Re: nfsd4_is_junction

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

On Sat, Oct 29, 2011 at 01:58:02PM -0400, Chuck Lever wrote:
> 
> On Oct 29, 2011, at 1:55 PM, J. Bruce Fields wrote:
> 
> > On Sat, Oct 29, 2011 at 01:47:55PM -0400, Chuck Lever wrote:
> >> 
> >> On Oct 29, 2011, at 6:16 AM, Christoph Hellwig wrote:
> >> 
> >>> This function should probably grow a check for S_NOSEC instead of
> >>> hitting the filesystems with an xattr read for every lookup.
> >> 
> >> It doesn't perform the xattr read on _every_ lookup, but only when the object has a special set of permission bits.
> >> 
> >> But actually, it probably doesn't need that 'type" attribute check at all.  It should delegate that check to mountd.  I assume many of those upcalls would hit in the export cache anyway?
> > 
> > You're suggesting doing an upcall on every path that passes the mode-bit
> > checks?
> > 
> > You'd end up populating the export cache with every path that passes
> > the mode-bit checks.  I don't think that's a good idea.
> 
> You just finished telling us that the mode bit checks would prevent doing an attribute read too often.  Again, how often do we think there will be directories with no execute bits set and the sticky bit set?  I think that's going to be exceptionally rare.

Yeah, probably so, but if there's a pathological case where we're wrong
then I think excessive xattr reads will be less annoying than excessive
upcalls and export cache entries.

And in the cached case: I doubt a lookup in the export cache is really
faster than a read of a cached xattr.  But I don't know.

--b.