Re: [PATCH 3/6] io_uring: allow allocated fixed files for openat/openat2

[Hao Xu <haoxu.linux@gmail.com>]

在 2022/5/9 下午11:50, Jens Axboe 写道:
> If the application passes in IORING_FILE_INDEX_ALLOC as the file_slot,
> then that's a hint to allocate a fixed file descriptor rather than have
> one be passed in directly.
> 
> This can be useful for having io_uring manage the direct descriptor space.
> 
> Normal open direct requests will complete with 0 for success, and < 0
> in case of error. If io_uring is asked to allocated the direct descriptor,
> then the direct descriptor is returned in case of success.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>   fs/io_uring.c                 | 32 +++++++++++++++++++++++++++++---
>   include/uapi/linux/io_uring.h |  9 +++++++++
>   2 files changed, 38 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 8c40411a7e78..ef999d0e09de 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -4697,7 +4697,7 @@ static int io_openat2_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>   	return __io_openat_prep(req, sqe);
>   }
>   
> -static int __maybe_unused io_file_bitmap_get(struct io_ring_ctx *ctx)
> +static int io_file_bitmap_get(struct io_ring_ctx *ctx)
>   {
>   	struct io_file_table *table = &ctx->file_table;
>   	unsigned long nr = ctx->nr_user_files;
> @@ -4722,6 +4722,32 @@ static int __maybe_unused io_file_bitmap_get(struct io_ring_ctx *ctx)
>   	return -ENFILE;
>   }
>   
> +static int io_fixed_fd_install(struct io_kiocb *req, unsigned int issue_flags,
> +			       struct file *file, unsigned int file_slot)
> +{
> +	int alloc_slot = file_slot == IORING_FILE_INDEX_ALLOC;
> +	struct io_ring_ctx *ctx = req->ctx;
> +	int ret;
> +
> +	if (alloc_slot) {
> +		io_ring_submit_lock(ctx, issue_flags);
> +		file_slot = io_file_bitmap_get(ctx);
> +		if (unlikely(file_slot < 0)) {
> +			io_ring_submit_unlock(ctx, issue_flags);
> +			return file_slot;
> +		}
> +	}
> +
> +	ret = io_install_fixed_file(req, file, issue_flags, file_slot);
> +	if (alloc_slot) {
> +		io_ring_submit_unlock(ctx, issue_flags);
> +		if (!ret)
> +			return file_slot;

Sorry, I missed onething, looks like this should be file_slot+1, as this
is returned to the userspace. I refer to the previous open/accept direct
feature, they see the file_index from userspace as number counted from
one, so it'd better to keep it consistent.

Regards,
Hao

> +	}
> +
> +	return ret;
> +}
> +
>   static int io_openat2(struct io_kiocb *req, unsigned int issue_flags)
>   {
>   	struct open_flags op;
> @@ -4777,8 +4803,8 @@ static int io_openat2(struct io_kiocb *req, unsigned int issue_flags)
>   	if (!fixed)
>   		fd_install(ret, file);
>   	else
> -		ret = io_install_fixed_file(req, file, issue_flags,
> -					    req->open.file_slot - 1);
> +		ret = io_fixed_fd_install(req, issue_flags, file,
> +						req->open.file_slot);
>   err:
>   	putname(req->open.filename);
>   	req->flags &= ~REQ_F_NEED_CLEANUP;
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index 06621a278cb6..b7f02a55032a 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -63,6 +63,15 @@ struct io_uring_sqe {
>   	__u64	__pad2[2];
>   };
>   
> +/*
> + * If sqe->file_index is set to this for opcodes that instantiate a new
> + * direct descriptor (like openat/openat2/accept), then io_uring will allocate
> + * an available direct descriptor instead of having the application pass one
> + * in. The picked direct descriptor will be returned in cqe->res, or -ENFILE
> + * if the space is full.
> + */
> +#define IORING_FILE_INDEX_ALLOC		(~0U)
> +
>   enum {
>   	IOSQE_FIXED_FILE_BIT,
>   	IOSQE_IO_DRAIN_BIT,