Re: [PATCH 2/3] NFS: Allocate a scratch page for READ_PLUS

From: Chuck Lever <chuck.lever@oracle.com>
To: Trond Myklebust <trondmy@hammerspace.com>
Cc: Anna Schumaker <schumaker.anna@gmail.com>,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH 2/3] NFS: Allocate a scratch page for READ_PLUS
Date: Fri, 4 Dec 2020 08:14:48 -0800 (PST)	[thread overview]
Message-ID: <8D97BD94-8518-4AAC-9789-996D956A02CF@oracle.com> (raw)
In-Reply-To: <dc191256b163d0c824c911fe7272d5443f2fd045.camel@hammerspace.com>

> On Dec 3, 2020, at 3:39 PM, Trond Myklebust <trondmy@hammerspace.com> wrote:
> 
> On Thu, 2020-12-03 at 15:31 -0500, Anna Schumaker wrote:
>> On Thu, Dec 3, 2020 at 3:27 PM Chuck Lever <chuck.lever@oracle.com>
>> wrote:
>>> 
>>> 
>>> 
>>>> On Dec 3, 2020, at 3:18 PM, schumaker.anna@gmail.com wrote:
>>>> 
>>>> From: Anna Schumaker <Anna.Schumaker@Netapp.com>
>>>> 
>>>> We might need this to better handle shifting around data in the
>>>> reply
>>>> buffer.
>>>> 
>>>> Suggested-by: Trond Myklebust <trond.myklebust@hammerspace.com>
>>>> Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
>>>> ---
>>>> fs/nfs/nfs42xdr.c       |  2 ++
>>>> fs/nfs/read.c           | 13 +++++++++++--
>>>> include/linux/nfs_xdr.h |  1 +
>>>> 3 files changed, 14 insertions(+), 2 deletions(-)
>>>> 
>>>> diff --git a/fs/nfs/nfs42xdr.c b/fs/nfs/nfs42xdr.c
>>>> index 8432bd6b95f0..ef095a5f86f7 100644
>>>> --- a/fs/nfs/nfs42xdr.c
>>>> +++ b/fs/nfs/nfs42xdr.c
>>>> @@ -1297,6 +1297,8 @@ static int nfs4_xdr_dec_read_plus(struct
>>>> rpc_rqst *rqstp,
>>>>       struct compound_hdr hdr;
>>>>       int status;
>>>> 
>>>> +     xdr_set_scratch_buffer(xdr, page_address(res->scratch),
>>>> PAGE_SIZE);
>>> 
>>> I intend to submit this for v5.11:
>>> 
>>> https://git.linux-nfs.org/?p=cel/cel-2.6.git;a=commit;h=0ae4c3e8a64ace1b8d7de033b0751afe43024416
>> 
>> Thanks for the heads up! This patch can probably be deferred until
>> yours goes in.
>> 
>>> 
>>> But seems like enough callers need a scratch buffer that the XDR
>>> layer should set up it transparently for all requests.
>> 
>> That could work too, and save some headache.
>> 
> 
> Why not just integrate it into xdr_init_decode_pages(), and then add a
> matching xdr_exit_decode_pages() to free up any allocated page?

Sounds OK.

For comparison, to support xdr_stream decoding on the server, I've
changed svc_rqst_alloc() to grab a page that stays around until the
nfsd thread dies. There is a new svcxdr_init_decode() API that
attaches that page for use as the decoding scratch buffer.

Since it's a new API, the call sites that set up new streams with
xdr_init_decode() are not affected.

See:

https://git.linux-nfs.org/?p=cel/cel-2.6.git;a=commit;h=5191955d6fc65e6d4efe8f4f10a6028298f57281

--
Chuck Lever