on Wed May 28 16:10:13 2003
# cat xxx.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/fcntl.h>
#include <sys/stat.h>
#include <sys/wait.h>

int main(void);
static const char fifo[]="/alt/FIFO";
void reaper(int unused)
{
    while(wait3(&unused, WNOHANG, NULL) > 0)
        ;
}
int main()
{
    int fd;
    int i;
    char buf[0x1000];
    struct stat sb;

    (void)mkfifo(fifo, 0644);
    (void)signal(SIGCHLD, reaper);
    if((fd = open(fifo, O_RDWR)) < 0)
        exit(EXIT_FAILURE);
    switch(fork())
    {
    case 0:
        for(;;)
        {
            if(read(fd, buf, sizeof(buf)) < 0)
                exit(EXIT_FAILURE);
            if(*buf == (char) 0xa5) exit(EXIT_SUCCESS);
        }
        break;  /* Not reached */
    case -1:
        fprintf(stderr, "Can't fork()\n");
        exit(EXIT_FAILURE);
        break; /* Not reached */
    default:
        break; /* Now required */
    }
    memset(buf, 0x00, sizeof(buf));
    for(i=0; i< 0x10; i++)
    {
        (void)write(fd, buf, sizeof(buf));
        (void)fstat(fd, &sb);
        printf("atime = %08lx\n", sb.st_atime);
        printf("mtime = %08lx\n", sb.st_mtime);
        printf("ctime = %08lx\n", sb.st_ctime);
        sleep(1);
    }
    *buf = (char)0xa5;
    (void)write(fd, buf, 0x01);
    (void)close(fd);
//    (void)unlink(fifo);
    return 0;
}

# gcc -O2 -o xxx -Wall xxx.c
# ./xxx
atime = 3ed51750
mtime = 3ed517c5
ctime = 3ed517c5
atime = 3ed51750
mtime = 3ed517c6
ctime = 3ed517c6
atime = 3ed51750
mtime = 3ed517c7
ctime = 3ed517c7
atime = 3ed51750
mtime = 3ed517c8
ctime = 3ed517c8
atime = 3ed51750
mtime = 3ed517c9
ctime = 3ed517c9
atime = 3ed51750
mtime = 3ed517ca
ctime = 3ed517ca
atime = 3ed51750
mtime = 3ed517cb
ctime = 3ed517cb
atime = 3ed51750
mtime = 3ed517cc
ctime = 3ed517cc
atime = 3ed51750
mtime = 3ed517cd
ctime = 3ed517cd
atime = 3ed51750
mtime = 3ed517ce
ctime = 3ed517ce
atime = 3ed51750
mtime = 3ed517cf
ctime = 3ed517cf
atime = 3ed51750
mtime = 3ed517d0
ctime = 3ed517d0
atime = 3ed51750
mtime = 3ed517d1
ctime = 3ed517d1
atime = 3ed51750
mtime = 3ed517d2
ctime = 3ed517d2
atime = 3ed51750
mtime = 3ed517d3
ctime = 3ed517d3
atime = 3ed51750
mtime = 3ed517d4
ctime = 3ed517d4
# >/alt/foo
bash: /alt/foo: Read-only file system
# exit
exit
Script done on Wed May 28 16:11:12 2003

As you can clearly see, access time (atime) is not changed.
However, both ctime and mtime are both changed with every
FIFO access. Since this FIFO is provably on a R/O file system,
nothing should change.

Now, somebody will probably claim that this is the correct
POSIX defined behavior <sigh> so you might have to make some
work-around like use a pipe or socket instead of the FIFO??

Cheers,
Dick Johnson
Penguin : Linux version 2.4.20 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Rob van Nieuwkerk]

Hi Richard,

> > > > > It turns out that Linux is updating inode timestamps of fifos (named
> > > > > pipes) that are written to while residing on a read-only filesystem.
> > > > > It is not only updating in-ram info, but it will issue *physical*
> > > > > writes to the read-only fs on the disk !

> > > FYI, I created a FIFO with mkfifo, remounted the file-system
> > > R/O, executed `cat` with it's input coming from the FIFO, and
> > > then waited for a few minutes. I then wrote to the FIFO.
> > > The atime did not change with 2.4.20.
> >
> > Just did the same here (on my workstation).  And the times *did* change ..
> > More precisely: the "modification" & "change" were updated, the "access"
> > time remained unchanged.
> >
> 
> Okay. I can now verify the problem. There are two problems as this

Yeah !, I'm no longer alone .. :-)
	.
	.
> As you can clearly see, access time (atime) is not changed.
> However, both ctime and mtime are both changed with every
> FIFO access. Since this FIFO is provably on a R/O file system,
> nothing should change.

