netdev - Re: [PATCH RFC 28/30] NFSD: Set up an rhashtable for the filecache

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Date:   Fri, 24 Jun 2022 01:29:32 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Chuck Lever III <chuck.lever@...cle.com>
Cc:     Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>,
        "tgraf@...g.ch" <tgraf@...g.ch>, Jeff Layton <jlayton@...hat.com>
Subject: Re: [PATCH RFC 28/30] NFSD: Set up an rhashtable for the filecache

On Fri, Jun 24, 2022 at 12:14:53AM +0000, Chuck Lever III wrote:
> 
> > On Jun 23, 2022, at 7:51 PM, Chuck Lever III <chuck.lever@...cle.com> wrote:
> > 
> >> On Jun 23, 2022, at 6:56 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >> 
> >> On Wed, Jun 22, 2022 at 10:15:50AM -0400, Chuck Lever wrote:
> >> 
> >>> +static u32 nfsd_file_obj_hashfn(const void *data, u32 len, u32 seed)
> >>> +{
> >>> +	const struct nfsd_file *nf = data;
> >>> +
> >>> +	return jhash2((const u32 *)&nf->nf_inode,
> >>> +		      sizeof_field(struct nfsd_file, nf_inode) / sizeof(u32),
> >>> +		      seed);
> >> 
> >> Out of curiosity - what are you using to allocate those?  Because if
> >> it's a slab, then middle bits of address (i.e. lower bits of
> >> (unsigned long)data / L1_CACHE_BYTES) would better be random enough...
> > 
> > 261 static struct nfsd_file *
> > 262 nfsd_file_alloc(struct nfsd_file_lookup_key *key, unsigned int may)
> > 263 {
> > 264         static atomic_t nfsd_file_id;
> > 265         struct nfsd_file *nf;
> > 266 
> > 267         nf = kmem_cache_alloc(nfsd_file_slab, GFP_KERNEL);
> > 
> > Was wondering about that. pahole says struct nfsd_file is 112
> > bytes on my system.
> 
> Oops. nfsd_file_obj_hashfn() is supposed to be generating the
> hash value based on the address stored in the nf_inode field.
> So it's an inode pointer, alloced via kmem_cache_alloc by default.

inode pointers are definitely "divide by L1_CACHE_BYTES and take lower
bits" fodder...