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.