Note that the fact that you see the times changing in the fs while it
is mounted doesn't imply a problem in itself: serial and tty device
nodes get their time-stamps updated too on a read-only fs when they
are written.  But these changes are in ram only: when you reboot you
get the old values back.

But with FIFOs the changes *do* get written out to the read-only fs !

Hmm, wonder what happens if you try it on a real read-only medium like
a CDR.  Maybe kernel errors/panic ..

> Now, somebody will probably claim that this is the correct
> POSIX defined behavior <sigh> so you might have to make some
> work-around like use a pipe or socket instead of the FIFO??

Seems very stupid to me if POSIX specifies this.
I don't have the POSIX spec, but maybe it specifies what "read-only"
is supposed to mean somewhere too ..

But let's wait & see .. :-)

	greetings,
	Rob van Nieuwkerk

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Stephen C. Tweedie]

[-- Attachment #1: Type: text/plain, Size: 455 bytes --]

Hi,

On Wed, May 28, 2003 at 10:52:30PM +0200, Rob van Nieuwkerk wrote:

> I don't have the POSIX spec, but maybe it specifies what "read-only"
> is supposed to mean somewhere too ..

SingleUnix says:

http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap04.html#tag_04_07

"Marks for update, and updates themselves, are not done for files on
read-only file systems"

So we're wrong here.  Patch below fixes it for me for 2.4.

Cheers,
 Stephen

[-- Attachment #2: 4202-vfs-mctime-rofs.patch --]
[-- Type: text/plain, Size: 1602 bytes --]

--- linux-2.4-odirect/fs/inode.c.=K0004=.orig
+++ linux-2.4-odirect/fs/inode.c
@@ -1194,6 +1194,24 @@ void update_atime (struct inode *inode)
 	mark_inode_dirty_sync (inode);
 }   /*  End Function update_atime  */
 
+/**
+ *	update_mctime	-	update the mtime and ctime
+ *	@inode: inode accessed
+ *
+ *	Update the modified and changed times on an inode for writes to special
+ *	files such as fifos.  No change is forced if the timestamps are already
+ *	up-to-date or if the filesystem is readonly.
+ */
+ 
+void update_mctime (struct inode *inode)
+{
+	if (inode->i_mtime == CURRENT_TIME && inode->i_ctime == CURRENT_TIME)
+		return;
+	if ( IS_RDONLY (inode) ) return;
+	inode->i_ctime = inode->i_mtime = CURRENT_TIME;
+	mark_inode_dirty (inode);
+}   /*  End Function update_mctime  */
+
 
 /*
  *	Quota functions that want to walk the inode lists..
--- linux-2.4-odirect/fs/pipe.c.=K0004=.orig
+++ linux-2.4-odirect/fs/pipe.c
@@ -230,8 +230,7 @@ pipe_write(struct file *filp, const char
 	/* Signal readers asynchronously that there is more data.  */
 	wake_up_interruptible(PIPE_WAIT(*inode));
 
-	inode->i_ctime = inode->i_mtime = CURRENT_TIME;
-	mark_inode_dirty(inode);
+	update_mctime(inode);
 
 out:
 	up(PIPE_SEM(*inode));
--- linux-2.4-odirect/include/linux/fs.h.=K0004=.orig
+++ linux-2.4-odirect/include/linux/fs.h
@@ -201,6 +201,7 @@ extern int leases_enable, dir_notify_ena
 #include <asm/byteorder.h>
 
 extern void update_atime (struct inode *);
+extern void update_mctime (struct inode *);
 #define UPDATE_ATIME(inode) update_atime (inode)
 
 extern void buffer_init(unsigned long);

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Christoph Hellwig]

On Fri, May 30, 2003 at 02:21:12PM +0100, Stephen C. Tweedie wrote:
> +void update_mctime (struct inode *inode)
> +{
> +	if (inode->i_mtime == CURRENT_TIME && inode->i_ctime == CURRENT_TIME)
> +		return;
> +	if ( IS_RDONLY (inode) ) return;
> +	inode->i_ctime = inode->i_mtime = CURRENT_TIME;
> +	mark_inode_dirty (inode);
> +}   /*  End Function update_mctime  */
> +

Yikes, this looks like devfs code!  Please try to use proper kernel style..

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Stephen C. Tweedie]

[-- Attachment #1: Type: text/plain, Size: 287 bytes --]

Hi,

On Fri, 2003-05-30 at 15:58, Christoph Hellwig wrote:

> Yikes, this looks like devfs code!  Please try to use proper kernel style..

It's pure cut-and-paste from the update_atime immediately above it.  But
sure, we can clean them both up while we're at it if you want.

