Re: [PATCH V1] nvme-pci: Fix NULL pointer dereference in nvme_pci_prp_iter_next

[Keith Busch <kbusch@kernel.org>]

On Tue, Feb 03, 2026 at 01:50:12PM +0000, Robin Murphy wrote:
> > Can dev->dma_skip_sync be modified in parallel with this check?
> > If so, dma_need_unmap() may return different results depending on the
> > time at which it is invoked.
> 
> It can if another thread is making mappings in parallel, however as things
> currently stand that would only lead to the current thread thinking it must
> save the unmap state for the mappings it's already made even if it
> technically didn't need to.
> 
> In principle it could also change back the other way if another thread reset
> the device's DMA mask, but doing that with active mappings would
> fundamentally break things in regard to the dma_skip_sync mechanism anyway.

We can handle a change in dma_needs_unmap() from false -> true. Worst
case scenario is we save off a descriptor when we didn't really need to,
but it's just a small memory use for the lifetime of the IO.

We don't correctly handle a transition from true -> false, though. We'd
currently leak memory if that happened. It sounds like that transition
is broken for other reasons too, so I won't bother trying to handle it.