Re: [PATCH RFC 5/5] block, nvme: add failed_bio callback for multipath bio failover

[Keith Busch <kbusch@kernel.org>]

On Wed, May 20, 2026 at 09:07:49AM -0600, Keith Busch wrote:
> On Wed, May 20, 2026 at 09:27:46AM +0200, Christoph Hellwig wrote:
> > On Tue, May 19, 2026 at 10:23:26AM -0700, Keith Busch wrote:
> > > From: Keith Busch <kbusch@kernel.org>
> > >
> > > The nvme driver has long utilized a zero capacity to indicate the path
> > > isn't reachable, which creates a race condition with IO dispatch when
> > > paths are being detached on a live system: when the block layer rejects
> > > a bio early due to a capacity check failure, drivers with multipath
> > > support using the original bio have no interception point to redirect
> > > the bio to another path.
> >
> > Trying to reverse-engineer - the problem is that the block-layer
> > code catches being beyond the capacity and directly completes the bio,
> > right?
> 
> Yes, and in the case being addressed here, the "zero capacity" setting
> is path specific, hence the driver wants to attempt a failover. I
> imagine general capacity violations are not path specific though, so
> this is kind of a weird case.

Oh, and it's not just the zero capacity IO error that multipath wants to
hanlde. It's also that we've marked the path's disk dead, so there's a
race if bio_queue_enter() will call bio_io_error() that this patch
handles. I should have mentioned that case too, which wasn't handled
with the BIO_REMAPPED flag suggestion.