--Stephen


[-- Attachment #2: 4202-vfs-mctime-rofs.patch --]
[-- Type: text/plain, Size: 1936 bytes --]

--- linux-2.4-odirect/fs/inode.c.=K0001=.orig
+++ linux-2.4-odirect/fs/inode.c
@@ -1187,12 +1187,34 @@ void update_atime (struct inode *inode)
 {
 	if (inode->i_atime == CURRENT_TIME)
 		return;
-	if ( IS_NOATIME (inode) ) return;
-	if ( IS_NODIRATIME (inode) && S_ISDIR (inode->i_mode) ) return;
-	if ( IS_RDONLY (inode) ) return;
+	if (IS_NOATIME(inode))
+		return;
+	if (IS_NODIRATIME(inode) && S_ISDIR(inode->i_mode)) 
+		return;
+	if (IS_RDONLY(inode)) 
+		return;
 	inode->i_atime = CURRENT_TIME;
 	mark_inode_dirty_sync (inode);
-}   /*  End Function update_atime  */
+}
+
+/**
+ *	update_mctime	-	update the mtime and ctime
+ *	@inode: inode accessed
+ *
+ *	Update the modified and changed times on an inode for writes to special
+ *	files such as fifos.  No change is forced if the timestamps are already
+ *	up-to-date or if the filesystem is readonly.
+ */
+ 
+void update_mctime (struct inode *inode)
+{
+	if (inode->i_mtime == CURRENT_TIME && inode->i_ctime == CURRENT_TIME)
+		return;
+	if (IS_RDONLY(inode))
+		return;
+	inode->i_ctime = inode->i_mtime = CURRENT_TIME;
+	mark_inode_dirty (inode);
+}
 
 
 /*
--- linux-2.4-odirect/fs/pipe.c.=K0001=.orig
+++ linux-2.4-odirect/fs/pipe.c
@@ -230,8 +230,7 @@ pipe_write(struct file *filp, const char
 	/* Signal readers asynchronously that there is more data.  */
 	wake_up_interruptible(PIPE_WAIT(*inode));
 
-	inode->i_ctime = inode->i_mtime = CURRENT_TIME;
-	mark_inode_dirty(inode);
+	update_mctime(inode);
 
 out:
 	up(PIPE_SEM(*inode));
--- linux-2.4-odirect/include/linux/fs.h.=K0001=.orig
+++ linux-2.4-odirect/include/linux/fs.h
@@ -201,6 +201,7 @@ extern int leases_enable, dir_notify_ena
 #include <asm/byteorder.h>
 
 extern void update_atime (struct inode *);
+extern void update_mctime (struct inode *);
 #define UPDATE_ATIME(inode) update_atime (inode)
 
 extern void buffer_init(unsigned long);

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Andrew Morton]

"Stephen C. Tweedie" <sct@redhat.com> wrote:
>
> It's pure cut-and-paste from the update_atime immediately above it.  But
> sure, we can clean them both up while we're at it if you want.

2.5 seems to have gained a handy library function.

diff -puN fs/pipe.c~pipe-rofs-fix fs/pipe.c
--- 25/fs/pipe.c~pipe-rofs-fix	2003-05-30 09:33:29.000000000 -0700
+++ 25-akpm/fs/pipe.c	2003-05-30 09:34:08.000000000 -0700
@@ -208,10 +208,8 @@ pipe_write(struct file *filp, const char
 		wake_up_interruptible(PIPE_WAIT(*inode));
 		kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN);
 	}
-	if (ret > 0) {
-		inode->i_ctime = inode->i_mtime = CURRENT_TIME;
-		mark_inode_dirty(inode);
-	}
+	if (ret > 0)
+		inode_update_time(inode, 1);	/* mtime and ctime */
 	return ret;
 }
 

_

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Nuno Silva]

Rob van Nieuwkerk wrote:
> Hi all,
> 
> It turns out that Linux is updating inode timestamps of fifos (named
> pipes) that are written to while residing on a read-only filesystem.
> It is not only updating in-ram info, but it will issue *physical*
> writes to the read-only fs on the disk !

Hi!

I can't give a solution but the workaround is obvious:
mount -t ramfs none /myFifos

Regards,
Nuno Silva

Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Rob van Nieuwkerk]

> 
> 
> Rob van Nieuwkerk wrote:
> > Hi all,
> > 
> > It turns out that Linux is updating inode timestamps of fifos (named
> > pipes) that are written to while residing on a read-only filesystem.
> > It is not only updating in-ram info, but it will issue *physical*
> > writes to the read-only fs on the disk !
> 
> Hi!
> 
> I can't give a solution but the workaround is obvious:
> mount -t ramfs none /myFifos

Hi Nuno,

Yup, already thought of that.
But would be nice if the bug was fixed too !

	greetings,
	Rob van Nieuwkerk