Re: [PATCH v2 0/5] fs, xfs: block map immutable files for dax, dma-to-storage, and swap

[Andy Lutomirski]

On Fri, Aug 11, 2017 at 3:26 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Fri, Aug 11, 2017 at 3:44 AM, Christoph Hellwig <hch@xxxxxx> wrote:
>> Please explain how this interface allows for any sort of safe userspace
>> DMA.
>
> So this is where I continue to see S_IOMAP_IMMUTABLE being able to
> support applications that MAP_SYNC does not. Dave mentioned userspace
> pNFS4 servers, but there's also Samba and other protocols that want to
> negotiate a direct path to pmem outside the kernel. Xen support has
> thus far not been able to follow in the footsteps of KVM enabling due
> to a dependence on static M2P tables that assume a static
> guest-physical to host-physical relationship [1]. Immutable files
> would allow Xen to follow the same "mmap a file" semantic as KVM.

One thing that makes me quite nervous about S_IOMAP_IMMUTABLE is the
degree to which things go badly if one program relies on it while
another program clears the flag: you risk corrupting unrelated
filesystem metadata.  I think a userspace interface to pin the extent
mapping of a file really wants a way to reliably keep it pinned (or to
reliably zap the userspace application if it gets unpinned).