Date: Tue, 17 Apr 2001 11:28:03 -0700 (PDT) From: Matt Dillon <dillon@earth.backplane.com> To: Poul-Henning Kamp <phk@critter.freebsd.dk> Cc: Kirk McKusick <mckusick@mckusick.com>, Julian Elischer <julian@elischer.org>, Rik van Riel <riel@conectiva.com.br>, freebsd-hackers@FreeBSD.ORG, David Xu <bsddiy@21cn.com> Subject: Re: vm balance Message-ID: <200104171828.f3HIS3E96964@earth.backplane.com> References: <24891.987530101@critter>
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:In message <200104171745.f3HHj2K95255@earth.backplane.com>, Matt Dillon writes:
:>:> I don't think NFS relies on vnodes never being freed.
:>:
:>:It does, in some case nfs stashes a vnode pointer and the v_id
:>:value away, and some time later tries to use that pair to try to
:>:refind the vnode again. If you free vnodes, it will still think
:>:the pointer is a vnode and if junk happens to be right it will
:>:think it is still a vnode. QED: Bad things (TM) will happen.
:>:
:>:# cd /sys/nfs
:>:# grep v_id *
:>:nfs_nqlease.c: vpid = vp->v_id;
:>:nfs_nqlease.c: if (vpid == vp->v_id) {
:>:nfs_nqlease.c: if (vpid == vp->v_id &&
:>:nfs_vnops.c: vpid = newvp->v_id;
:>:nfs_vnops.c: if (vpid == newvp->v_id) {
:>:
:>:--
:>:Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
:>
:> hahahahahahahaha.. Look at the code more closely. v_id is not
:> managed by NFS, it's managed by vfs_cache.c. There's a big XXX
:> comment just before cache_purge() that explains it. Believe me,
:> NFS is the least of your worries here.
:
:Matt, you try to free vnodes back to the malloc pool and you will
:see what happens OK ?
:
:--
:Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
ok ok... lets see. Oh, ok I see what it's doing. Actually I think
you just found a bug.
if ((error = cache_lookup(dvp, vpp, cnp)) && error != ENOENT) {
struct vattr vattr;
int vpid;
if ((error = VOP_ACCESS(dvp, VEXEC, cnp->cn_cred, p)) != 0) {
*vpp = NULLVP;
return (error);
}
newvp = *vpp;
vpid = newvp->v_id;
This is totally bogus. VOP_ACCESS can block, so even using vpid above
to check that the vnode hasn't been ripped out from under the code won't
work.
Also, take a look at the vput() later on, and also the vput() in
kern/vfs_cache.c/vfs_cache_lookup() - that looks bogus to me too and
would probably crash the machine.
The easiest solution here is to make cache_lookup bump the ref count
on the returned vnode and require that all users of cache_lookup vrele()
it.
-Matt
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