On Fri, Feb 8, 2019 at 6:35 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
> If we have fixed user buffers, we can map them into the kernel when we
> setup the io_uring. That avoids the need to do get_user_pages() for
> each and every IO.
>
> To utilize this feature, the application must call io_uring_register()
> after having setup an io_uring instance, passing in
> IORING_REGISTER_BUFFERS as the opcode. The argument must be a pointer to
> an iovec array, and the nr_args should contain how many iovecs the
> application wishes to map.
>
> If successful, these buffers are now mapped into the kernel, eligible
> for IO. To use these fixed buffers, the application must use the
> IORING_OP_READ_FIXED and IORING_OP_WRITE_FIXED opcodes, and then
> set sqe->index to the desired buffer index. sqe->addr..sqe->addr+seq->len
> must point to somewhere inside the indexed buffer.
>
> The application may register buffers throughout the lifetime of the
> io_uring instance. It can call io_uring_register() with
> IORING_UNREGISTER_BUFFERS as the opcode to unregister the current set of
> buffers, and then register a new set. The application need not
> unregister buffers explicitly before shutting down the io_uring
> instance.
>
> It's perfectly valid to setup a larger buffer, and then sometimes only
> use parts of it for an IO. As long as the range is within the originally
> mapped region, it will work just fine.
>
> For now, buffers must not be file backed. If file backed buffers are
> passed in, the registration will fail with -1/EOPNOTSUPP. This
> restriction may be relaxed in the future.
>
> RLIMIT_MEMLOCK is used to check how much memory we can pin. A somewhat
> arbitrary 1G per buffer size is also imposed.
[...]
> static int io_import_iovec(struct io_ring_ctx *ctx, int rw,
> const struct sqe_submit *s, struct iovec **iovec,
> struct iov_iter *iter)
> @@ -711,6 +763,15 @@ static int io_import_iovec(struct io_ring_ctx *ctx, int rw,
> const struct io_uring_sqe *sqe = s->sqe;
> void __user *buf = u64_to_user_ptr(READ_ONCE(sqe->addr));
> size_t sqe_len = READ_ONCE(sqe->len);
> + u8 opcode;
(You could add a comment here if you want, something like "We're
reading ->opcode for the second time, but the first read doesn't care
whether it's _FIXED or not, so it doesn't matter whether ->opcode
changes concurrently. The first read does care about whether it is a
READ or a WRITE, so we don't trust this read for that purpose and
instead let the caller pass in the read/write flag.")
> + opcode = READ_ONCE(sqe->opcode);
> + if (opcode == IORING_OP_READ_FIXED ||
> + opcode == IORING_OP_WRITE_FIXED) {
> + ssize_t ret = io_import_fixed(ctx, rw, sqe, iter);
> + *iovec = NULL;
> + return ret;
> + }
>
> if (!s->has_user)
> return EFAULT;
[...]
> @@ -1242,6 +1334,187 @@ static unsigned long ring_pages(unsigned sq_entries, unsigned cq_entries)
> return (bytes + PAGE_SIZE - 1) / PAGE_SIZE;
> }
>
> +static int io_sqe_buffer_unregister(struct io_ring_ctx *ctx)
> +{
> + int i, j;
> +
> + if (!ctx->user_bufs)
> + return -ENXIO;
> +
> + for (i = 0; i < ctx->sq_entries; i++) {
->sq_entries? Shouldn't this be ->nr_user_bufs?
> + struct io_mapped_ubuf *imu = &ctx->user_bufs[i];
> +
> + for (j = 0; j < imu->nr_bvecs; j++)
> + put_page(imu->bvec[j].bv_page);
> +
> + io_unaccount_mem(ctx->user, imu->nr_bvecs);
> + kfree(imu->bvec);
> + imu->nr_bvecs = 0;
> + }
> +
> + kfree(ctx->user_bufs);
> + ctx->user_bufs = NULL;
(It isn't really necessary, but you could set nr_user_bufs=0 here.)
> + return 0;
> +}
[...]
