Re: [PATCH v7 07/12] dma-mapping: introduce dma_has_iommu()

[Christoph Hellwig <hch@lst.de>]

On Thu, Oct 12, 2017 at 01:10:33PM -0700, Dan Williams wrote:
> On Thu, Oct 12, 2017 at 11:27 AM, Jason Gunthorpe
> <jgunthorpe@obsidianresearch.com> wrote:
> > On Tue, Oct 10, 2017 at 01:17:26PM -0700, Dan Williams wrote:
> >
> >> Also keep in mind that what triggers the lease break is another
> >> application trying to write or punch holes in a file that is mapped
> >> for RDMA. So, if the hardware can't handle the iommu mapping getting
> >> invalidated asynchronously and the application can't react in the
> >> lease break timeout period then the administrator should arrange for
> >> the file to not be written or truncated while it is mapped.
> >
> > That makes sense, but why not return ENOSYS or something to the app
> > trying to alter the file if the RDMA hardware can't support this
> > instead of having the RDMA app deal with this lease break weirdness?
> 
> That's where I started, an inode flag that said "hands off, this file
> is busy", but Christoph pointed out that we should reuse the same
> mechanisms that pnfs is using. The pnfs protection scheme uses file
> leases, and once the kernel decides that a lease needs to be broken /
> layout needs to be recalled there is no stopping it, only delaying.

That was just a suggestion - the important statement is that a hands
off flag is just a no-go.

> However, chatting this over with a few more people I have an alternate
> solution that effectively behaves the same as how non-ODP hardware
> handles this case of hole punch / truncation today. So, today if this
> scenario happens on a page-cache backed mapping, the file blocks are
> unmapped and the RDMA continues into pinned pages that are no longer
> part of the file. We can achieve the same thing with the iommu, just
> re-target the I/O into memory that isn't part of the file. That way
> hardware does not see I/O errors and the DAX data consistency model is
> no worse than the page-cache case.

Yikes.

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