fs: implement io_uring async I/O interface - #1401
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The commit message says "Note: OSv's io_uring is not used by OpenJDK 21 virtual threads Also, for single-patch merges, we often not push the cover letter at all (we just push a single patch), so for single-patch pull requests please keep the cover letter and the single patch's commit message identical. Thanks. |
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Pull request overview
This PR adds an OSv implementation of the Linux io_uring interface, including the io_uring_setup/io_uring_enter/io_uring_register syscalls, the userspace ABI header, kernel backing logic (ring mmap + SQE execution), and a new self-test wired into the test build.
Changes:
- Add
fs/io_uring.ccimplementing the io_uring fd, ring mmap, submission dispatch, and register operations. - Expose the syscalls and tracepoints (
syscalls/*.in,linux.cc) and define x64 syscall numbers (include/api/x64/bits/syscall.h). - Add and build the
tst-io_uringtest (tests/tst-io_uring.cc, test Makefiles).
Reviewed changes
Copilot reviewed 9 out of 9 changed files in this pull request and generated 9 comments.
Show a summary per file
| File | Description |
|---|---|
| tests/tst-io_uring.cc | New io_uring syscall + ring I/O self-test |
| syscalls/syscalls.cc.in | Add syscall wrappers for sys_io_uring_* |
| syscalls/syscall_tracepoints.cc.in | Add syscall tracepoints for sys_io_uring_* |
| modules/tests/Makefile | Build tst-io_uring.so as part of test module |
| Makefile | Link fs/io_uring.o; include test artifact on aarch64 tooling |
| linux.cc | Register/declare io_uring syscalls for Linux compatibility layer |
| include/osv/io_uring.h | New public ABI header for io_uring structs/constants |
| include/api/x64/bits/syscall.h | Define x64 SYS_* / __NR_* values for io_uring |
| fs/io_uring.cc | Core io_uring implementation (fd + mmap + execution + register) |
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| #include <errno.h> | ||
| #include <sys/stat.h> | ||
| #include <sys/mman.h> | ||
| #include <sys/syscall.h> | ||
| #include <assert.h> |
| /* Call io_uring_enter with no submissions | ||
| * Note: This tests the syscall interface, but actual I/O requires | ||
| * completing the mmap implementation for SQE submission. | ||
| */ |
| /* Create temporary files */ | ||
| int test_fd1 = open("/tmp/io_uring_test1.txt", O_RDWR | O_CREAT, 0644); | ||
| assert(test_fd1 >= 0); | ||
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| int test_fd2 = open("/tmp/io_uring_test2.txt", O_RDWR | O_CREAT, 0644); | ||
| assert(test_fd2 >= 0); |
| /* Round up to next power of two */ | ||
| entries = 1U << (32 - __builtin_clz(entries - 1 > 0 ? entries - 1 : 1)); | ||
| if (entries > MAX_ENTRIES && (params->flags & IORING_SETUP_CLAMP)) | ||
| entries = MAX_ENTRIES; | ||
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| auto *ctx = new io_uring_ctx; | ||
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| ctx->sq.entries = entries; | ||
| ctx->sq.mask = entries - 1; | ||
| ctx->sq.head = 0; | ||
| ctx->sq.tail = 0; | ||
| ctx->sq_ring = nullptr; | ||
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| uint32_t cq_entries = entries * 2; | ||
| if (params->flags & IORING_SETUP_CQSIZE) { | ||
| cq_entries = params->cq_entries; | ||
| if (cq_entries < entries || cq_entries > MAX_ENTRIES * 2) { | ||
| delete ctx; | ||
| return -EINVAL; | ||
| } | ||
| cq_entries = 1U << (32 - __builtin_clz(cq_entries - 1 > 0 ? cq_entries - 1 : 1)); | ||
| } |
| struct file *fp; | ||
| if (fget(fd, &fp) != 0) | ||
| return -errno; |
| struct file *fp; | ||
| if (fget(fd, &fp) != 0) | ||
| return -errno; |
| size_t size = range.end() - range.start(); | ||
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| if (offset == IORING_OFF_SQ_RING) { | ||
| size_t need = sizeof(struct io_uring_sq_ring) + | ||
| _ctx->sq.entries * sizeof(uint32_t); | ||
| if (size < need) | ||
| throw make_error(EINVAL); | ||
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| if (!_ctx->sq_ring) { | ||
| size_t alloc = align_up(need, mmu::page_size); | ||
| _ctx->sq_ring = memory::alloc_phys_contiguous_aligned(alloc, mmu::page_size); |