> +static int io_sqe_buffer_register(struct io_ring_ctx *ctx, void __user *arg,
> + unsigned nr_args)
> +{
> + struct vm_area_struct **vmas = NULL;
> + struct page **pages = NULL;
> + int i, j, got_pages = 0;
> + int ret = -EINVAL;
> +
> + if (ctx->user_bufs)
> + return -EBUSY;
> + if (!nr_args || nr_args > UIO_MAXIOV)
> + return -EINVAL;
> +
> + ctx->user_bufs = kcalloc(nr_args, sizeof(struct io_mapped_ubuf),
> + GFP_KERNEL);
> + if (!ctx->user_bufs)
> + return -ENOMEM;
> +
> + for (i = 0; i < nr_args; i++) {
> + struct io_mapped_ubuf *imu = &ctx->user_bufs[i];
> + unsigned long off, start, end, ubuf;
> + int pret, nr_pages;
> + struct iovec iov;
> + size_t size;
> +
> + ret = io_copy_iov(ctx, &iov, arg, i);
> + if (ret)
> + break;
> +
> + /*
> + * Don't impose further limits on the size and buffer
> + * constraints here, we'll -EINVAL later when IO is
> + * submitted if they are wrong.
> + */
> + ret = -EFAULT;
> + if (!iov.iov_base || !iov.iov_len)
> + goto err;
> +
> + /* arbitrary limit, but we need something */
> + if (iov.iov_len > SZ_1G)
> + goto err;
> +
> + ubuf = (unsigned long) iov.iov_base;
> + end = (ubuf + iov.iov_len + PAGE_SIZE - 1) >> PAGE_SHIFT;
> + start = ubuf >> PAGE_SHIFT;
> + nr_pages = end - start;
> +
> + ret = io_account_mem(ctx->user, nr_pages);
Technically, this accounting is probably a bit off; I think if you
pass in a vector of 4K areas from 1G hugepages, you're going to pin
factor 0x40000 more memory than you think you're pinning.
(get_user_pages() counts references against the head page of a
compound page; nothing in the kernel can tell afterwards which part of
the hugepage you're using.) I'm not sure how much of a problem that
is, but it should probably at least be documented. Unless I'm just
missing something?
> + if (ret)
> + goto err;
> +
> + if (!pages || nr_pages > got_pages) {
> + kfree(vmas);
> + kfree(pages);
> + pages = kmalloc_array(nr_pages, sizeof(struct page *),
> + GFP_KERNEL);
> + vmas = kmalloc_array(nr_pages,
> + sizeof(struct vma_area_struct *),
> + GFP_KERNEL);
> + if (!pages || !vmas) {
> + ret = -ENOMEM;
> + io_unaccount_mem(ctx->user, nr_pages);
> + goto err;
> + }
> + got_pages = nr_pages;
> + }
> +
> + imu->bvec = kmalloc_array(nr_pages, sizeof(struct bio_vec),
> + GFP_KERNEL);
> + if (!imu->bvec) {
> + io_unaccount_mem(ctx->user, nr_pages);
> + goto err;
> + }
> +
> + down_write(¤t->mm->mmap_sem);
Weren't you planning to make this down_read()?
> + pret = get_user_pages_longterm(ubuf, nr_pages, FOLL_WRITE,
> + pages, vmas);
> + if (pret == nr_pages) {
> + /* don't support file backed memory */
> + for (j = 0; j < nr_pages; j++) {
> + struct vm_area_struct *vma = vmas[j];
> +
> + if (vma->vm_file &&
> + !is_file_hugepages(vma->vm_file)) {
> + ret = -EOPNOTSUPP;
> + break;
> + }
> + }
> + } else {
> + ret = pret < 0 ? pret : -EFAULT;
> + }
> + up_write(¤t->mm->mmap_sem);
[...]
> +}
[...]
> diff --git a/include/linux/sched/user.h b/include/linux/sched/user.h
> index 39ad98c09c58..c7b5f86b91a1 100644
> --- a/include/linux/sched/user.h
> +++ b/include/linux/sched/user.h
> @@ -40,7 +40,7 @@ struct user_struct {
> kuid_t uid;
>
> #if defined(CONFIG_PERF_EVENTS) || defined(CONFIG_BPF_SYSCALL) || \
> - defined(CONFIG_NET)
> + defined(CONFIG_NET) || defined(CONFIG_IO_URING)
> atomic_long_t locked_vm;
> #endif
You're already using locked_vm in patch 5, right? I think that means
that from patch 5 up to this patch, some kernel configs will fail to
build.
