Re: [PATCH v4 2/5] NFSD: Add READ_PLUS data support

["J. Bruce Fields" <bfields@fieldses.org>]

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.

--b.