Re: [PATCH] io_uring: fix blocking inline submission

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

在 2021/6/9 下午7:07, Pavel Begunkov 写道:
> There is a complaint against sys_io_uring_enter() blocking if it submits
> stdin reads. The problem is in __io_file_supports_async(), which
> sees that it's a cdev and allows it to be processed inline.
> 
> Punt char devices using generic rules of io_file_supports_async(),
> including checking for presence of *_iter() versions of rw callbacks.
> Apparently, it will affect most of cdevs with some exceptions like
> null and zero devices.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
> 
> "...For now, just ensure that anything potentially problematic is done
> inline". I believe this part is outdated, but what use cases we miss?
> Anything that we care about?
> 
> IMHO the best option is to do like in this patch and add
> (read,write)_iter(), to places we care about.
> 
> /dev/[u]random, consoles, any else?
> 
This reminds me another thing, once I did nowait read on a brd(block
ramdisk), I saw a 10%~30% regression after __io_file_supports_async()
added. brd is bio based device (block layer doesn't support nowait IO
for this kind of device), so theoretically it makes sense to punt it to
iowq threads in advance in __io_file_supports_async(), but actually
what originally happen is: IOCB_NOWAIT is not delivered to block
layer(REQ_NOWAIT) and then the IO request is executed inline (It seems
brd device won't block). This finally makes 'check it in advance'
slower..
>   fs/io_uring.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 42380ed563c4..44d1859f0dfb 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2616,7 +2616,7 @@ static bool __io_file_supports_async(struct file *file, int rw)
>   			return true;
>   		return false;
>   	}
> -	if (S_ISCHR(mode) || S_ISSOCK(mode))
> +	if (S_ISSOCK(mode))
>   		return true;
>   	if (S_ISREG(mode)) {
>   		if (IS_ENABLED(CONFIG_BLOCK) &&
>