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 10:45:35PM +0100, Andreas Gruenbacher wrote:
> On Tue, Feb 21, 2017 at 10:37 PM, J. Bruce Fields <bfields@redhat.com> wrote:
> > 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.
> 
> Just be careful not to return a length from getxattr(path, name, NULL,
> 0) that will cause getxattr(path, name, buffer, size) to fail with
> ERANGE, please. Otherwise, user space might get very confused.

Ugh, OK.  So there could be userspace code that does something like

	while (getxattr(path, name, buf, size) == -ERANGE) {
		/* oops, must have raced with a size change */
		size = getxattr(path, name, NULL, 0);
		buf = realloc(buf, size);
	}

and you'd consider that a kernel bug not a userspace bug?

I suspect that can happen both before and after my changes.

So what do we want for that case?  Just -EIO?

--b.