linux-kernel - Re: [PATCH] nfsd: don't free files unconditionally in __nfsd_file_cache

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Message-ID: <C04F7C2C-848A-47C5-9C4E-9B9E5E82BAFF@oracle.com>
Date:   Fri, 20 Jan 2023 20:21:18 +0000
From:   Chuck Lever III <chuck.lever@...cle.com>
To:     Jeff Layton <jlayton@...nel.org>
CC:     Ruben Vestergaard <rubenv@...mr.dk>,
        Torkil Svensgaard <torkil@...mr.dk>,
        Shachar Kagan <skagan@...dia.com>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nfsd: don't free files unconditionally in
 __nfsd_file_cache_purge



> On Jan 20, 2023, at 2:52 PM, Jeff Layton <jlayton@...nel.org> wrote:
> 
> nfsd_file_cache_purge is called when the server is shutting down, in
> which case, tearing things down is generally fine, but it also gets
> called when the exports cache is flushed.

Yeah... cache flush is probably the case we've been missing.


> Instead of walking the cache and freeing everything unconditionally,
> handle it the same as when we have a notification of conflicting access.
> 
> Fixes: ac3a2585f018 ("nfsd: rework refcounting in filecache")
> Reported-by: Ruben Vestergaard <rubenv@...mr.dk>
> Reported-by: Torkil Svensgaard <torkil@...mr.dk>
> Reported-by: Shachar Kagan <skagan@...dia.com>
> Signed-off-by: Jeff Layton <jlayton@...nel.org>
> ---
> fs/nfsd/filecache.c | 61 +++++++++++++++++++++++++++------------------
> 1 file changed, 37 insertions(+), 24 deletions(-)
> 
> v2: use the same method to purge entries from the cache as we do when
>    there is a notification of conflicting access.
> 
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> index 58ac93e7e680..397ae212b98d 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -661,6 +661,40 @@ static struct shrinker	nfsd_file_shrinker = {
> 	.seeks = 1,
> };
> 
> +/**
> + * maybe_queue_nfsd_file - try to unhash and queue a nfsd_file to be freed
> + * @nf: nfsd_file to attempt to queue
> + * @dispose: private list to queue successfully-put objects
> + *
> + * Unhash an nfsd_file, try to get a reference to it, and then put that
> + * reference. If it's the last reference, queue it to the dispose list.
> + *
> + * The caller MUST hold the rcu_read_lock() !

__nfsd_file_cache_purge() isn't holding rcu_read_lock(), it's
holding the nfsd_mutex. Is this comment incorrect, or is it just
too specific? Or did I miss something obvious?


> + */
> +static void
> +maybe_queue_nfsd_file(struct nfsd_file *nf, struct list_head *dispose)

I prefer the name nfsd_file_try_to_queue() or nfsd_file_try_to_dispose().
nfsd_file_ should be the prefix where possible. Unless you're
redriving, I can fix that.


> +{
> +	int decrement = 1;
> +
> +	/* If we raced with someone else unhashing, ignore it */
> +	if (!nfsd_file_unhash(nf))
> +		return;
> +
> +	/* If we can't get a reference, ignore it */
> +	if (!nfsd_file_get(nf))
> +		return;
> +
> +	/* Extra decrement if we remove from the LRU */
> +	if (nfsd_file_lru_remove(nf))
> +		++decrement;
> +
> +	/* If refcount goes to 0, then put on the dispose list */
> +	if (refcount_sub_and_test(decrement, &nf->nf_ref)) {
> +		list_add(&nf->nf_lru, dispose);
> +		trace_nfsd_file_closing(nf);
> +	}
> +}
> +
> /**
>  * nfsd_file_queue_for_close: try to close out any open nfsd_files for an inode
>  * @inode:   inode on which to close out nfsd_files
> @@ -688,30 +722,12 @@ nfsd_file_queue_for_close(struct inode *inode, struct list_head *dispose)
> 
> 	rcu_read_lock();
> 	do {
> -		int decrement = 1;
> -
> 		nf = rhashtable_lookup(&nfsd_file_rhash_tbl, &key,
> 				       nfsd_file_rhash_params);
> 		if (!nf)
> 			break;
> 
> -		/* If we raced with someone else unhashing, ignore it */
> -		if (!nfsd_file_unhash(nf))
> -			continue;
> -
> -		/* If we can't get a reference, ignore it */
> -		if (!nfsd_file_get(nf))
> -			continue;
> -
> -		/* Extra decrement if we remove from the LRU */
> -		if (nfsd_file_lru_remove(nf))
> -			++decrement;
> -
> -		/* If refcount goes to 0, then put on the dispose list */
> -		if (refcount_sub_and_test(decrement, &nf->nf_ref)) {
> -			list_add(&nf->nf_lru, dispose);
> -			trace_nfsd_file_closing(nf);
> -		}
> +		maybe_queue_nfsd_file(nf, dispose);
> 	} while (1);
> 	rcu_read_unlock();
> }
> @@ -928,11 +944,8 @@ __nfsd_file_cache_purge(struct net *net)
> 
> 		nf = rhashtable_walk_next(&iter);
> 		while (!IS_ERR_OR_NULL(nf)) {
> -			if (!net || nf->nf_net == net) {
> -				nfsd_file_unhash(nf);
> -				nfsd_file_lru_remove(nf);
> -				list_add(&nf->nf_lru, &dispose);
> -			}
> +			if (!net || nf->nf_net == net)
> +				maybe_queue_nfsd_file(nf, &dispose);
> 			nf = rhashtable_walk_next(&iter);
> 		}
> 
> -- 
> 2.39.0
> 

--
Chuck Lever