Setxattr
~~~~~~~~

The 'setxattr' VFS method is used to modify the extended attributes
associated with a resource. The 'fsetxattr()' system call induces the
'setxattr' variant of the MDS_REINT RPC.

.Setxattr RPCs for Modifying a Resource's Extended Attributes
[[setxattr-rpcs]]
image::setxattr_rpcs.png["setxattr RPCs for modifying a resource's Extended Attributes",height=100]

//////////////////////////////////////////////////////////////////////
The setxattr_rpcs.png diagram resembles this text art:

Time
Step Client         MDT          OST
     -------        -------      -------
1    MDS_REINT------->
2            <-------MDS_REINT
//////////////////////////////////////////////////////////////////////

*1 - The client issues an MDS_REINT with the REINT_SETXATTR
sub-operation.*

In addition to the 'ptlrpc_body' (RPC message header), the MDS_REINT
request RPC from the client has the REINT structure
'mdt_rec_setxattr', the name of the extended attribute in question,
the extended attribute data to put in place, and a lock request
'ldlm_request'. For a detailed discussion of all the fields in the
'mdt_rec_setxattr' and 'ldlm_request' refer to <<mdt-rec-setxattr>>
and <<struct-ldlm-request>>.

.MDS_REINT:REINT_SETXATTR Request Packet Structure
image::mds-reint-setxattr-request.png["MDS_REINT:REINT_SETXATTR Request Packet Structure",height=50]

//////////////////////////////////////////////////////////////////////
The mds-reint-setxattr-request.png diagram resembles this text art:

        MDS_REINT:
      --REINT_SETXATTR-request-------------------------
      | ptlrpc_body | mdt_rec_setxattr | ldlm_request |
      -------------------------------------------------
//////////////////////////////////////////////////////////////////////

The 'setxattr' wants to set the 'ctime' associated with the EA update
as well, so the one 'sx_valid' flag it uses is OBD_MD_FLCTIME
(0x00008). The 'sx_valid' field also has the OBD_MD_FLXATTR
(0x01000000000) flag set, though that is never checked or used.

The 'ldlm_request' structure encompasses an early lock cancellation
(see <<early-lock-cancellation>>) on the lock that the client had
previously acquired for the target resource. The lock handle
identifies this lock. Only lock_count and lock_handle are used, and
the rest of the ldlm_request is cleared, i.e. all fields set to
zero. In some instances the ldlm_request is empty (all zeros)
indicating early lock cancellation is not being used. This may be a
bug in the code for that instance.

*2 - The MDS_REINT reply acknowledges the updated attributes.*

In addition to the 'ptlrpc_body' (RPC message header), the MDS_REINT
reply RPC to the client has the 'mdt_body' structure. For a detailed
discussion of the fields in the 'mdt_body' refer to <<struct-mdt-body>>.

.MDS_REINT:REINT_SETXATTR Reply Packet Structure
image::mds-reint-setxattr-reply.png["MDS_REINT:REINT_SETXATTR Reply Packet Structure",height=50]

//////////////////////////////////////////////////////////////////////
The mds-reint-setxattr-reply.png diagram resembles this text art:

      --REINT_SETXATTR-reply-----
      | ptlrpc_body | mdt_body |
      --------------------------
//////////////////////////////////////////////////////////////////////

The reply from the MDT after the setxattr operation has no valid
'mdt_body' fields, 'mbo_valid = 0x0.