[PATCH] nanosecond filesystem support???

[PATCH] nanosecond filesystem support???

[Jeff Chua]

On 2/12/07, Andi Kleen <ak@suse.de> wrote:

> The bigger problem is getting a file system that support it.


Andi,

It seems that the part that's not returning nanosecond is in the code 
below. I've modified it, and now stat() is returning st_mtim.tv_nsec 
correctly.

I've tested it on ext2 and reiserfs, and both seems to be working.


I don't know why "t.tv_nsec = 0;" was set in the code. Any idea?



Thanks,
Jeff.


--- linux/kernel/time.c.org	2007-02-13 08:43:08 +0800
+++ linux/kernel/time.c	2007-02-13 08:52:29 +0800
@@ -86,7 +86,6 @@
  	if (get_user(tv.tv_sec, tptr))
  		return -EFAULT;

-	tv.tv_nsec = 0;

  	err = security_settime(&tv, NULL);
  	if (err)
@@ -269,7 +269,6 @@
  	if (gran <= jiffies_to_usecs(1) * 1000) {
  		/* nothing */
  	} else if (gran == 1000000000) {
-		t.tv_nsec = 0;
  	} else {
  		t.tv_nsec -= t.tv_nsec % gran;
  	}

Re: [PATCH] nanosecond filesystem support???

[Arjan van de Ven]

On Tue, 2007-02-13 at 16:38 +0800, Jeff Chua wrote:
> On 2/12/07, Andi Kleen <ak@suse.de> wrote:
> 
> > The bigger problem is getting a file system that support it.
> 
> 
> Andi,
> 
> It seems that the part that's not returning nanosecond is in the code 
> below. I've modified it, and now stat() is returning st_mtim.tv_nsec 
> correctly.
> 
> I've tested it on ext2 and reiserfs, and both seems to be working.
> 
> 
> I don't know why "t.tv_nsec = 0;" was set in the code. Any idea?

it was there to avoid the following situation:

on disk it's still in seconds
kernel has the inode in memory, with a recently updated times
user asks for it, gets nanoseconds data
kernel evicts inode to disk
user asks again,
kernel reads from disk (in whole seconds only on disk)
kernel reports a DIFFERENT time than it did before
user program gets confused

this actually happens, make is one of those cases that get confused for
example..

-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org

Re: [PATCH] nanosecond filesystem support???

[Andi Kleen]

> It seems that the part that's not returning nanosecond is in the code 
> below. I've modified it, and now stat() is returning st_mtim.tv_nsec 
> correctly.
> 
> I've tested it on ext2 and reiserfs, and both seems to be working.
> 
> 
> I don't know why "t.tv_nsec = 0;" was set in the code. Any idea?

The first hunk doesn't make any sense. stime() will never know anything about
nanoseconds. You're just putting uninitialized memory in there.

The truncation you removed in the second was deliberately added for the old file
systems that don't support higher granuality *times. Otherwise
the time can jump backwards between a inode that is still cached
in memory and one that is saved/restored from disk. Several programs
(including make) assume this cannot happen, so the kernel prevents it
by always rounding.

If you want ns resolution you need a file system that supports it:
that's currently XFS, JFS, NTFS/CIFS (resolution is lower, but < 1s), NFSv[34], UDF, 
with suitable servers,  tmpfs/ramfs/hugetlbfs. ext4 ns support is being worked on,

BTW the real max resolution supported in the kernel right now is jiffies.

-Andi

Re: [PATCH] nanosecond filesystem support???

[Jeff Chua]

On 2/13/07, Arjan van de Ven <arjan@infradead.org> wrote:
> it was there to avoid the following situation:
> on disk it's still in seconds


On 2/13/07, Andi Kleen <andi@firstfloor.org> wrote:

> If you want ns resolution you need a file system that supports it:
> that's currently XFS, JFS, NTFS/CIFS (resolution is lower, but < 1s), NFSv[34], UDF,
> with suitable servers,  tmpfs/ramfs/hugetlbfs. ext4 ns support is being worked on,
>
> BTW the real max resolution supported in the kernel right now is jiffies.


Andy, Arjan,

Ok, I got it now. Well, after reboot, I ran stat() and since the info
has to come from disk, now the nanosec returned is zero!

Thanks for all your help.

Jeff.