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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: <20200904135259.GB26706@fieldses.org>
Date:   Fri, 4 Sep 2020 09:56:19 -0400
Cc:     Bruce Fields <bfields@redhat.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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References: <20200817165310.354092-1-Anna.Schumaker@Netapp.com>
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 <20200901191854.GD12082@fieldses.org> <20200904135259.GB26706@fieldses.org>
To:     Bruce Fields <bfields@fieldses.org>,
        Anna Schumaker <schumaker.anna@gmail.com>
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> 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.

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 ?


--
Chuck Lever