On Tue, 11 Oct 2022, Chuck Lever III wrote:

quoted

On Oct 10, 2022, at 8:16 PM, NeilBrown [off-list ref] wrote:

On Fri, 07 Oct 2022, Chuck Lever wrote:

quoted

-static unsigned int file_hashval(struct svc_fh *fh)
+/*
+ * The returned hash value is based solely on the address of an in-code
+ * inode, a pointer to a slab-allocated object. The entropy in such a
+ * pointer is concentrated in its middle bits.

I think you need more justification than that for throwing away some of
the entropy, even if you don't think it is much.

We might have that justification:

https://lore.kernel.org/linux-nfs/YrUFbLJ5uVbWtZbf@ZenIV/ (local)

Actually I believe we are not discarding /any/ entropy in
this function. The bits we discard are invariant.

And, note that this is exactly the same situation we just merged
in the filecache overhaul, and is a common trope amongst other
hash tables that base their function on the inode's address.

Common?  I searched for ">> *L1_CACHE_SHIFT".  Apart from the nfsd
filecache you mentioned I find two.  One in quota and one in reiserfs.
Both work with traditional hash tables which are more forgiving of
longer chains.
Do you have other evidence of this being a common trope?

Thanks,
NeilBrown