Re: Some thoughts on NFS

On Tue, Feb 19, 2019 at 8:03 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> A theoretical question I thought of is whether there are any
> interleavings of operations that allow a checkpoint to complete
> bogusly, while a concurrent close() in a regular backend fails with
> EIO for data that was included in the checkpoint, and panics.  I
> *suspect* the answer is that every interleaving is safe for 4.16+
> kernels that report IO errors to every descriptor.  In older kernels I
> wonder if there could be a schedule where an arbitrary backend eats
> the error while closing, then the checkpointer calls fsync()
> successfully and then logs a checkpoint, and then then the arbitrary
> backend panics (too late).  I suspect EIO on close() doesn't happen in
> practice on regular local filesystems, which is why I mention it in
> the context of NFS, but I could be wrong about that.

Ugh.  It looks like Linux NFS doesn't even use the new errseq_t
machinery in 4.16+.  So even if we had the fd-passing patch, I think
there may be a dangerous schedule like this:

A: close() -> EIO, clears AS_EIO flag
B: fsync() -> SUCCESS, log a checkpoint
A: panic!  (but it's too late, we already logged a checkpoint but
didn't flush all the dirty data the belonged to it)

-- 
Thomas Munro
https://enterprisedb.com