[Linux-cachefs] Adventures in NFS re-exporting

[bfields]

On Thu, Nov 12, 2020 at 11:05:57PM +0000, Daire Byrne wrote:
> So, I can't lay claim to identifying the exact optimisation/hack that
> improves the retention of the re-export server's client cache when
> re-exporting an NFSv3 server (which is then read by many clients). We
> were working with an engineer at the time who showed an interest in
> our use case and after we supplied a reproducer he suggested modifying
> the nfs/inode.c
> 
> -		if (!inode_eq_iversion_raw(inode, fattr->change_attr)) {
> +		if (inode_peek_iversion_raw(inode) < fattr->change_attr)
> {
> 
> His reasoning at the time was:
> 
> "Fixes inode invalidation caused by read access. The least important
> bit is ORed with 1 and causes the inode version to differ from the one
> seen on the NFS share. This in turn causes unnecessary re-download
> impacting the performance significantly. This fix makes it only
> re-fetch file content if inode version seen on the server is newer
> than the one on the client."
> 
> But I've always been puzzled by why this only seems to be the case
> when using knfsd to re-export the (NFSv3) client mount. Using multiple
> processes on a standard client mount never causes any similar
> re-validations. And this happens with a completely read-only share
> which is why I started to think it has something to do with atimes as
> that could perhaps still cause a "write" modification even when
> read-only?

Ah-hah!  So, it's inode_query_iversion() that's modifying a nfs inode's
i_version.  That's a special thing that only nfsd would do.

I think that's totally fixable, we'll just have to think a little about
how....

--b.