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

[Bruce Fields <bfields@fieldses.org>]

On Fri, Sep 04, 2020 at 09:56:19AM -0400, Chuck Lever wrote:
> 
> 
> > On Sep 4, 2020, at 9:52 AM, J. Bruce Fields <bfields@fieldses.org> wrote:
> > 
> > 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.
> 
> 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 ?

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?

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.

But it might be a good idea to test anyway.

--b.