Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 4 Sep 2020 09:52:59 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Anna Schumaker <schumaker.anna@gmail.com>
Cc:     "J. Bruce Fields" <bfields@redhat.com>,
        Chuck Lever <chuck.lever@oracle.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH v4 2/5] NFSD: Add READ_PLUS data support
Message-ID: <20200904135259.GB26706@fieldses.org>
References: <20200817165310.354092-1-Anna.Schumaker@Netapp.com>
 <20200817165310.354092-3-Anna.Schumaker@Netapp.com>
 <20200828212521.GA33226@pick.fieldses.org>
 <20200828215627.GB33226@pick.fieldses.org>
 <CAFX2Jfn3LN9Zc-=4mAm1mQ3k8PN6C1yF4xqh6B-yyXCxFnp7hQ@mail.gmail.com>
 <20200901164938.GC12082@fieldses.org>
 <CAFX2Jf=vmnfV_4=401=BFnmZJCOqfEWTQRPHzRHePpJrTCcb7w@mail.gmail.com>
 <20200901191854.GD12082@fieldses.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200901191854.GD12082@fieldses.org>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-nfs-owner@vger.kernel.org
Precedence: bulk

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:
> > >
> > > 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?
> > > >
> > > > The minimum size seems to be PAGE_SIZE from everything I've seen.
> > >
> > > OK, can we make that assumption explicit?  It'd simplify stuff like
> > > this.
> > 
> > I'm okay with that, but it's technically up to the underlying filesystem.
> 
> Maybe we should ask on linux-fsdevel.
> 
> 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.
> 
> Minimum hole alignment might be the right idea.
> 
> 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.

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.

--b.