Re: [RFC 1/1] io_uring: improve register file feature's usability

[Pavel Begunkov <asml.silence@xxxxxxxxx>]

On 10/12/21 09:48, Xiaoguang Wang wrote:
> The idea behind register file feature is good and straightforward, but
> there is a very big issue that it's hard to use for user apps. User apps
> need to bind slot info to file descriptor. For example, user app wants
> to register a file, then it first needs to find a free slot in register
> file infrastructure, that means user app needs to maintain slot info in
> userspace, which is a obvious burden for userspace developers.

Slot allocation is specifically entirely given away to the userspace,
the userspace has more info and can use it more efficiently, e.g.
if there is only a small managed set of registered files they can
always have O(1) slot "lookup", and a couple of more use cases.

If userspace wants to mimic a fdtable into io_uring's registered table,
it's possible to do as is and without extra fdtable tracking

fd = open();
io_uring_update_slot(off=fd, fd=fd);

For the dual wanting an fd both in the normal fdtable and fixed table
with same indexes, not sure how viable that is but "direct open" can
be extended if needed.


> Actually, file descriptor can be a good candidate slot info. If app wants
> to register a file, it can use this file's fd as valid slot, there'll
> definitely be no conflicts and very easy for user apps.
>
> To support to pass fd as slot info, we'll need to automatically resize
> io_file_table if passed fd is greater than current io_file_table size,
> just like how fd table extends.
>
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
> ---
>   fs/io_uring.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 53 insertions(+), 8 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 73135c5c6168..be7abd89c0b0 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -7768,6 +7768,21 @@ static bool io_alloc_file_tables(struct io_file_table *table, unsigned nr_files)
>   	return !!table->files;
>   }
>   
> +static int io_resize_file_tables(struct io_ring_ctx *ctx, unsigned old_files,
> +				 unsigned new_files)
> +{
> +	size_t oldsize = sizeof(ctx->file_table.files[0]) * old_files;
> +	size_t newsize = sizeof(ctx->file_table.files[0]) * new_files;
> +
> +	ctx->file_table.files = kvrealloc(ctx->file_table.files, oldsize, newsize,
> +					   GFP_KERNEL_ACCOUNT);
> +	if (!ctx->file_table.files)
> +		return -ENOMEM;
> +
> +	ctx->nr_user_files = new_files;
> +	return 0;
> +}
> +
>   static void io_free_file_tables(struct io_file_table *table)
>   {
>   	kvfree(table->files);
> @@ -8147,6 +8162,25 @@ static void io_rsrc_put_work(struct work_struct *work)
>   	}
>   }
>   
> +static inline int io_calc_file_tables_size(__s32 __user *fds, unsigned nr_files)
> +{
> +	int i, fd, max_fd = 0;
> +
> +	for (i = 0; i < nr_files; i++) {
> +		if (copy_from_user(&fd, &fds[i], sizeof(fd)))
> +			return -EFAULT;
> +		if (fd == -1)
> +			continue;
> +		if (fd > max_fd)
> +			max_fd = fd;
> +	}
> +
> +	max_fd++;
> +	if (max_fd < nr_files)
> +		max_fd = nr_files;
> +	return max_fd;
> +}
> +
>   static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>   				 unsigned nr_args, u64 __user *tags)
>   {
> @@ -8154,6 +8188,7 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>   	struct file *file;
>   	int fd, ret;
>   	unsigned i;
> +	int num_files;
>   
>   	if (ctx->file_data)
>   		return -EBUSY;
> @@ -8171,8 +8206,12 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>   	if (ret)
>   		return ret;
>   
> +	num_files = io_calc_file_tables_size(fds, nr_args);
> +	if (num_files < 0)
> +		goto out_free;
> +
>   	ret = -ENOMEM;
> -	if (!io_alloc_file_tables(&ctx->file_table, nr_args))
> +	if (!io_alloc_file_tables(&ctx->file_table, num_files))
>   		goto out_free;
>   
>   	for (i = 0; i < nr_args; i++, ctx->nr_user_files++) {
> @@ -8204,7 +8243,7 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>   			fput(file);
>   			goto out_fput;
>   		}
> -		io_fixed_file_set(io_fixed_file_slot(&ctx->file_table, i), file);
> +		io_fixed_file_set(io_fixed_file_slot(&ctx->file_table, fd), file);
>   	}
>   
>   	ret = io_sqe_files_scm(ctx);
> @@ -8390,15 +8429,22 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
>   	struct io_rsrc_data *data = ctx->file_data;
>   	struct io_fixed_file *file_slot;
>   	struct file *file;
> -	int fd, i, err = 0;
> +	int fd, err = 0;
>   	unsigned int done;
>   	bool needs_switch = false;
> +	int num_files;
>   
>   	if (!ctx->file_data)
>   		return -ENXIO;
>   	if (up->offset + nr_args > ctx->nr_user_files)
>   		return -EINVAL;
>   
> +	num_files = io_calc_file_tables_size(fds, nr_args);
> +	if (num_files < 0)
> +		return -EFAULT;
> +	if (io_resize_file_tables(ctx, ctx->nr_user_files, num_files) < 0)
> +		return -ENOMEM;
> +
>   	for (done = 0; done < nr_args; done++) {
>   		u64 tag = 0;
>   
> @@ -8414,12 +8460,11 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
>   		if (fd == IORING_REGISTER_FILES_SKIP)
>   			continue;
>   
> -		i = array_index_nospec(up->offset + done, ctx->nr_user_files);
> -		file_slot = io_fixed_file_slot(&ctx->file_table, i);
> +		file_slot = io_fixed_file_slot(&ctx->file_table, fd);
>   
>   		if (file_slot->file_ptr) {
>   			file = (struct file *)(file_slot->file_ptr & FFS_MASK);
> -			err = io_queue_rsrc_removal(data, up->offset + done,
> +			err = io_queue_rsrc_removal(data, fd,
>   						    ctx->rsrc_node, file);
>   			if (err)
>   				break;
> @@ -8445,9 +8490,9 @@ static int __io_sqe_files_update(struct io_ring_ctx *ctx,
>   				err = -EBADF;
>   				break;
>   			}
> -			*io_get_tag_slot(data, up->offset + done) = tag;
> +			*io_get_tag_slot(data, fd) = tag;
>   			io_fixed_file_set(file_slot, file);
> -			err = io_sqe_file_register(ctx, file, i);
> +			err = io_sqe_file_register(ctx, file, fd);
>   			if (err) {
>   				file_slot->file_ptr = 0;
>   				fput(file);

--
Pavel Begunkov