Re: [PATCH 0/3] NFSD patches to support junctions

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Sep 02, 2011 at 12:38:13PM -0400, Chuck Lever wrote:
> Sometime soon we are going to have easy-to-install user space FedFS
> components.  Here are kernel patches needed to make server-side FedFS
> support work.  Please consider these for the 3.2 kernel.
> 
> The third patch introduces a potentially expensive check to see if
> a junction has been encountered during a mountpoint lookup.  An object
> is a junction iff it has the requisite set of extended attributes.
> However, reading an extended attribute is expensive on some file
> systems.
> 
> To mitigate the cost of this check, junctions always have their sticky
> bit set.  The expensive extended attribute part of the junction test
> is done only if the sticky bit is present.
> 
> Note that today junctions are directories, but someday symlinks might
> also act as junctions (for SMB2 support).  And very few files have the
> sticky bit set.  So we avoid doing a directory test here.
> 
> Also, junctions ostensibly have all zero mode bits to hide their local
> contents.  I don't think the kernel needs to be concerned about the
> permissions as long as the sticky bit is set.  This allows some
> flexibility in how junctions are represented.  However, Jeff thinks
> that having nfsd4_is_junction() also consider mode bits would make the
> expensive part of this test still less frequent.
> 
> Any thoughts about this?

Hm, right, a sticky bit set on a directory is a normal thing.  I thought
Trond's original idea was to check for the sticky bit and not
executable?  Which is a pointless combination so should be very rare.

On a typical system maybe directories with the sticky bit are normally
somewhat rare, but that's a question of policy and there could be cases
where it's common.

--b.

> 
> ---
> 
> Trond Myklebust (3):
>       NFSD: Add a cache for fs_locations information
>       NFSD: Remove the ex_pathname field from struct svc_export
>       NFSD: Cleanup for nfsd4_path()
> 
> 
>  fs/nfsd/export.c            |   15 +-----
>  fs/nfsd/nfs4xdr.c           |  106 ++++++++++++++++++++++++++++++++-----------
>  fs/nfsd/nfsd.h              |    7 +++
>  fs/nfsd/vfs.c               |   16 ++++++
>  include/linux/nfsd/export.h |    2 -
>  5 files changed, 104 insertions(+), 42 deletions(-)
> 
> -- 
> Chuck Lever
> --
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