Re: [PATCH V2 2/2] NFSD: allow client to use write delegation stateid for READ

[Chuck Lever <chuck.lever@oracle.com>]

On 2/24/25 4:11 PM, Dai Ngo wrote:
> 
> On 2/24/25 6:48 AM, Chuck Lever wrote:
>> On 2/21/25 6:42 PM, Dai Ngo wrote:
>>> Allow READ using write delegation stateid granted on OPENs with
>>> OPEN4_SHARE_ACCESS_WRITE only, to accommodate clients whose WRITE
>>> implementation may unavoidably do (e.g., due to buffer cache
>>> constraints).
>>>
>>> When the server offers a write delegation for an OPEN with
>>> OPEN4_SHARE_ACCESS_WRITE, the file access mode, the nfs4_file
>>> and nfs4_ol_stateid are upgraded as if the OPEN was sent with
>>> OPEN4_SHARE_ACCESS_BOTH.
>>>
>>> When this delegation is returned or revoked, the corresponding open
>>> stateid is looked up and if it's found then the file access mode,
>>> the nfs4_file and nfs4_ol_stateid are downgraded to remove the read
>>> access.
>> I probably don't understand something. The patch description seems to
>> suggest that a WR_ONLY OPEN state ID is also granted read in this case?
> 
> Currently nfsd allows a WR_ONLY OPEN state ID to do READ. The access check
> is done in access_permit_read:
> 
> static inline int
> access_permit_read(struct nfs4_ol_stateid *stp)
> {
>         return test_access(NFS4_SHARE_ACCESS_READ, stp) ||
>                 test_access(NFS4_SHARE_ACCESS_BOTH, stp) ||
>                 test_access(NFS4_SHARE_ACCESS_WRITE, stp);       <<====
> }
> 
> Is this behavior intentional or is it a bug?

RFC 8881 Section 9.1.2 makes an exception for this case, so not a bug.

One assumes this is to permit clients to perform RMW, but a comment
above this helper would have alleviated some confusion amongst us
software historians.


