On Fri, Aug 11, 2017 at 3:44 AM, Christoph Hellwig <hch@xxxxxx> wrote: > On Sun, Aug 06, 2017 at 11:51:50AM -0700, Dan Williams wrote: >> Of course it's a useful API. An application already needs to worry >> about the block map, that's why we have fallocate, msync, fiemap >> and... > > Fallocate and msync do not expose the block map in any way. Proof: > they work just fine over say nfs. Right, but they let userspace make inferences about the state of metadata relative to I/O to a given storage address. In this regard S_IOMAP_IMMUTABLE is no different than MAP_SYNC, but 'immutable' goes a step further to let an application infer that the storage address is stable. This enables applications that MAP_SYNC does not, see below. > fiemap does indeed expose the block map, which is the whole point. > But it's a debug tool that we don't event have a man page for. And > it's not usable for anything else, if only for the fact that it doesn't > tell you what device your returned extents are relative to. True, one couldn't just use immutable + fiemap and expect to have the right storage device. > >> > We've been through this a few times but let me repeat it: The only >> > sensible API gurantee is one that is observable and usable. >> >> I'm missing how block-map immutable files violate this observable and >> usable constraint? > > What is the observable behavior of an extent map change? How can you > describe your immutable extent map behavior so that when I violate > them by e.g. moving one extent to a different place on disk you can > observe that in userspace? The violation is blocked, it's immutable. Using this feature means the application is taking away some of the kernel's freedom. That is a valid / safe tradeoff for the set of applications that would otherwise resort to raw device access. > >> This immutable approach should also go in, it solves the same problem >> without the the latency drawback, > > How is your latency going to be any different from MAP_SYNC on > a fully allocated and pre-zeroed file? So, I went back and read Jan's patches, and in the pre-allocated case I don't think we can get stuck behind a backlog of dirty metada flushing since the implementation only seems to take the synchronous fault path if the fault dirtied the block map. >> Beyond flush from userspace it also >> can be used to solve the swapfile problems you highlighted > > Which swapfile problem? The TOCTOU problem of enabling swap vs reflink that you mentioned in your criticism of the daxctl syscall, but now that I look your comments were based on the *general* case use of bmap(), However, xfs in particular as of commits: eb5e248d502b xfs: don't allow bmap on rt files db1327b16c2b xfs: report shared extent mappings to userspace correctly ...doesn't appear to have this problem. That said Dave's idea to use immutable + unwritten extents for swap makes sense to me. That's a feature, not a bug fix, but I went ahead and appended a proof-of-concept implementation to the v3 posting. >> and it >> allows safe ongoing dma to a filesystem-dax mapping beyond what we can >> already do with direct-I/O. > > 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. Applications that just want flush from userspace can use MAP_SYNC, those that need to temporarily pin the block for RDMA can use the in-kernel pNFS server, and those that need to coordinate both from userspace can use S_IOMAP_IMMUTABLE. It's a continuum, not a competition. [1]: https://lists.xen.org/archives/html/xen-devel/2017-04/msg00427.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
