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Subject: Re: [PATCH 3/3] NFS: Avoid copy of xdr padding in read()
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <6f13978155f7f6fd6cc885f9efdb13c0e890faf3.camel@hammerspace.com>
Date:   Thu, 19 Nov 2020 17:58:16 -0500
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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Message-Id: <F2A105B4-6395-45ED-ABFF-DD6A0EBE1D79@oracle.com>
References: <20201118221939.20715-1-trondmy@kernel.org>
 <20201118221939.20715-2-trondmy@kernel.org>
 <20201118221939.20715-3-trondmy@kernel.org>
 <42FFB4EC-5E31-4002-92FC-7CA329479D78@oracle.com>
 <57b085d32f624986412770d10cc4daa8211ee0f4.camel@hammerspace.com>
 <D322F599-E680-4715-AD9A-CC6017AFF8E0@oracle.com>
 <6f13978155f7f6fd6cc885f9efdb13c0e890faf3.camel@hammerspace.com>
To:     Trond Myklebust <trondmy@hammerspace.com>
Precedence: bulk


> On Nov 19, 2020, at 9:34 AM, Trond Myklebust <trondmy@hammerspace.com> =
wrote:
>=20
> On Thu, 2020-11-19 at 09:31 -0500, Chuck Lever wrote:
>>=20
>>=20
>>> On Nov 19, 2020, at 9:30 AM, Trond Myklebust <
>>> trondmy@hammerspace.com> wrote:
>>>=20
>>> On Thu, 2020-11-19 at 09:17 -0500, Chuck Lever wrote:
>>>>=20
>>>>=20
>>>>> On Nov 18, 2020, at 5:19 PM, trondmy@kernel.org wrote:
>>>>>=20
>>>>> From: Trond Myklebust <trond.myklebust@hammerspace.com>
>>>>>=20
>>>>> When doing a read() into a page, we also don't care if the nul
>>>>> padding
>>>>> stays in that last page when the data length is not 32-bit
>>>>> aligned.
>>>>=20
>>>> What if the READ payload lands in the middle of a file? The
>>>> pad on the end will overwrite file content just past where
>>>> the READ payload lands.
>>>=20
>>> If the size > buf->page_len, then it gets truncated in
>>> xdr_align_pages() afaik.
>>=20
>> I will need to check how RPC/RDMA behaves. It might build a
>> chunk that includes the pad in this case, which would break
>> things.
>=20
> That would be a bug in the existing code too, then. It shouldn't be
> writing beyond the buffer size we set in the NFS layer.

Testing now with xfstests, which should include fsx with direct
I/O of odd sizes. So far I haven't seen any unexpected behavior.

But I'm not sure what copy you're trying to avoid. This one in
xdr_align_pages() ?

1189         else if (nwords < xdr->nwords) {
1190                 /* Truncate page data and move it into the tail */
1191                 offset =3D buf->page_len - len;
1192                 copied =3D xdr_shrink_pagelen(buf, offset);
1193                 trace_rpc_xdr_alignment(xdr, offset, copied);
1194                 xdr->nwords =3D XDR_QUADLEN(buf->len - cur);
1195         }

We set up the receive buffer already to avoid this copy. It should
rarely, if ever, happen. That's the point of rpc_prepare_reply_pages().


>>>>> Signed-off-by: Trond Myklebust
>>>>> <trond.myklebust@hammerspace.com>
>>>>> ---
>>>>> fs/nfs/nfs2xdr.c | 2 +-
>>>>> fs/nfs/nfs3xdr.c | 2 +-
>>>>> fs/nfs/nfs4xdr.c | 2 +-
>>>>> 3 files changed, 3 insertions(+), 3 deletions(-)
>>>>>=20
>>>>> diff --git a/fs/nfs/nfs2xdr.c b/fs/nfs/nfs2xdr.c
>>>>> index db9c265ad9e1..468bfbfe44d7 100644
>>>>> --- a/fs/nfs/nfs2xdr.c
>>>>> +++ b/fs/nfs/nfs2xdr.c
>>>>> @@ -102,7 +102,7 @@ static int decode_nfsdata(struct xdr_stream
>>>>> *xdr, struct nfs_pgio_res *result)
>>>>>         if (unlikely(!p))
>>>>>                 return -EIO;
>>>>>         count =3D be32_to_cpup(p);
>>>>> -       recvd =3D xdr_read_pages(xdr, count);
>>>>> +       recvd =3D xdr_read_pages(xdr, xdr_align_size(count));
>>>>>         if (unlikely(count > recvd))
>>>>>                 goto out_cheating;
>>>>> out:
>>>>> diff --git a/fs/nfs/nfs3xdr.c b/fs/nfs/nfs3xdr.c
>>>>> index d3e1726d538b..8ef7c961d3e2 100644
>>>>> --- a/fs/nfs/nfs3xdr.c
>>>>> +++ b/fs/nfs/nfs3xdr.c
>>>>> @@ -1611,7 +1611,7 @@ static int decode_read3resok(struct
>>>>> xdr_stream *xdr,
>>>>>         ocount =3D be32_to_cpup(p++);
>>>>>         if (unlikely(ocount !=3D count))
>>>>>                 goto out_mismatch;
>>>>> -       recvd =3D xdr_read_pages(xdr, count);
>>>>> +       recvd =3D xdr_read_pages(xdr, xdr_align_size(count));
>>>>>         if (unlikely(count > recvd))
>>>>>                 goto out_cheating;
>>>>> out:
>>>>> diff --git a/fs/nfs/nfs4xdr.c b/fs/nfs/nfs4xdr.c
>>>>> index 755b556e85c3..5baa767106dc 100644
>>>>> --- a/fs/nfs/nfs4xdr.c
>>>>> +++ b/fs/nfs/nfs4xdr.c
>>>>> @@ -5202,7 +5202,7 @@ static int decode_read(struct xdr_stream
>>>>> *xdr, struct rpc_rqst *req,
>>>>>                 return -EIO;
>>>>>         eof =3D be32_to_cpup(p++);
>>>>>         count =3D be32_to_cpup(p);
>>>>> -       recvd =3D xdr_read_pages(xdr, count);
>>>>> +       recvd =3D xdr_read_pages(xdr, xdr_align_size(count));
>>>>>         if (count > recvd) {
>>>>>                 dprintk("NFS: server cheating in read reply: "
>>>>>                                 "count %u > recvd %u\n", count,
>>>>> recvd);
>>>>> --=20
>>>>> 2.28.0
>=20
> --=20
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
>=20
>=20

--
Chuck Lever