> -Dai
> 
>>
>>
>>> Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
>>> ---
>>>   fs/nfsd/nfs4state.c | 62 +++++++++++++++++++++++++++++++++++++++++++++
>>>   fs/nfsd/state.h     |  2 ++
>>>   2 files changed, 64 insertions(+)
>>>
>>> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
>>> index b533225e57cf..0c14f902c54c 100644
>>> --- a/fs/nfsd/nfs4state.c
>>> +++ b/fs/nfsd/nfs4state.c
>>> @@ -6126,6 +6126,51 @@ nfs4_delegation_stat(struct nfs4_delegation
>>> *dp, struct svc_fh *currentfh,
>>>       return rc == 0;
>>>   }
>>>   +/*
>>> + * Upgrade file access mode to include FMODE_READ. This is called
>>> only when
>>> + * a write delegation is granted for an OPEN with
>>> OPEN4_SHARE_ACCESS_WRITE.
>>> + */
>>> +static void
>>> +nfs4_upgrade_rdwr_file_access(struct nfs4_ol_stateid *stp)
>>> +{
>>> +    struct nfs4_file *fp = stp->st_stid.sc_file;
>>> +    struct nfsd_file *nflp;
>>> +    struct file *file;
>>> +
>>> +    spin_lock(&fp->fi_lock);
>>> +    nflp = fp->fi_fds[O_WRONLY];
>>> +    file = nflp->nf_file;
>>> +    file->f_mode |= FMODE_READ;
>>> +    swap(fp->fi_fds[O_RDWR], fp->fi_fds[O_WRONLY]);
>>> +    clear_access(NFS4_SHARE_ACCESS_WRITE, stp);
>>> +    set_access(NFS4_SHARE_ACCESS_BOTH, stp);
>>> +    __nfs4_file_get_access(fp, NFS4_SHARE_ACCESS_READ);    /* incr
>>> fi_access[O_RDONLY] */
>>> +    spin_unlock(&fp->fi_lock);
>>> +}
>>> +
>>> +/*
>>> + * Downgrade file access mode to remove FMODE_READ. This is called when
>>> + * a write delegation, granted for an OPEN with
>>> OPEN4_SHARE_ACCESS_WRITE,
>>> + * is returned.
>>> + */
>>> +static void
>>> +nfs4_downgrade_wronly_file_access(struct nfs4_ol_stateid *stp)
>>> +{
>>> +    struct nfs4_file *fp = stp->st_stid.sc_file;
>>> +    struct nfsd_file *nflp;
>>> +    struct file *file;
>>> +
>>> +    spin_lock(&fp->fi_lock);
>>> +    nflp = fp->fi_fds[O_RDWR];
>>> +    file = nflp->nf_file;
>>> +    file->f_mode &= ~FMODE_READ;
>>> +    swap(fp->fi_fds[O_WRONLY], fp->fi_fds[O_RDWR]);
>>> +    clear_access(NFS4_SHARE_ACCESS_BOTH, stp);
>>> +    set_access(NFS4_SHARE_ACCESS_WRITE, stp);
>>> +    spin_unlock(&fp->fi_lock);
>>> +    nfs4_file_put_access(fp, NFS4_SHARE_ACCESS_READ);    /* decr.
>>> fi_access[O_RDONLY] */
>>> +}
>>> +
>>>   /*
>>>    * The Linux NFS server does not offer write delegations to NFSv4.0
>>>    * clients in order to avoid conflicts between write delegations and
>>> @@ -6207,6 +6252,11 @@ nfs4_open_delegation(struct nfsd4_open *open,
>>> struct nfs4_ol_stateid *stp,
>>>           dp->dl_cb_fattr.ncf_cur_fsize = stat.size;
>>>           dp->dl_cb_fattr.ncf_initial_cinfo =
>>> nfsd4_change_attribute(&stat);
>>>           trace_nfsd_deleg_write(&dp->dl_stid.sc_stateid);
>>> +
>>> +        if ((open->op_share_access & NFS4_SHARE_ACCESS_BOTH) ==
>>> NFS4_SHARE_ACCESS_WRITE) {
>>> +            dp->dl_stateid = stp->st_stid.sc_stateid;
>>> +            nfs4_upgrade_rdwr_file_access(stp);
>>> +        }
>>>       } else {
>>>           open->op_delegate_type = deleg_ts ?
>>> OPEN_DELEGATE_READ_ATTRS_DELEG :
>>>                               OPEN_DELEGATE_READ;
>>> @@ -7710,6 +7760,8 @@ nfsd4_delegreturn(struct svc_rqst *rqstp,
>>> struct nfsd4_compound_state *cstate,
>>>       struct nfs4_stid *s;
>>>       __be32 status;
>>>       struct nfsd_net *nn = net_generic(SVC_NET(rqstp), nfsd_net_id);
>>> +    struct nfs4_ol_stateid *stp;
>>> +    struct nfs4_stid *stid;
>>>         if ((status = fh_verify(rqstp, &cstate->current_fh, S_IFREG,
>>> 0)))
>>>           return status;
>>> @@ -7724,6 +7776,16 @@ nfsd4_delegreturn(struct svc_rqst *rqstp,
>>> struct nfsd4_compound_state *cstate,
>>>         trace_nfsd_deleg_return(stateid);
>>>       destroy_delegation(dp);
>>> +
>>> +    if (dp->dl_stateid.si_generation && dp-
>>> >dl_stateid.si_opaque.so_id) {
>>> +        if (!nfsd4_lookup_stateid(cstate, &dp->dl_stateid,
>>> +                SC_TYPE_OPEN, 0, &stid, nn)) {
>>> +            stp = openlockstateid(stid);
>>> +            nfs4_downgrade_wronly_file_access(stp);
>>> +            nfs4_put_stid(stid);
>>> +        }
>>> +    }
>>> +
>>>       smp_mb__after_atomic();
>>>       wake_up_var(d_inode(cstate->current_fh.fh_dentry));
>>>   put_stateid:
>>> diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
>>> index 74d2d7b42676..3f2f1b92db66 100644
>>> --- a/fs/nfsd/state.h
>>> +++ b/fs/nfsd/state.h
>>> @@ -207,6 +207,8 @@ struct nfs4_delegation {
>>>         /* for CB_GETATTR */
>>>       struct nfs4_cb_fattr    dl_cb_fattr;
>>> +
>>> +    stateid_t        dl_stateid;  /* open stateid */
>>>   };
>>>     static inline bool deleg_is_read(u32 dl_type)
>>


-- 
Chuck Lever