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From:   "NeilBrown" <neilb@suse.de>
To:     "Jeff Layton" <jlayton@kernel.org>
Cc:     chuck.lever@oracle.com, linux-nfs@vger.kernel.org
Subject: Re: [PATCH v2 2/3] nfsd: only keep unused entries on the LRU
In-reply-to: <20221027215213.138304-3-jlayton@kernel.org>
References: <20221027215213.138304-1-jlayton@kernel.org>,
 <20221027215213.138304-3-jlayton@kernel.org>
Date:   Fri, 28 Oct 2022 09:20:59 +1100
Message-id: <166690925944.13915.14734120966513564215@noble.neil.brown.name>
Precedence: bulk

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


>=20
> 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.
>=20
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/nfsd/filecache.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
>=20
> 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;
>  }
> =20
> -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;
>  }
> =20
> +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;
> --=20
> 2.37.3
>=20
>=20