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From:   Chuck Lever <chuck.lever@oracle.com>
To:     Bruce Fields <bfields@fieldses.org>
CC:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v1 19/42] SUNRPC: Fix xdr_get_next_encode_buffer() page
 boundary handling
Thread-Topic: [PATCH v1 19/42] SUNRPC: Fix xdr_get_next_encode_buffer() page
 boundary handling
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Date:   Wed, 3 Mar 2021 18:30:40 +0000
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> On Mar 3, 2021, at 1:27 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
>=20
>=20
>=20
>> On Mar 3, 2021, at 1:25 PM, Bruce Fields <bfields@fieldses.org> wrote:
>>=20
>> On Wed, Mar 03, 2021 at 06:19:33PM +0000, Chuck Lever wrote:
>>>=20
>>>=20
>>>> On Mar 3, 2021, at 11:52 AM, Bruce Fields <bfields@fieldses.org> wrote=
:
>>>>=20
>>>> On Wed, Mar 03, 2021 at 03:43:28PM +0000, Chuck Lever wrote:
>>>>> Why would that not be OK? the next call to xdr_get_next_encode_buffer=
()
>>>>> should do the right thing and bounce the new encoded data from the
>>>>> next page into this one again.
>>>>>=20
>>>>> So far I have not encountered any problems. Would such a problem show
>>>>> up with some frequency under normal use, or would it be especially
>>>>> subtle?
>>>>=20
>>>> I mainly just want to make sure we've got a coherent idea what this co=
de
>>>> is doing....
>>>=20
>>> Agreed, that's a good thing.
>>=20
>> I'm also a little vague on what exactly the problem is you're running
>> into.  (Probably because I haven't really looked at the v3 readdir
>> encoding.)
>>=20
>> Is it approaching the end of a page, or is it running out of buflen?
>> How exactly does it fail?
>=20
> I don't recall exactly, it was a late last summer when I wrote all these.
>=20
> Approaching the end of a page, IIRC, the unpatched code would leave
> a gap in the directory entry stream.

Well, when I converted the entry encoders to use xdr_stream, it would
have a problem around the end of a page. The existing encoders are
open-coded to deal with this case.


>> --b.
>>=20
>>>=20
>>>=20
>>>> For testing: large replies that aren't just read data are readdir and
>>>> getacl.  So reading large directories with lots of variably-named file=
s
>>>> might be good. Also pynfs could easily send a single compound with lot=
s
>>>> of variable-sized reads, that might be interesting.
>>>=20
>>> I've run the full git regression suite over NFSv3 many many times with
>>> this patch applied. That includes unpacking via tar, a full build from
>>> scratch, and then running thousands of individual tests.
>>>=20
>>> So that doesn't exercise a particular corner case, but it does reflect
>>> a broad variety of directory operations.
>>>=20
>>>=20
>>>> Constructing a compound that will result in xdr_reserve_space calls th=
at
>>>> exactly hit the various corner cases may be hard.  But maybe if we jus=
t
>>>> send a bunch of compounds that vary over some range we can still
>>>> guarantee hitting those cases.
>>>>=20
>>>> --b.
>>>=20
>>> --
>>> Chuck Lever
>=20
> --
> Chuck Lever

--
Chuck Lever