Re: [PATCH 6/6] NFSv4: allow getacl rpc to allocate pages on demand

["J. Bruce Fields" <bfields@redhat.com>]

On Tue, Feb 21, 2017 at 10:21:05PM +0100, Andreas Gruenbacher wrote:
> On Tue, Feb 21, 2017 at 7:46 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
> > Hi Andreas-
> >
> >
> >> On Feb 20, 2017, at 4:31 PM, Andreas Gruenbacher <agruenba@redhat.com> wrote:
> >>
> >> On Mon, Feb 20, 2017 at 6:15 PM, J. Bruce Fields <bfields@redhat.com> wrote:
> >>> On Mon, Feb 20, 2017 at 11:42:31AM -0500, Chuck Lever wrote:
> >>>>
> >>>>> On Feb 20, 2017, at 11:09 AM, J. Bruce Fields <bfields@redhat.com> wrote:
> >>>>>
> >>>>> On Sun, Feb 19, 2017 at 02:29:03PM -0500, Chuck Lever wrote:
> >>>>>>
> >>>>>>> On Feb 18, 2017, at 9:07 PM, J. Bruce Fields <bfields@redhat.com> wrote:
> >>>>>>>
> >>>>>>> From: Weston Andros Adamson <dros@netapp.com>
> >>>>>>>
> >>>>>>> Instead of preallocating pags, allow xdr_partial_copy_from_skb() to
> >>>>>>> allocate whatever pages we need on demand.  This is what the NFSv3 ACL
> >>>>>>> code does.
> >>>>>>
> >>>>>> The patch description does not explain why this change is
> >>>>>> being done.
> >>>>>
> >>>>> The only justification I see is avoiding allocating pages unnecessarily.
> >>>>
> >>>> That makes sense. Is there a real world workload that has seen
> >>>> a negative effect?
> >>>>
> >>>>
> >>>>> Without this patch, for each getacl, we allocate 17 pages (if I'm
> >>>>> calculating correctly) and probably rarely use most of them.
> >>>>>
> >>>>> In the v3 case I think it's 7 pages instead of 17.
> >>>>
> >>>> I would have guessed 9. Out of curiosity, is there a reason
> >>>> documented for these size limits?
> >>>
> >>>
> >>> In the v4 case:
> >>>
> >>>        #define NFS4ACL_MAXPAGES DIV_ROUND_UP(XATTR_SIZE_MAX, PAGE_SIZE)
> >>>
> >>> And I believe XATTR_SIZE_MAX is a global maximum on the size of any
> >>> extend attribute value.
> >>
> >> XATTR_SIZE_MAX is the maximum size of an extended attribute. NFSv4
> >> ACLs are passed through unchanged in "system.nfs4_acl".
> >
> > "Extended attribute" means this is a Linux-specific limit?
> 
> Yes.
> 
> > Is there anything that prevents a non-Linux system from constructing
> > or returning an ACL that is larger than that?
> 
> No.

In the >=v4.1 case there are session limits, but they'll typically be
less.  In the 4.0 case I think there's no explicit limit at all.  In
practice I bet other systems are similar to Linux in that the assume
peers won't send rpc replies or requests larger than about the
maximum-sized read or write.  But again that'll usually be a higher
limit than our ACL limit.

> > What happens on a Linux client when a server returns an ACL that does
> > not fit in this allotment?
> 
> I would hope an error, but I haven't tested it.

I haven't tested either, but it looks to me like the rpc layer receives
a truncated request, the xdr decoding recognizes that it's truncated,
and the result is an -ERANGE.

Looking now I think that my "NFSv4: simplify getacl decoding" changes
that to an -EIO.  More importantly, it makes that an EIO even when the
calling application was only asking for the length, not the actual ACL
data.  I'll fix that.

--b.