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From:   Chuck Lever III <chuck.lever@oracle.com>
To:     Neil Brown <neilb@suse.de>
CC:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v2 5/9] NFSD: Add an NFSD_FILE_GC flag to enable nfsd_file
 garbage collection
Thread-Topic: [PATCH v2 5/9] NFSD: Add an NFSD_FILE_GC flag to enable
 nfsd_file garbage collection
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Date:   Tue, 11 Oct 2022 13:03:47 +0000
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> On Oct 10, 2022, at 7:50 PM, NeilBrown <neilb@suse.de> wrote:
>=20
> On Fri, 07 Oct 2022, Chuck Lever wrote:
>> NFSv4 operations manage the lifetime of nfsd_file items they use by
>> means of NFSv4 OPEN and CLOSE. Hence there's no need for them to be
>> garbage collected.
>>=20
>> Introduce a mechanism to enable garbage collection for nfsd_file
>> items used only by NFSv2/3 callers.
>>=20
>> Note that the change in nfsd_file_put() ensures that both CLOSE and
>> DELEGRETURN will actually close out and free an nfsd_file on last
>> reference of a non-garbage-collected file.
>>=20
>> Link: https://bugzilla.linux-nfs.org/show_bug.cgi?id=3D394
>> Suggested-by: Trond Myklebust <trond.myklebust@hammerspace.com>
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>> fs/nfsd/filecache.c |   61 +++++++++++++++++++++++++++++++++++++++++++++=
------
>> fs/nfsd/filecache.h |    3 +++
>> fs/nfsd/nfs3proc.c  |    4 ++-
>> fs/nfsd/trace.h     |    3 ++-
>> fs/nfsd/vfs.c       |    4 ++-
>> 5 files changed, 63 insertions(+), 12 deletions(-)
>>=20
>> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
>> index b7aa523c2010..01c27deabf83 100644
>> --- a/fs/nfsd/filecache.c
>> +++ b/fs/nfsd/filecache.c
>> @@ -63,6 +63,7 @@ struct nfsd_file_lookup_key {
>> 	struct net			*net;
>> 	const struct cred		*cred;
>> 	unsigned char			need;
>> +	unsigned char			gc:1;
>> 	enum nfsd_file_lookup_type	type;
>> };
>>=20
>> @@ -162,6 +163,8 @@ static int nfsd_file_obj_cmpfn(struct rhashtable_com=
pare_arg *arg,
>> 			return 1;
>> 		if (!nfsd_match_cred(nf->nf_cred, key->cred))
>> 			return 1;
>> +		if (test_bit(NFSD_FILE_GC, &nf->nf_flags) !=3D key->gc)
>> +			return 1;
>> 		if (test_bit(NFSD_FILE_HASHED, &nf->nf_flags) =3D=3D 0)
>> 			return 1;
>> 		break;
>> @@ -297,6 +300,8 @@ nfsd_file_alloc(struct nfsd_file_lookup_key *key, un=
signed int may)
>> 		nf->nf_flags =3D 0;
>> 		__set_bit(NFSD_FILE_HASHED, &nf->nf_flags);
>> 		__set_bit(NFSD_FILE_PENDING, &nf->nf_flags);
>> +		if (key->gc)
>> +			__set_bit(NFSD_FILE_GC, &nf->nf_flags);
>> 		nf->nf_inode =3D key->inode;
>> 		/* nf_ref is pre-incremented for hash table */
>> 		refcount_set(&nf->nf_ref, 2);
>> @@ -428,16 +433,27 @@ nfsd_file_put_noref(struct nfsd_file *nf)
>> 	}
>> }
>>=20
>> +static void
>> +nfsd_file_unhash_and_put(struct nfsd_file *nf)
>> +{
>> +	if (nfsd_file_unhash(nf))
>> +		nfsd_file_put_noref(nf);
>> +}
>> +
>> void
>> nfsd_file_put(struct nfsd_file *nf)
>> {
>> 	might_sleep();
>>=20
>> -	nfsd_file_lru_add(nf);
>> +	if (test_bit(NFSD_FILE_GC, &nf->nf_flags) =3D=3D 1)
>> +		nfsd_file_lru_add(nf);
>> +	else if (refcount_read(&nf->nf_ref) =3D=3D 2)
>> +		nfsd_file_unhash_and_put(nf);
>> +
>> 	if (test_bit(NFSD_FILE_HASHED, &nf->nf_flags) =3D=3D 0) {
>> 		nfsd_file_flush(nf);
>> 		nfsd_file_put_noref(nf);
>> -	} else if (nf->nf_file) {
>> +	} else if (nf->nf_file && test_bit(NFSD_FILE_GC, &nf->nf_flags) =3D=3D=
 1) {
>> 		nfsd_file_put_noref(nf);
>> 		nfsd_file_schedule_laundrette();
>> 	} else
>> @@ -1017,12 +1033,14 @@ nfsd_file_is_cached(struct inode *inode)
>>=20
>> static __be32
>> nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
>> -		     unsigned int may_flags, struct nfsd_file **pnf, bool open)
>> +		     unsigned int may_flags, struct nfsd_file **pnf,
>> +		     bool open, int want_gc)
>> {
>> 	struct nfsd_file_lookup_key key =3D {
>> 		.type	=3D NFSD_FILE_KEY_FULL,
>> 		.need	=3D may_flags & NFSD_FILE_MAY_MASK,
>> 		.net	=3D SVC_NET(rqstp),
>> +		.gc	=3D want_gc,
>> 	};
>> 	bool open_retry =3D true;
>> 	struct nfsd_file *nf;
>> @@ -1117,14 +1135,35 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, str=
uct svc_fh *fhp,
>> 	 * then unhash.
