Re: [PATCH 2/6] io_uring: add IORING_OP_PROVIDE_BUFFERS

[Jens Axboe <axboe@kernel.dk>]

On 3/9/20 11:28 AM, Andres Freund wrote:
> Hi,
> 
> On 2020-03-09 11:17:46 -0600, Jens Axboe wrote:
>>>> +static int io_add_buffers(struct io_provide_buf *pbuf, struct list_head *list)
>>>> +{
>>>> +	struct io_buffer *buf;
>>>> +	u64 addr = pbuf->addr;
>>>> +	int i, bid = pbuf->bid;
>>>> +
>>>> +	for (i = 0; i < pbuf->nbufs; i++) {
>>>> +		buf = kmalloc(sizeof(*buf), GFP_KERNEL);
>>>> +		if (!buf)
>>>> +			break;
>>>> +
>>>> +		buf->addr = addr;
>>>> +		buf->len = pbuf->len;
>>>> +		buf->bid = bid;
>>>> +		list_add(&buf->list, list);
>>>> +		addr += pbuf->len;
>>>> +		bid++;
>>>> +	}
>>>> +
>>>> +	return i;
>>>> +}
>>>
>>> Hm, aren't you loosing an error here if you kmalloc fails for i > 0?
>>> Afaict io_provide_buffes() only checks for ret != 0. I think userland
>>> should know that a PROVIDE_BUFFERS failed, even if just partially (I'd
>>> just make it fail wholesale).
>>
>> The above one does have the issue that we're losing the error for i ==
>> 0, current one does:
>>
>> return i ? i : -ENOMEM;
>>
>> But this is what most interfaces end up doing, return the number
>> processed, if any, or error if none of them were added. Like a short
>> read, for example, and you'd get EIO if you forwarded and tried again.
>> So I tend to prefer doing it like that, at least to me it seems more
>> logical than unwinding. The application won't know what buffer caused
>> the error if you unwind, whereas it's perfectly clear if you asked to
>> add 128 and we return 64 that the issue is with the 65th buffer.
> 
> Fair enough. I was/am thinking that this'd pretty much always be a fatal
> error for the application. Which does seem a bit different from the
> short read/write case, where there are plenty reasons to handle them
> "silently" during normal operation.
> 
> But I can error out with the current interface, so ...

Even if it is most likely a fatal condition for the application, it
would usually indicate that the application is doing something wrong.
Which means you want to debug it, and that's a lot more approachable
if you know exactly which buffer caused the issue. There's merit to
saying "buffer N isn't valid" rather than "One of the N buffers
submitted has an issue, good luck!".

-- 
Jens Axboe