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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: <6d072290a67fc60b3cdd146dbb16cd529c29a8ec.camel@hammerspace.com>
Date:   Fri, 20 Nov 2020 12:51:48 -0500
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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Message-Id: <1404959A-B680-4413-9E6F-7ED6ABB4F3B4@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>
 <F2A105B4-6395-45ED-ABFF-DD6A0EBE1D79@oracle.com>
 <40f07903bf0cb1b80c6fa99f3465c4b1d771b027.camel@hammerspace.com>
 <6A3A6033-F7AC-4518-9D84-B420D05CFA4E@oracle.com>
 <6d072290a67fc60b3cdd146dbb16cd529c29a8ec.camel@hammerspace.com>
To:     Trond Myklebust <trondmy@hammerspace.com>
Precedence: bulk


> On Nov 20, 2020, at 12:14 PM, Trond Myklebust =
<trondmy@hammerspace.com> wrote:
>=20
> On Fri, 2020-11-20 at 09:52 -0500, Chuck Lever wrote:
>>=20
>>=20
>>> On Nov 19, 2020, at 7:58 PM, Trond Myklebust <
>>> trondmy@hammerspace.com> wrote:
>>>=20
>>> On Thu, 2020-11-19 at 17:58 -0500, Chuck Lever wrote:
>>>>=20
>>>>=20
>>>>> 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.
>>>>=20
>>>> Testing now with xfstests, which should include fsx with direct
>>>> I/O of odd sizes. So far I haven't seen any unexpected behavior.
>>>>=20
>>>> But I'm not sure what copy you're trying to avoid. This one in
>>>> xdr_align_pages() ?
>>>>=20
>>>> 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         }
>>>>=20
>>>> 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().
>>>=20
>>>=20
>>> ...and the point of padding here is to avoid unaligned access to
>>> memory. That is completely broken by this whole mechanism, which
>>> causes
>>> us to place the real data in the tail in an unaligned buffer that
>>> follows this padding.
>>=20
>> I've never seen run-time complaints about unaligned accesses of
>> the tail data. Do you have a reproducer? (And obviously this
>> applies only to NFSv4 COMPOUND results, right?)
>=20
> I don't think we can trigger it anywhere right now mainly because =
we're
> careful never to put any further ops after a read, readdir or =
readlink,
> even though we probably should append a VERIFY to some of those so as
> to protect data caching (particularly for readlink).
>=20
>>=20
>>> Furthermore, rpc_prepare_reply_pages() only ever places the padding
>>> in
>>> the tail _if_ our buffer size is already not 32-bit aligned.
>>> Otherwise,
>>> we're engaging in this pointless exercise of making the tail buffer
>>> data unaligned after the fact.
>>=20
>> Architecturally, I agree that it would be best if the tail buffer
>> presented the XDR data items aligned on 4 bytes.
>>=20
>> But I do not agree that a pad goes in the pages. Some transports do
>> not send an XDR pad for unaligned data payloads. Transports have to
>> have a way of avoiding that pad. If it no longer goes in the tail,
>> then how will they do that?
>>=20
>> We could perhaps have a new flag, XDRBUF_IMPLICIT_PAD, that means
>> the transport has to add a pad to xdr->pages on send, or did not add
>> a pad on receive.
>>=20
>=20
> I don't see why we would need any of this.

Right. I was conflating the send and receive side. You're talking
about only receive. This would be a problem when the structure of
the reply doesn't allow the client to align the pages and tail[]
in advance.

So, READ/READLINK/READDIR/LISTXATTRS, on either TCP, or with RDMA
and krb5i and krb5p where XDR pads have to stay inline. I'm still
a little troubled by what happens to the pad during a short READ.

The common case for READ requests is page-aligned payloads, so I
don't think tail content alignment is a performance issue. Do you
concur?


> If the pad is being directly
> placed in the tail by the RPC transport layer, then there is nothing =
in
> xdr_read_pages() that will move it or change the alignment of the
> existing code in the tail. The call just notes that xdr_align_pages()
> returned a length that was shorter than the one we supplied, and
> continues to skip the padding in the tail.

That's what I observe today. I'm trying to understand the usage
scenario where the tail content is not word-aligned. Probably can
happen on server-side receives too.


> The one thing that perhaps is broken here is that if we supply a =
length
> to xdr_read_pages() that is in fact larger than xdr_align_size(xdr-
>> buf->page_len), then we should skip additional data that was placed =
in
> the tail, but that is a bug with the existing xdr_read_pages() and is
> unaffected by these patches.

I think I understand what these patches do well enough to drop my
objection, pending further exercise with a testing barrage.


>>>>>>>>> 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
>>>>=20
>>>> --
>>>> Chuck Lever
>>>>=20
>>>>=20
>>>>=20
>>>=20
>>> --=20
>>> Trond Myklebust
>>> Linux NFS client maintainer, Hammerspace
>>> trond.myklebust@hammerspace.com
>>=20
>> --
>> Chuck Lever
>>=20
>>=20
>>=20
>=20
> --=20
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com

--
Chuck Lever