>> 	 */
>> 	if (status !=3D nfs_ok || key.inode->i_nlink =3D=3D 0)
>> -		if (nfsd_file_unhash(nf))
>> -			nfsd_file_put_noref(nf);
>> +		nfsd_file_unhash_and_put(nf);
>> 	clear_bit_unlock(NFSD_FILE_PENDING, &nf->nf_flags);
>> 	smp_mb__after_atomic();
>> 	wake_up_bit(&nf->nf_flags, NFSD_FILE_PENDING);
>> 	goto out;
>> }
>>=20
>> +/**
>> + * nfsd_file_acquire_gc - Get a struct nfsd_file with an open file
>> + * @rqstp: the RPC transaction being executed
>> + * @fhp: the NFS filehandle of the file to be opened
>> + * @may_flags: NFSD_MAY_ settings for the file
>> + * @pnf: OUT: new or found "struct nfsd_file" object
>> + *
>> + * The nfsd_file object returned by this API is reference-counted
>> + * and garbage-collected. The object is retained for a few
>> + * seconds after the final nfsd_file_put() in case the caller
>> + * wants to re-use it.
>> + *
>> + * Returns nfs_ok and sets @pnf on success; otherwise an nfsstat in
>> + * network byte order is returned.
>> + */
>> +__be32
>> +nfsd_file_acquire_gc(struct svc_rqst *rqstp, struct svc_fh *fhp,
>> +		     unsigned int may_flags, struct nfsd_file **pnf)
>> +{
>> +	return nfsd_file_do_acquire(rqstp, fhp, may_flags, pnf, true, 1);
>> +}
>> +
>> /**
>>  * nfsd_file_acquire - Get a struct nfsd_file with an open file
>>  * @rqstp: the RPC transaction being executed
>> @@ -1132,6 +1171,10 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, stru=
ct svc_fh *fhp,
>>  * @may_flags: NFSD_MAY_ settings for the file
>>  * @pnf: OUT: new or found "struct nfsd_file" object
>>  *
>> + * The nfsd_file_object returned by this API is reference-counted
>> + * but not garbage-collected. The object is released one RCU grace
>> + * period after the final nfsd_file_put().
>> + *
>>  * Returns nfs_ok and sets @pnf on success; otherwise an nfsstat in
>>  * network byte order is returned.
>>  */
>> @@ -1139,7 +1182,7 @@ __be32
>> nfsd_file_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
>> 		  unsigned int may_flags, struct nfsd_file **pnf)
>> {
>> -	return nfsd_file_do_acquire(rqstp, fhp, may_flags, pnf, true);
>> +	return nfsd_file_do_acquire(rqstp, fhp, may_flags, pnf, true, 0);
>> }
>>=20
>> /**
>> @@ -1149,6 +1192,10 @@ nfsd_file_acquire(struct svc_rqst *rqstp, struct =
svc_fh *fhp,
>>  * @may_flags: NFSD_MAY_ settings for the file
>>  * @pnf: OUT: new or found "struct nfsd_file" object
>>  *
>> + * The nfsd_file_object returned by this API is reference-counted
>> + * but not garbage-collected. The object is released immediately
>> + * one RCU grace period after the final nfsd_file_put().
>=20
> While that last sentence is correct, I think it is missing the point.
> The reference to grace periods is really an internal implementation
> details.

Fair enough, document policy and not implementation. I'll rework this
based on your suggestion below.


> The important point is that on final nfsd_file_put(), the object is
> removed from the hash table.  This contrasts with the other version
> where on final nfsd_file_put(), the object is added to the lru.
>=20
> The code all seems good, just the comment seemed odd.
>=20
> Thanks,
> NeilBrown
>=20
>=20
>> + *
>>  * Returns nfs_ok and sets @pnf on success; otherwise an nfsstat in
>>  * network byte order is returned.
