Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
From:   "NeilBrown" <neilb@suse.de>
To:     "Chuck Lever III" <chuck.lever@oracle.com>
Cc:     "Linux NFS Mailing List" <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v2 7/9] NFSD: Use rhashtable for managing nfs4_file objects
In-reply-to: <EB08B095-BF02-4B5E-8CD2-12B0201328D2@oracle.com>
References: <166507275951.1802.13184584115155050247.stgit@manet.1015granger.net>,
 <166507324882.1802.884870684212914640.stgit@manet.1015granger.net>,
 <166544739751.14457.9018300177489236723@noble.neil.brown.name>,
 <EB08B095-BF02-4B5E-8CD2-12B0201328D2@oracle.com>
Date:   Wed, 12 Oct 2022 17:45:34 +1100
Message-id: <166555713439.32740.4569758527894347740@noble.neil.brown.name>
Precedence: bulk

On Tue, 11 Oct 2022, Chuck Lever III wrote:
> > On Oct 10, 2022, at 8:16 PM, NeilBrown <neilb@suse.de> wrote:
> > 
> > On Fri, 07 Oct 2022, Chuck Lever wrote:
> >> 
> >> -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/
> 
> 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