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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 18:27:54 +0000
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> 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 cod=
e
>>> 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?

I don't recall exactly, it was a late last summer when I wrote all these.

Approaching the end of a page, IIRC, the unpatched code would leave
a gap in the directory entry stream.


> --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 files
>>> might be good. Also pynfs could easily send a single compound with lots
>>> 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 tha=
t
>>> exactly hit the various corner cases may be hard.  But maybe if we just
>>> send a bunch of compounds that vary over some range we can still
>>> guarantee hitting those cases.
>>>=20
>>> --b.
>>=20
>> --
>> Chuck Lever

--
Chuck Lever