[pnfs] device export ops

[dean hildebrand]

Hello,

At Bakeathon we had a discussion regarding the device export ops.
Beyond updating them for draft-13, we talked reducing the number of
export ops and data copies it requires to retrieve device information.
 The current ops retrieve the device, encode it, and them free it,
which is quite cumbersome and requires extra data copies.  We noticed
that readdir only has a single vfs op, so why can't we do something
similar.  In addition, nfs readdir only calls this vfs op at xdr time,
not in nfs4proc.

To that end, Benny suggested moving to an iterator concept.  To
retrieve a device list, NFSD continually calls the same export op,
each time returning the next device in the list (using cookies, etc as
an index)  Benny had suggested 2 ops, but I think we might need just
one.

To get feedback on the plan, here is the plan and the proposed data
structures.  The main goals of this plan is to:
a) Reduce the number of export ops
b) Copy device information only once (onto the xdr buffer).

The current 4 export ops related to devices would be replaced with a
single *get_device* export op.  *get_device* would take as arguments:
1. superblock
2. nfsd4_pnfs_getdev struct: Filled with the client arguments.  For
getdeviceinfo, it has a deviceid.  For getdevicelist, it has a valid
cookie.
3. A xdr stream to encode the device
4. A function pointer, used mainly for the file layout type.  The
function will be called by the underlying file system to encode a
device on the xdr stream. (This is also similar to readdir)

Essentially GETDEVICEINFO and GETDEVICELIST will do the same thing on
the server except:

For GETDEVICEINFO:
1. Set gd_devid to a non-zero value and call *get_device* once.  This
will encode the device info for gd_devid onto the output stream. (This
assumes devices cannot have a id of 0.  Is this ok?  If not, gd_devid
could be a pointer.)

For GETDEVICELIST:
1. Set gd_device to 0 and set gd_cookie to be the value from the
client. (If gd_device is a pointer, we would set it to NULL)
2. Call *get_device* multiple times to encode all the devices.  Stop
when either nfserr_toosmall, nfserr_eof, or another error is returned.


DATA STRUCTURES:

typedef int (*nfsd41_encodedev_t)(struct nfsd4_export_xdr* xdr, void *device);

struct nfsd4_export_xdr {
        u32 *p;            /* in */
        u32 *end;          /* in */
        u32 bytes_written; /* out */
};

struct nfsd4_pnfs_devcookie {
        u64                             gd_cookie;
        u64                             gd_verf;
};

/* Filled in via the getdevicelist and getdeviceinfo arguments from
the client */
struct nfsd4_pnfs_getdev {
        u32                             gd_type;           /* request */
        u32                             gd_maxcount;    /* request */
        u32                             gd_devid;          /* request */
        struct nfsd4_pnfs_devcookie     gd_cookie;   /* req/resp */
};

/* pNFS:
 * Args: sb - Current superblock
 *       getdev - type, maxcount and either a devid or a cookie
 *       xdr    - begin and end of xdr buffer, outputs num of bytes written
 *       encdev - Function which does encoding of device
 * Return: nfs_ok, nfserr_toosmall, nfserr, nfserr_eof */
int (*get_device) (struct super_block *sb,
                          void *getdev,  /* struct nfsd4_pnfs_getdev */
                          struct nfsd4_export_xdr *xdr,
                          nfsd41_encodedev_t encdev);


Dean