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Date:   Fri, 4 Dec 2020 08:14:48 -0800 (PST)
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
References: <20201203201841.103294-1-Anna.Schumaker@Netapp.com>
 <20201203201841.103294-3-Anna.Schumaker@Netapp.com>
 <8CE5CE8C-9301-4250-B077-ACE23969C19A@oracle.com>
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 <dc191256b163d0c824c911fe7272d5443f2fd045.camel@hammerspace.com>
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> On Dec 3, 2020, at 3:39 PM, Trond Myklebust <trondmy@hammerspace.com> =
wrote:
>=20
> 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:
>>>=20
>>>=20
>>>=20
>>>> On Dec 3, 2020, at 3:18 PM, schumaker.anna@gmail.com wrote:
>>>>=20
>>>> From: Anna Schumaker <Anna.Schumaker@Netapp.com>
>>>>=20
>>>> We might need this to better handle shifting around data in the
>>>> reply
>>>> buffer.
>>>>=20
>>>> 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(-)
>>>>=20
>>>> 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;
>>>>=20
>>>> +     xdr_set_scratch_buffer(xdr, page_address(res->scratch),
>>>> PAGE_SIZE);
>>>=20
>>> I intend to submit this for v5.11:
>>>=20
>>> =
https://git.linux-nfs.org/?p=3Dcel/cel-2.6.git;a=3Dcommit;h=3D0ae4c3e8a64a=
ce1b8d7de033b0751afe43024416
>>=20
>> Thanks for the heads up! This patch can probably be deferred until
>> yours goes in.
>>=20
>>>=20
>>> But seems like enough callers need a scratch buffer that the XDR
>>> layer should set up it transparently for all requests.
>>=20
>> That could work too, and save some headache.
>>=20
>=20
> 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=3Dcel/cel-2.6.git;a=3Dcommit;h=3D5191955d6fc6=
5e6d4efe8f4f10a6028298f57281


--
Chuck Lever