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Subject: Re: [PATCH v4 2/5] NFSD: Add READ_PLUS data support
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <20200904140324.GC26706@fieldses.org>
Date:   Fri, 4 Sep 2020 10:07:22 -0400
Cc:     Anna Schumaker <schumaker.anna@gmail.com>,
        Bruce Fields <bfields@redhat.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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To:     Bruce Fields <bfields@fieldses.org>
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> On Sep 4, 2020, at 10:03 AM, Bruce Fields <bfields@fieldses.org> =
wrote:
>=20
> On Fri, Sep 04, 2020 at 09:56:19AM -0400, Chuck Lever wrote:
>>=20
>>=20
>>> On Sep 4, 2020, at 9:52 AM, J. Bruce Fields <bfields@fieldses.org> =
wrote:
>>>=20
>>> On Tue, Sep 01, 2020 at 03:18:54PM -0400, J. Bruce Fields wrote:
>>>> On Tue, Sep 01, 2020 at 01:40:16PM -0400, Anna Schumaker wrote:
>>>>> On Tue, Sep 1, 2020 at 12:49 PM J. Bruce Fields =
<bfields@fieldses.org> wrote:
>>>>>>=20
>>>>>> On Mon, Aug 31, 2020 at 02:16:26PM -0400, Anna Schumaker wrote:
>>>>>>> On Fri, Aug 28, 2020 at 5:56 PM J. Bruce Fields =
<bfields@redhat.com> wrote:
>>>>>>>> We really don't want to bother encoding small holes.  I doubt
>>>>>>>> filesystems want to bother with them either.  Do they give us =
any
>>>>>>>> guarantees as to the minimum size of a hole?
>>>>>>>=20
>>>>>>> The minimum size seems to be PAGE_SIZE from everything I've =
seen.
>>>>>>=20
>>>>>> OK, can we make that assumption explicit?  It'd simplify stuff =
like
>>>>>> this.
>>>>>=20
>>>>> I'm okay with that, but it's technically up to the underlying =
filesystem.
>>>>=20
>>>> Maybe we should ask on linux-fsdevel.
>>>>=20
>>>> Maybe minimum hole length isn't the right question: suppose at time =
1 a
>>>> file has a single hole at bytes 100-200, then it's modified so at =
time 2
>>>> it has a hole at bytes 50-150.  If you lseek(fd, 0, SEEK_HOLE) at =
time
>>>> 1, you'll get 100.  Then if you lseek(fd, 100, SEEK_DATA) at time =
2,
>>>> you'll get 150.  So you'll encode a 50-byte hole in the READ_PLUS =
reply
>>>> even though the file never had a hole smaller than 100 bytes.
>>>>=20
>>>> Minimum hole alignment might be the right idea.
>>>>=20
>>>> If we can't get that: maybe just teach encode_read to stop when it
>>>> *either* returns maxcount worth of file data (and holes) *or* =
maxcount
>>>> of encoded xdr data, just to prevent a weird filesystem from =
triggering
>>>> a bug.
>>>=20
>>> Alternatively, if it's easier, we could enforce a minimum alignment =
by
>>> rounding up the result of SEEK_HOLE to the nearest multiple of (say) =
512
>>> bytes, and rounding down the result of SEEK_DATA.
>>=20
>> Perhaps it goes without saying, but is there an effort to
>> ensure that the set of holes is represented in exactly the
>> same way when accessing a file via READ_PLUS and
>> SEEK_DATA/HOLE ?
>=20
> So you're thinking of something like a pynfs test that creates a file
> with holes and then tries reading through it with READ_PLUS and SEEK =
and
> comparing the results?

I hadn't considered a particular test platform, but yes, a regression
test like that would be appropriate.


> There are lots of legitimate reasons that test might "fail"--servers
> aren't required to support holes at all, and have a lot of lattitude
> about how to report them.

Agreed that the test would need to account for server support for holes.


> But it might be a good idea to test anyway.

My primary concern is that the result of a file copy operation should
look the same on NFS/TCP (with READ_PLUS) and NFS/RDMA (with =
SEEK_DATA/HOLE).


--
Chuck Lever