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

From: "J. Bruce Fields" <bfields@redhat.com>
To: Chuck Lever <chuck.lever@oracle.com>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>,
	Anna Schumaker <schumakeranna@gmail.com>,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	Dros Adamson <dros@primarydata.com>,
	Weston Andros Adamson <dros@netapp.com>
Subject: Re: [PATCH 6/6] NFSv4: allow getacl rpc to allocate pages on demand
Date: Mon, 20 Feb 2017 12:15:19 -0500	[thread overview]
Message-ID: <20170220171519.GE12335@parsley.fieldses.org> (raw)
In-Reply-To: <4824B968-4ED6-44AA-A935-3D309D76EFFF@oracle.com>

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.

In the v3 case:

	/* Maximum number of ACL entries over NFS */
	#define NFS_ACL_MAX_ENTRIES     1024

	#define NFSACL_MAXPAGES         ((2*(8+12*NFS_ACL_MAX_ENTRIES) + PAGE_SIZE-1) \
	                                 >> PAGE_SHIFT)

No idea where that 1024 comes from.

--b.