Re: [PATCH v2 2/3] nfsd: only keep unused entries on the LRU

NeilBrown
2022-10-27T22:21:12Z

On Fri, 28 Oct 2022, Jeff Layton wrote:
> Currently, nfsd_files live on the LRU once they are added until they are
> unhashed. There's no need to keep ones that are actively in use there.

Is that true?
nfsd_file_do_acquire() calls nfsd_file_lru_remove()
Isn't that enough to keep the file off the lru while it is active?

Thanks,
NeilBrown

>
> Before incrementing the refcount, do a lockless check for nf_lru being
> empty. If it's not then attempt to remove the entry from the LRU. If
> that's successful, claim the LRU reference and return it. If the removal
> fails (or if the list_head was empty), then just increment the counter
> as we normally would.
>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> fs/nfsd/filecache.c | 23 ++++++++++++++++++++---
> 1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> index e63534f4b9f8..d2bbded805d4 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -420,14 +420,31 @@ nfsd_file_unhash(struct nfsd_file *nf)
> return false;
> }
>
> -struct nfsd_file *
> -nfsd_file_get(struct nfsd_file *nf)
> +static struct nfsd_file *
> +__nfsd_file_get(struct nfsd_file *nf)
> {
> if (likely(refcount_inc_not_zero(&nf->nf_ref)))
> return nf;
> return NULL;
> }
>
> +struct nfsd_file *
> +nfsd_file_get(struct nfsd_file *nf)
> +{
> + /*
> + * Do a lockless list_empty check first, before attempting to
> + * remove it, so we can avoid the spinlock when it's not on the
> + * list.
> + *
> + * If we successfully remove it from the LRU, then we can just
> + * claim the LRU reference and return it. Otherwise, we need to
> + * bump the counter the old-fashioned way.
> + */
> + if (!list_empty(&nf->nf_lru) && nfsd_file_lru_remove(nf))
> + return nf;
> + return __nfsd_file_get(nf);
> +}
> +
> /**
> * nfsd_file_unhash_and_queue - unhash a file and queue it to the dispose list
> * @nf: nfsd_file to be unhashed and queued
> @@ -449,7 +466,7 @@ nfsd_file_unhash_and_queue(struct nfsd_file *nf, struct list_head *dispose)
> * to take a reference. If that fails, just ignore
> * the file altogether.
> */
> - if (!nfsd_file_lru_remove(nf) && !nfsd_file_get(nf))
> + if (!nfsd_file_lru_remove(nf) && !__nfsd_file_get(nf))
> return false;
> list_add(&nf->nf_lru, dispose);
> return true;
> --
> 2.37.3
>
>