Re: [PATCH v1 19/42] SUNRPC: Fix xdr_get_next_encode_buffer() page boundary handling

[Bruce Fields <bfields@fieldses.org>]

On Wed, Mar 03, 2021 at 06:19:33PM +0000, Chuck Lever wrote:
> 
> 
> > On Mar 3, 2021, at 11:52 AM, Bruce Fields <bfields@fieldses.org> wrote:
> > 
> > On Wed, Mar 03, 2021 at 03:43:28PM +0000, Chuck Lever wrote:
> >> Why would that not be OK? the next call to xdr_get_next_encode_buffer()
> >> should do the right thing and bounce the new encoded data from the
> >> next page into this one again.
> >> 
> >> So far I have not encountered any problems. Would such a problem show
> >> up with some frequency under normal use, or would it be especially
> >> subtle?
> > 
> > I mainly just want to make sure we've got a coherent idea what this code
> > is doing....
> 
> Agreed, that's a good thing.

I'm also a little vague on what exactly the problem is you're running
into.  (Probably because I haven't really looked at the v3 readdir
encoding.)

Is it approaching the end of a page, or is it running out of buflen?
How exactly does it fail?

--b.

> 
> 
> > For testing: large replies that aren't just read data are readdir and
> > getacl.  So reading large directories with lots of variably-named files
> > might be good. Also pynfs could easily send a single compound with lots
> > of variable-sized reads, that might be interesting.
> 
> I've run the full git regression suite over NFSv3 many many times with
> this patch applied. That includes unpacking via tar, a full build from
> scratch, and then running thousands of individual tests.
> 
> So that doesn't exercise a particular corner case, but it does reflect
> a broad variety of directory operations.
> 
> 
> > Constructing a compound that will result in xdr_reserve_space calls that
> > exactly hit the various corner cases may be hard.  But maybe if we just
> > send a bunch of compounds that vary over some range we can still
> > guarantee hitting those cases.
> > 
> > --b.
> 
> --
> Chuck Lever
> 
>