btrfs and default directory link count

btrfs and default directory link count

[Andreas Schneider]

Hello,

I did run the Samba testsuite and have a failing test 
(samba.vfstest.stream_depot). It revealed that it only fails on btrfs. The 
reason is that a simple check fails:

if (smb_fname_base->st.st_ex_nlink == 2)

If you create a directory on btrfs and check stat:

$ mkdir x
$ stat x
  File: ‘x’
  Size: 0               Blocks: 0          IO Block: 4096   directory
Device: 2bh/43d Inode: 3834720     Links: 1
Access: (0755/drwxr-xr-x)  Uid: ( 1000/     asn)   Gid: (  100/   users)
Access: 2013-11-08 11:54:32.431040963 +0100
Modify: 2013-11-08 11:54:32.430040956 +0100
Change: 2013-11-08 11:54:32.430040956 +0100
 Birth: -

then you see Links: 1. On ext4 or other filesystems:

mkdir x
stat x
  File: ‘x’
  Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: fd00h/64768d    Inode: 8126886     Links: 2
Access: (0755/drwxr-xr-x)  Uid: ( 1000/     asn)   Gid: (  100/   users)
Access: 2013-11-08 11:54:55.428212340 +0100
Modify: 2013-11-08 11:54:55.427212319 +0100
Change: 2013-11-08 11:54:55.427212319 +0100
 Birth: -

the link count for a directory differs: Links: 2.

Why is btrfs different here? Could someone explain this?


Thanks,


	-- andreas

-- 
Andreas Schneider                   GPG-ID: CC014E3D
www.cryptomilk.org                asn@cryptomilk.org

Re: btrfs and default directory link count

[cwillu]

On Fri, Nov 8, 2013 at 5:07 AM, Andreas Schneider <asn@cryptomilk.org> wrote:
> Hello,
>
> I did run the Samba testsuite and have a failing test
> (samba.vfstest.stream_depot). It revealed that it only fails on btrfs. The
> reason is that a simple check fails:
>
> if (smb_fname_base->st.st_ex_nlink == 2)
>
> If you create a directory on btrfs and check stat:
>
> $ mkdir x
> $ stat x
>   File: ‘x’
>   Size: 0               Blocks: 0          IO Block: 4096   directory
> Device: 2bh/43d Inode: 3834720     Links: 1
> Access: (0755/drwxr-xr-x)  Uid: ( 1000/     asn)   Gid: (  100/   users)
> Access: 2013-11-08 11:54:32.431040963 +0100
> Modify: 2013-11-08 11:54:32.430040956 +0100
> Change: 2013-11-08 11:54:32.430040956 +0100
>  Birth: -
>
> then you see Links: 1. On ext4 or other filesystems:
>
> mkdir x
> stat x
>   File: ‘x’
>   Size: 4096            Blocks: 8          IO Block: 4096   directory
> Device: fd00h/64768d    Inode: 8126886     Links: 2
> Access: (0755/drwxr-xr-x)  Uid: ( 1000/     asn)   Gid: (  100/   users)
> Access: 2013-11-08 11:54:55.428212340 +0100
> Modify: 2013-11-08 11:54:55.427212319 +0100
> Change: 2013-11-08 11:54:55.427212319 +0100
>  Birth: -
>
> the link count for a directory differs: Links: 2.
>
> Why is btrfs different here? Could someone explain this?

As I understand it, inferring the number of directory entries from
st_nlink is an optimization that isn't universally valid. If that
count is 1, it must be considered invalid, and programs that don't
handle this correctly are broken.  Coreutils handle this, at least...

Re: btrfs and default directory link count

[Andreas Schneider]

On Friday 08 November 2013 06:24:01 cwillu wrote:
> On Fri, Nov 8, 2013 at 5:07 AM, Andreas Schneider <asn@cryptomilk.org> 
wrote:
> > Hello,
> > 
> > I did run the Samba testsuite and have a failing test
> > (samba.vfstest.stream_depot). It revealed that it only fails on btrfs. The
> > reason is that a simple check fails:
> > 
> > if (smb_fname_base->st.st_ex_nlink == 2)
> > 
> > If you create a directory on btrfs and check stat:
> > 
> > $ mkdir x
> > $ stat x
> > 
> >   File: ‘x’
> >   Size: 0               Blocks: 0          IO Block: 4096   directory
> > 
> > Device: 2bh/43d Inode: 3834720     Links: 1
> > Access: (0755/drwxr-xr-x)  Uid: ( 1000/     asn)   Gid: (  100/   users)
> > Access: 2013-11-08 11:54:32.431040963 +0100
> > Modify: 2013-11-08 11:54:32.430040956 +0100
> > Change: 2013-11-08 11:54:32.430040956 +0100
> > 
> >  Birth: -
> > 
> > then you see Links: 1. On ext4 or other filesystems:
> > 
> > mkdir x
> > stat x
> > 
> >   File: ‘x’
> >   Size: 4096            Blocks: 8          IO Block: 4096   directory
> > 
> > Device: fd00h/64768d    Inode: 8126886     Links: 2
> > Access: (0755/drwxr-xr-x)  Uid: ( 1000/     asn)   Gid: (  100/   users)
> > Access: 2013-11-08 11:54:55.428212340 +0100
> > Modify: 2013-11-08 11:54:55.427212319 +0100
> > Change: 2013-11-08 11:54:55.427212319 +0100
> > 
> >  Birth: -
> > 
> > the link count for a directory differs: Links: 2.
> > 
> > Why is btrfs different here? Could someone explain this?
> 
> As I understand it, inferring the number of directory entries from
> st_nlink is an optimization that isn't universally valid. If that
> count is 1, it must be considered invalid, and programs that don't
> handle this correctly are broken.  Coreutils handle this, at least...

Thanks for the explanation, I will fix our code.


	-- andreas

-- 
Andreas Schneider                   GPG-ID: CC014E3D
www.cryptomilk.org                asn@cryptomilk.org