Changes since v3 [1]: * Move from an fallocate(2) interface to a new mmap(2) flag and rename 'immutable' to 'sealed'. * Do not record the sealed state in permanent metadata it is now purely a temporary state for as long as a MAP_DIRECT vma is referencing the inode (Christoph) * Drop the CAP_IMMUTABLE requirement, but do require a PROT_WRITE mapping. [1]: https://lwn.net/Articles/730570/ --- This is the next revision of a patch series that aims to enable applications that otherwise need to resort to DAX mapping a raw device file to instead move to a filesystem. In the course of reviewing a previous posting, Christoph said: That being said I think we absolutely should support RDMA memory registrations for DAX mappings. I'm just not sure how S_IOMAP_IMMUTABLE helps with that. We'll want a MAP_SYNC | MAP_POPULATE to make sure all the blocks are populated and all ptes are set up. Second we need to make sure get_user_page works, which for now means we'll need a struct page mapping for the region (which will be really annoying for PCIe mappings, like the upcoming NVMe persistent memory region), and we need to guarantee that the extent mapping won't change while the get_user_pages holds the pages inside it. I think that is true due to side effects even with the current DAX code, but we'll need to make it explicit. And maybe that's where we need to converge - "sealing" the extent map makes sense as such a temporary measure that is not persisted on disk, which automatically gets released when the holding process exits, because we sort of already do this implicitly. It might also make sense to have explicitly breakable seals similar to what I do for the pNFS blocks kernel server, as any userspace RDMA file server would also need those semantics. So, this is an attempt to converge on the idea that we need an explicit and process-lifetime-temporary mechanism for a process to be able to make assumptions about the mapping to physical page to dax-file-offset relationship. The "explicitly breakable seals" aspect is not addressed in these patches, but I wonder if it might be a voluntary mechanism that can implemented via userfaultfd. These pass a basic smoke test and are meant to just gauge 'right track' / 'wrong track'. The main question it seems is whether the pinning done in this patchset is too early (applies before get_user_pages()) and too coarse (applies to the whole file). Perhaps this is where I discarded too easily Jan's suggestion to look at Peter Z's mm_mpin() syscall [2]? On the other hand, the coarseness and simple lifetime rules of MAP_DIRECT make it an easy mechanism to implement and explain. Another reason I kept the scope of S_IOMAP_SEALED coarsely defined was to support Dave's desired use case of sealing for operating on reflinked files [3]. Suggested mmap(2) man page edits are included in the changelog of patch 3. [2]: https://lwn.net/Articles/600502/ [3]: https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1467677.html --- Dan Williams (3): fs, xfs: introduce S_IOMAP_SEALED mm: introduce MAP_VALIDATE a mechanism for adding new mmap flags fs, xfs: introduce MAP_DIRECT for creating block-map-sealed file ranges fs/attr.c | 10 +++ fs/dax.c | 2 + fs/open.c | 6 ++ fs/read_write.c | 3 + fs/xfs/libxfs/xfs_bmap.c | 5 + fs/xfs/xfs_bmap_util.c | 3 + fs/xfs/xfs_file.c | 107 ++++++++++++++++++++++++++++++++ fs/xfs/xfs_inode.h | 1 fs/xfs/xfs_ioctl.c | 6 ++ fs/xfs/xfs_super.c | 1 include/linux/fs.h | 9 +++ include/linux/mm.h | 2 - include/linux/mm_types.h | 1 include/linux/mman.h | 3 + include/uapi/asm-generic/mman-common.h | 2 + mm/filemap.c | 5 + mm/mmap.c | 22 ++++++- 17 files changed, 183 insertions(+), 5 deletions(-)
