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

[Dan Williams]

On Fri, Aug 11, 2017 at 8:57 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> 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).

In the current patches, mapping_mapped() pins the immutable state.