Re: [PATCH 8/8] nfsd: hold net namespace reference in nfsd_file

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Jun 01, 2026 at 01:31:11PM -0400, Jeff Layton wrote:
> Take a net-namespace reference in nfsd_file_alloc() (get_net) and
> release it in nfsd_file_free() (put_net), so that nf_net is always
> valid for files that the GC or shrinker has isolated from the hash
> table and LRU -- which __nfsd_file_cache_purge() cannot see.
> 
> Without this, nf_net can dangle for in-flight files whose net namespace
> is torn down concurrently, causing a use-after-free when
> nfsd_file_dispose_list_delayed() calls net_generic(nf->nf_net, ...).
> 
> Because nfsd_file_free() now calls put_net(nf->nf_net), the old
> nfsd_file_put_local() pattern of returning nf->nf_net after
> nfsd_file_put() is unsafe -- put_net() could theoretically drop the
> last net namespace reference, leaving the returned pointer stale.
> Fix this by moving the nfsd_net_put() call into nfsd_file_put_local()
> itself, before the nfsd_file_put() that may trigger nfsd_file_free().
> The function now returns void and the caller no longer needs to handle
> the net reference.

That means that each nfsd_file_alloc()/nfsd_file_free() is now touching
the same cacheline on kernels with netns enabled.  Scalability implications
might be interesting...