Re: Too many ENOSPC errors

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2023-06-12 at 19:04 +0000, Trond Myklebust wrote:
> On Mon, 2023-06-12 at 13:30 -0400, Jeff Layton wrote:
> > On Mon, 2023-06-12 at 11:58 -0400, Jeff Layton wrote:
> > 
> > > 
> > > Got it: I think I see what's happening. filemap_sample_wb_err just
> > > calls
> > > errseq_sample, which does this:
> > > 
> > > errseq_t errseq_sample(errseq_t
> > > *eseq)                                                             
> > > {                                                                  
> > >                                  
> > >         errseq_t old =
> > > READ_ONCE(*eseq);                                                  
> > >           
> > >                                                                    
> > >                                  
> > >         /* If nobody has seen this error yet, then we can be the
> > > first. */                          
> > >         if (!(old &
> > > ERRSEQ_SEEN))                                                      
> > >              
> > >                 old =
> > > 0;                                                                 
> > >            
> > >         return
> > > old;                                                               
> > >                   
> > > }                                                          
> > > 
> > > Because no one has seen that error yet (ERRSEQ_SEEN is clear), the
> > > write
> > > ends up being the first to see it and it gets back a 0, even though
> > > the
> > > error happened before the sample.
> > > 
> > > The above behavior is what we want for the sample that we do at
> > > open()
> > > time, but not what's needed for this use-case. We need a new helper
> > > that
> > > samples the value regardless of whether it has already been seen:
> > > 
> > > errseq_t errseq_peek(errseq_t *eseq)
> > > {
> > >         return READ_ONCE(*eseq);
> > > }
> > > 
> > > ...but we'll also need to fix up errseq_check to handle differences
> > > between the SEEN bit.
> > > 
> > > I'll see if I can spin up a patch for that. Stay tuned.
> > 
> > This may not be fixable with the way that NFS is trying to use
> > errseq_t.
> > 
> > The fundamental problem is that we need to mark the errseq_t in the
> > mapping as SEEN when we sample it, to ensure that a later error is
> > recorded and not ignored.
> > 
> > But...if the error hasn't been reported yet and we mark it SEEN here,
> > and then a later error doesn't occur, then a later open won't have
> > its
> > errseq_t set to 0, and that unseen error could be lost.
> > 
> > It's a bit of a pity: as originally envisioned, the errseq_t
> > mechanism
> > would provide for this sort of use case, but we added this patch not
> > long after the original code went in, and it changed those semantics:
> > 
> >     b4678df184b3 errseq: Always report a writeback error once
> > 
> > I don't see a good way to do this using the current errseq_t
> > mechanism,
> > given these competing needs. I'll keep thinking about it though.
> > Maybe
> > we could add some sort of store and forward mechanism for fsync on
> > NFS?
> > That could get rather complex though.
> > 
> > Cheers,
> 
> Does RHEL-8 have commit 6c984083ec24, 064109db53ec, d95b26650e86,
> e6005436f6cc, 9641d9bc9b75, and cea9ba7239dc applied?
> 

Ben is working on backporting those as we speak. Hopefully we can get
RHEL8's state closer to where upstream is.

I'm also working on a patch for upstream that should give Chris the
expected behavior in this test.
-- 
Jeff Layton <jlayton@kernel.org>