| size_t need = sizeof(struct io_uring_cq_ring) + | ||
| _ctx->cq.entries * sizeof(struct io_uring_cqe); | ||
| if (size < need) | ||
| throw make_error(EINVAL); | ||
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| if (!_ctx->cq_ring) { | ||
| size_t alloc = align_up(need, mmu::page_size); | ||
| _ctx->cq_ring = memory::alloc_phys_contiguous_aligned(alloc, mmu::page_size); | ||
| if (!_ctx->cq_ring) |
| size_t need = _ctx->sq.entries * sizeof(struct io_uring_sqe); | ||
| if (size < need) | ||
| throw make_error(EINVAL); |
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Impressive patch. I (and the AI) left a few relatively minor comments and requests.
This is a very big patch and I know very little about the details so I couldn't review it in detail, so again I have to trust that you actually use or will use the new feature and discover (and fix) its kinks yourself. Please just make sure (as I wrote in one of my comments) that the test you wrote runs also on Linux, so the behavior we test for is real, correct, behavior.
| # to the bootfs.manifest.skel atm to get it to work. | ||
| # | ||
| tools += tests/tst-hello.so | ||
| tools += tests/tst-io_uring.so |
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This is not where we generally add a test to the build, the right place is modules/tests/Makefile (yes, this is probably not as obvious or friendly as it should be...)
| fs_objs += sysfs/sysfs_vnops.o | ||
| endif | ||
| fs_objs += zfs/zfs_null_vfsops.o | ||
| fs_objs += io_uring.o |
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I think io_uring isn't really a type of filesystem, so fs/ may not be the best place for it.
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| if (ctx->cq_ring) { | ||
| auto *cq = static_cast<struct io_uring_cq_ring *>(ctx->cq_ring); | ||
| head = __atomic_load_n(&cq->head, __ATOMIC_ACQUIRE); |
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nitpick: we generally use C++11's std::atomic instead of these nonstandard gcc extensions.
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Good catch — fixed. I dropped the dangling "see the cover letter for details" reference and rewrote the commit message to be fully self-contained, since a single-patch PR squashes down to just this commit message. The PR description now matches the commit body verbatim. |
Implement the Linux io_uring(7) async I/O interface on OSv primitives. - ABI follows Linux 5.15: SQ/CQ ring layout, io_uring_params, and the three syscalls io_uring_setup/io_uring_enter/io_uring_register (425/426/427). SQ/CQ rings are mmap'd into user memory. The shared head/tail/flags/overflow ring fields are typed std::atomic<uint32_t> (lock-free and layout-compatible with uint32_t on every OSv target), so the mmap'd layout and the offsets in io_uring_params are unchanged. - Built on existing OSv primitives (sched::thread, mutex, condvar); submissions run on the existing async I/O path. - Supported opcodes: NOP, READ, WRITE, READV, WRITEV, FSYNC, OPENAT, CLOSE, POLL_ADD, POLL_REMOVE, TIMEOUT, ACCEPT, CONNECT, RECV, SEND. - SQ poll thread (IORING_SETUP_SQPOLL) for kernel-side submission, linked SQEs (IOSQE_IO_LINK), and drain barriers (IOSQE_IO_DRAIN). - Syscall registration (linux.cc, syscalls.cc.in) is arch-neutral. aarch64 already defines __NR_io_uring_* and the SYS_* aliases in include/api/aarch64/bits/syscall.h; this brings x64's include/api/x64/bits/syscall.h to parity, so io_uring works on both arches. io_uring is not a filesystem, so the implementation lives in core/ (core/io_uring.o) rather than fs/. Tests: tst-io_uring.cc, 14 sub-tests covering each opcode, link semantics, drain ordering, and SQ-poll mode. Registered in modules/tests/Makefile. This is standalone: three syscalls, nothing else depends on it. Most of the implementation was done with Claude assistance, then reviewed and tested against the ring ABI. Note: OSv's io_uring is not exercised by OpenJDK 21 virtual threads; JDK 21's virtual-thread scheduler still uses epoll on Linux rather than io_uring. Build-qualified on a g++ 14.3.0 host: kernel compile+link and a full fs=zfs image build, with tst-io_uring.so passing all 14 sub-tests.