>>  */
>> @@ -1156,7 +1203,7 @@ __be32
>> nfsd_file_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
>> 		 unsigned int may_flags, struct nfsd_file **pnf)
>> {
>> -	return nfsd_file_do_acquire(rqstp, fhp, may_flags, pnf, false);
>> +	return nfsd_file_do_acquire(rqstp, fhp, may_flags, pnf, false, 0);
>> }
>>=20
>> /*
>> diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
>> index f81c198f4ed6..0f6546bcd3e0 100644
>> --- a/fs/nfsd/filecache.h
>> +++ b/fs/nfsd/filecache.h
>> @@ -38,6 +38,7 @@ struct nfsd_file {
>> #define NFSD_FILE_HASHED	(0)
>> #define NFSD_FILE_PENDING	(1)
>> #define NFSD_FILE_REFERENCED	(2)
>> +#define NFSD_FILE_GC		(3)
>> 	unsigned long		nf_flags;
>> 	struct inode		*nf_inode;	/* don't deref */
>> 	refcount_t		nf_ref;
>> @@ -55,6 +56,8 @@ void nfsd_file_put(struct nfsd_file *nf);
>> struct nfsd_file *nfsd_file_get(struct nfsd_file *nf);
>> void nfsd_file_close_inode_sync(struct inode *inode);
>> bool nfsd_file_is_cached(struct inode *inode);
>> +__be32 nfsd_file_acquire_gc(struct svc_rqst *rqstp, struct svc_fh *fhp,
>> +		  unsigned int may_flags, struct nfsd_file **nfp);
>> __be32 nfsd_file_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
>> 		  unsigned int may_flags, struct nfsd_file **nfp);
>> __be32 nfsd_file_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
>> diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
>> index d12823371857..6a5ad6c91d8e 100644
>> --- a/fs/nfsd/nfs3proc.c
>> +++ b/fs/nfsd/nfs3proc.c
>> @@ -779,8 +779,8 @@ nfsd3_proc_commit(struct svc_rqst *rqstp)
>> 				(unsigned long long) argp->offset);
>>=20
>> 	fh_copy(&resp->fh, &argp->fh);
>> -	resp->status =3D nfsd_file_acquire(rqstp, &resp->fh, NFSD_MAY_WRITE |
>> -					 NFSD_MAY_NOT_BREAK_LEASE, &nf);
>> +	resp->status =3D nfsd_file_acquire_gc(rqstp, &resp->fh, NFSD_MAY_WRITE=
 |
>> +					    NFSD_MAY_NOT_BREAK_LEASE, &nf);
>> 	if (resp->status)
>> 		goto out;
>> 	resp->status =3D nfsd_commit(rqstp, &resp->fh, nf, argp->offset,
>> diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
>> index 9ebd67d461f9..4921144880d3 100644
>> --- a/fs/nfsd/trace.h
>> +++ b/fs/nfsd/trace.h
>> @@ -742,7 +742,8 @@ DEFINE_CLID_EVENT(confirmed_r);
>> 	__print_flags(val, "|",						\
>> 		{ 1 << NFSD_FILE_HASHED,	"HASHED" },		\
>> 		{ 1 << NFSD_FILE_PENDING,	"PENDING" },		\
>> -		{ 1 << NFSD_FILE_REFERENCED,	"REFERENCED"})
>> +		{ 1 << NFSD_FILE_REFERENCED,	"REFERENCED"},		\
>> +		{ 1 << NFSD_FILE_GC,		"GC"})
>>=20
>> DECLARE_EVENT_CLASS(nfsd_file_class,
>> 	TP_PROTO(struct nfsd_file *nf),
>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
>> index 44f210ba17cc..89d682a56fc4 100644
>> --- a/fs/nfsd/vfs.c
>> +++ b/fs/nfsd/vfs.c
>> @@ -1060,7 +1060,7 @@ __be32 nfsd_read(struct svc_rqst *rqstp, struct sv=
c_fh *fhp,
>> 	__be32 err;
>>=20
>> 	trace_nfsd_read_start(rqstp, fhp, offset, *count);
>> -	err =3D nfsd_file_acquire(rqstp, fhp, NFSD_MAY_READ, &nf);
>> +	err =3D nfsd_file_acquire_gc(rqstp, fhp, NFSD_MAY_READ, &nf);
>> 	if (err)
>> 		return err;
>>=20
>> @@ -1092,7 +1092,7 @@ nfsd_write(struct svc_rqst *rqstp, struct svc_fh *=
fhp, loff_t offset,
>>=20
>> 	trace_nfsd_write_start(rqstp, fhp, offset, *cnt);
>>=20
>> -	err =3D nfsd_file_acquire(rqstp, fhp, NFSD_MAY_WRITE, &nf);
>> +	err =3D nfsd_file_acquire_gc(rqstp, fhp, NFSD_MAY_WRITE, &nf);
>> 	if (err)
>> 		goto out;

--
Chuck Lever