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Date:      Thu, 15 Feb 2007 12:45:37 +0200
From:      Kostik Belousov <kostikbel@gmail.com>
To:        Tomas Olsson <tol@it.su.se>
Cc:        freebsd-fs@freebsd.org, "Rick C. Petty" <rick-arla@kiwi-computer.com>, arla-drinkers@stacken.kth.se
Subject:   Re: Arla on FreeBSD
Message-ID:  <20070215104537.GC39168@deviant.kiev.zoral.com.ua>
In-Reply-To: <lsrmz3f680v.fsf_-_@kashyyyk.ite.kth.se>
References:  <6FC9F9894A9F8C49A722CF9F2132FC2204C9DAB6@ms05.mailstreet2003.net> <45D1F30A.6080403@freebsd.org> <eqsut3$6a3$1@sea.gmane.org> <20070213192906.U726@chrishome.localnet> <20070214162938.GA96725@keira.kiwi-computer.com> <20070214173211.L1054@chrishome.localnet> <20070214170808.GC96725@keira.kiwi-computer.com> <lsrvei45pq1.fsf@kashyyyk.ite.kth.se> <20070215044707.GA39168@deviant.kiev.zoral.com.ua> <lsrmz3f680v.fsf_-_@kashyyyk.ite.kth.se>
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On Thu, Feb 15, 2007 at 11:16:00AM +0100, Tomas Olsson wrote:
> Kostik Belousov <kostikbel@gmail.com> writes:
> > > I'm already funded and can work full time on this, but a FreeBSD hack=
er
> > > would help a lot.  Any volunteers?
> >=20
> > Sorry for me pointing out obvious, why not continue to use fs@ as place
> > where to ask ?
> >
> You're very right, I'm just to shy to do it... Thanks.
>=20
> Anyway;
>=20
> Arla is built around a "small" caching fs driver (nnpfs) servicing user
> requests by asking the 'arlad' daemon for help or just operating on local
> files created/fetched by arlad. They communicate over a char device.
>=20
> A simple read would be handled as such:
>         getnode/getdata rpc to arlad
>         installnode/installdata + wakeup msgs from arlad
>         VOP_READ() on newly fetched cache file
>=20
> Subsequent reads on the same data would skip the rpc part, unless arlad h=
as
> invalidated the node.
>=20
> Previously, there was a 1:1 mapping between nnpfs vnode and cache file. T=
he
> installdata message was then handled by fetching the cache file's vnode (=
in
> arlad's context), storing it in the nnpfs_node for future reference/acces=
s.
> Now we ended up with one cache file per "block" (large) of data, and
> decided that it would be better to open/access/close the cache "block" fi=
le
> on each access.  The closest we could get to the olden ways was to open t=
he
> directory where a node's cache blocks reside, in arlad's context.
>=20
> The interesting part is how we open and access the cache files, and from
> what context. arlad is in chroot() to avoid recursive lookups across /, a=
nd
> it seems like a good idea to avoid such lookups now too.
>=20
> So the main question is how to properly do VOP_{LOOKUP,CREATE,WRITE} etc =
on
> cache files in this dual context world, without mixing identities in bad
> ways or confusing the OS too much.
>=20
> The currently messed up code lives in
> http://cvsweb.stacken.kth.se/cvsweb.cgi/arla/nnpfs/bsd/
>=20
> Most interesting is nnpfs_vnodeops-common.c (nnpfs_write_common()) and
> nnpfs_blocks.c (open_file())

I made really quick look at the places you mentioned. I have some
comment for open_file(). For FreeBSD >=3D 6.x, the right way to open vnode
from the kernel code is to use vn_open() (and then vn_close()) API.
Something along the lines (this is for already existing file):

	td =3D curthread;
	NDINIT(&nd, LOOKUP, FOLLOW | MPSAFE, UIO_USERSPACE, fname, td);
	flags =3D FREAD | FWRITE;
	error =3D vn_open(&nd, &flags, 0, -1);
	if (error)
		return (error);
	vfslocked =3D NDHASGIANT(&nd);
	NDFREE(&nd, NDF_ONLY_PNBUF);
	vp =3D nd.ni_vp;
vp is now locked, shall be unlocked by VOP_UNLOCK() before returning to
usermode. Giant is conditionally locked based on MP-safeness of the fs vp
belongs to. When Giant-protected region shall be leaved, use
	VFS_UNLOCK_GIANT(vfslocked);

To close the vnode, use
	vn_close(vp, FREAD|FWRITE, td->td_ucred, td);

See, for instance, kern/kern_ktrace.c, ufs/ufs/ufs_quota.c or
security/audit/audit_syscalls.c for real code that does this.

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