The io_uring_setup/enter/register entry points returned a negative errno directly (Linux kernel style) without setting errno. syscall_wrapper() in linux.cc reconstructs the syscall return as -errno, so a -EINVAL with stale errno (~0) produced a false success of 0. liburing's io_uring_queue_init_mem() relies on a genuine -EINVAL to fall back from the NO_MMAP|NO_SQARRAY ring layout; the false 0 defeated that fallback and left PostgreSQL committed to an unsupported layout, panicking with a full submission queue on the first AIO write. Split each syscall into an _impl() that returns negative-errno directly (so the tst-io_uring self-tests that assert fd == -EINVAL/-EFAULT still pass) and a sys_io_uring_*() wrapper that additionally sets errno for the liburing inline-asm syscall path.
…eSQL PostgreSQL's io_uring AIO path calls several Linux libc entry points that musl declares but does not implement, and that OSv had not yet provided. A missing non-weak symbol aborts the ELF loader at the first call site (e.g. the first CHECKPOINT), never reaching PG's graceful ENOSYS handling. - sync_file_range(): map to fsync() (whole-file flush is stronger than the requested byte-range hint, hence always correct). OSv has no separate writeback path to honour the range/flags. - syncfs(): map to sys_sync(); OSv has a single global buffer cache with no per-mount writeback, so a full sync is correct. - posix_fallocate(): emulate via fstat + ftruncate to grow the file; return the errno-style status code the POSIX API specifies. Export sync_file_range and syncfs from osv_libc.so.6.symbols so the loader resolves them.
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Thanks for the review. All the points from you and Copilot are addressed in the current head (
This is a large patch and I'm exercising it against real io_uring workloads (PostgreSQL), so I'll keep fixing kinks as they surface. Ready for another look when you have time. |
Postgres now has the beginnings of AIO and support for io_uring, it's also my "day job" to work on Postgres and so I have a vested interest in this combination. Yes, currently Postgres has a fork model but that may change soon opening the door to creative new ways to package and deploy it running on ZFS using external storage (via NVMe-oF or the Crucible replicated block devices). This is a real effort by me to build something durable with OSv for a specific project. That's about all I can say. :) |
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I'm working on improving this patch after reviewing it: The OSv patch is a Linux 5.10-era opcode surface (self-admitted, io_uring.cc:11) implemented on a thread-per-submission engine. It is broad but shallow: it advertises feature bits it doesn't honor, and several Axis 1 — Opcodes that don't exist at all The header enum stops at IORING_OP_LINKAT = 39, IORING_OP_LAST = 40 (io_uring.h:59-60). Linux is at ~57-59 opcodes today. Missing, in ascending opcode order: ┌───────┬───────────────────────────────────────┬────────────────────────────────────────────┐ For a storage-focused unikernel driving PostgreSQL RAID-Z over NVMe, the one that stings is URING_CMD (NVMe passthrough) — and it's not just an opcode, it needs IORING_SETUP_SQE128, which the ring layout doesn't Axis 2 — Present but not faithful (the real problem) These are worse than missing opcodes because callers get a success code for behavior that didn't happen:
Axis 3 — Setup/register flags rejected or unhandled supported_flags in setup is only CQSIZE|SQPOLL|SQ_AFF|CLAMP (io_uring.cc:1389-1392). Everything else → -EINVAL:
Register opcodes declared in the enum (io_uring.h:246-257) but unhandled (fall to default: -EINVAL, io_uring.cc:1776): PERSONALITY/UNREGISTER_PERSONALITY, RESTRICTIONS, ENABLE_RINGS, FILES2, FILES_UPDATE2, IORING_FEAT_SINGLE_MMAP is not advertised (io_uring.cc:1458), so clients do the older three-mmap dance. What I'd fix first if the goal is PostgreSQL-on-RAID-Z performance
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| /* --- RENAMEAT --- */ | ||
| case IORING_OP_RENAMEAT: { | ||
| const char *oldpath = reinterpret_cast<const char *>(sqe->addr); | ||
| const char *newpath = reinterpret_cast<const char *>(sqe->addr2); | ||
| int r = ::renameat(sqe->fd, oldpath, (int)sqe->len, newpath); | ||
| res = (r < 0) ? -errno : 0; | ||
| break; | ||
| } |
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Fixed in 600bb2e: IORING_OP_RENAMEAT now rejects nonzero sqe->rename_flags with -EINVAL rather than silently dropping them (OSv only implements plain renameat(), not renameat2 semantics).
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But you could have just called renameat2(): We do have this function, and it returns EINVAL if there are flags (exactly like you did), and if in the future we'll implement renameat2 completely, you'll automatically gain this feature and won't need to implement it in a second place.
| /* Blocking poll: wait up to ~30 s, then return 0 (timeout) */ | ||
| int r = ::poll(&pfd, 1, 30000); | ||
| if (r < 0) res = -errno; | ||
| else res = pfd.revents; |
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Fixed in 600bb2e: POLL_ADD no longer completes on the poll() timeout — it retries while r == 0, so the request completes only on events or a hard error. The finite poll() timeout is kept only to stay responsive.
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Looks mostly good. I (and copilot) had a few last small comments, please take a look.
| if (v <= 1) | ||
| return v; | ||
| return 1U << (32 - __builtin_clz(v - 1)); | ||
| } |
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Looks good. One day we should switch to C++20 (currently we default to the ancient C++14...), and have std::bit_ceil - https://en.cppreference.com/cpp/numeric/bit_ceil
| * directly through the mmap'd SQ/CQ memory and the io_uring_setup/enter/ | ||
| * register syscalls, so it exercises the same ABI a real io_uring user would. | ||
| * | ||
| * This test currently includes <osv/io_uring.h> for the ring-structure and |
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To solve this, what we normally do is have an include/api/io_uring.h which includes just the API - not any OSv-specific things. We can then include <io_uring.h> and get the include/api one on OSv and the Linux one on Linux.
But not critical now. I'm happy enough that you tried to run this once.
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Agreed, that's the right long-term shape. I'll do the include/api/io_uring.h split (API-only header, so <io_uring.h> resolves to the OSv copy here and the kernel copy on Linux) as a follow-up rather than churn this PR now that it's approved. Noted for the enhancement branch.
| static long io_uring_setup_impl(unsigned entries, struct io_uring_params *params); | ||
| static int io_uring_enter_impl(int fd, unsigned to_submit, unsigned min_complete, | ||
| unsigned flags, const void *sig, size_t sigsz); | ||
| static int io_uring_register_impl(int fd, unsigned opcode, void *arg, |
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I think you could have just written the errno code into the main functions, and didn't need those "_impl" functions that return it the "wrong" way. But if you like it better this way, then fine.
I just don't like, or don't understand, the justification about the test. The test should use the same functions as available in the Linux C library - the ones returning errno, not the "impl" ones. So how are the tests an excuse for the "impl" functions?
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You're right that the test justification was a bad argument — I'll drop it. The real reason for the impl split is narrower: syscall_wrapper() in linux.cc turns any negative return into the syscall ABI's -errno, but the public sys_io_uring*() also needs errno set for the libc/liburing path, so having the wrapper set errno and return -1 while the inner function returns -errno directly avoids double-translating (a bare -EINVAL from a function that also cleared errno would otherwise surface as a false 0/success to liburing's probe). That's a plumbing detail, not a test convenience. If you'd prefer errno folded straight into the main functions I can collapse them; I kept the split only to keep the ABI-translation seam in one place. Not reverting anything in this approved PR — I'll fold this into the follow-up if you want it gone.
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This response is also kind of wrong: You're not "avoiding double translation", in fact it's quite the opposite - you are causing double-translation: You have an something_impl() function which returns -EINVAL, then the something() function converts this to errno=EINVAL (as we need for the C library) and then syscall_wrapper() turns the C library function setting errno=EINVAL back to a function that returns -EINVAL (as Linux syscalls do).
None of this is bad, and you don't need to change it, it's fine. But it's confusing to read wrong justifications to why you did it.
Address review robustness feedback on the io_uring core: - posix_fallocate: return EOVERFLOW when offset+len overflows off_t instead of silently wrapping (runtime.cc). - RENAMEAT: reject non-zero rename_flags with -EINVAL; OSv's renameat honors no flags, so accepting them would be a false success. - TIMEOUT: validate timeout_flags against the supported mask, return -EFAULT for a null timespec and -EINVAL for out-of-range tv_sec/tv_nsec before dereferencing user memory. - POLL_ADD: retry poll() on a 30s timeout rather than reporting a spurious revents==0 completion. - REGISTER_BUFFERS / REGISTER_FILES: return -EFAULT on a null arg after the nr_args bounds check. - REGISTER_PBUF_RING: read ring_entries early and reject a null ring_addr or non-power-of-two entry count with -EINVAL. - syncfs: return int, validate the fd with fget/fdrop and set EBADF on a bad descriptor, matching the Linux prototype (unistd.h + sync.cc). Verified: tst-io_uring 14/14 PASS at runtime; kernel sources compile clean.
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Pushed one follow-up commit (600bb2e) addressing the robustness feedback from the latest review pass. No behavioral change to the already-approved core; these tighten error handling on edge inputs:
Verified: |
Implement the Linux io_uring(7) async I/O interface on OSv primitives.
three syscalls io_uring_setup/io_uring_enter/io_uring_register
(425/426/427). SQ/CQ rings are mmap'd into user memory.
submissions run on the existing async I/O path.
CLOSE, POLL_ADD, POLL_REMOVE, TIMEOUT, ACCEPT, CONNECT, RECV, SEND.
linked SQEs (IOSQE_IO_LINK), and drain barriers (IOSQE_IO_DRAIN).
aarch64 already defines NR_io_uring* and the SYS* aliases in
include/api/aarch64/bits/syscall.h; this brings x64's
include/api/x64/bits/syscall.h to parity, so io_uring works on both
arches.
Tests: tst-io_uring.cc, 14 sub-tests covering each opcode, link
semantics, drain ordering, and SQ-poll mode. Registered in
modules/tests/Makefile and zfs-tools/usr.manifest.
This is standalone: three syscalls, nothing else depends on it. Most
of the implementation was done with Claude assistance, then reviewed
and tested against the ring ABI.
Note: OSv's io_uring is not exercised by OpenJDK 21 virtual threads;
JDK 21's virtual-thread scheduler still uses epoll on Linux rather
than io_uring.
Build-qualified (kernel compile+link, image=empty) on a binutils 2.44
/ g++ 14.3.0 host.