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From: Jens Axboe <[email protected]>
To: Andres Freund <[email protected]>
Cc: [email protected]
Subject: Re: [PATCH 2/6] io_uring: add IORING_OP_PROVIDE_BUFFERS
Date: Tue, 10 Mar 2020 07:33:23 -0600	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

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


  reply	other threads:[~2020-03-10 13:33 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-28 20:30 [PATCHSET v3] io_uring support for automatic buffers Jens Axboe
2020-02-28 20:30 ` [PATCH 1/6] io_uring: buffer registration infrastructure Jens Axboe
2020-02-28 20:30 ` [PATCH 2/6] io_uring: add IORING_OP_PROVIDE_BUFFERS Jens Axboe
2020-02-29  0:43   ` Pavel Begunkov
2020-02-29  4:50     ` Jens Axboe
2020-02-29 11:36       ` Pavel Begunkov
2020-02-29 17:32         ` Jens Axboe
2020-02-29 12:08   ` Pavel Begunkov
2020-02-29 17:34     ` Jens Axboe
2020-02-29 18:11       ` Jens Axboe
2020-03-09 17:03   ` Andres Freund
2020-03-09 17:17     ` Jens Axboe
2020-03-09 17:28       ` Andres Freund
2020-03-10 13:33         ` Jens Axboe [this message]
2020-02-28 20:30 ` [PATCH 3/6] io_uring: support buffer selection Jens Axboe
2020-02-29 12:21   ` Pavel Begunkov
2020-02-29 17:35     ` Jens Axboe
2020-03-09 17:21   ` Andres Freund
2020-03-10 13:37     ` Jens Axboe
2020-02-28 20:30 ` [PATCH 4/6] io_uring: add IOSQE_BUFFER_SELECT support for IORING_OP_READV Jens Axboe
2020-02-28 20:30 ` [PATCH 5/6] net: abstract out normal and compat msghdr import Jens Axboe
2020-02-28 20:30 ` [PATCH 6/6] io_uring: add IOSQE_BUFFER_SELECT support for IORING_OP_RECVMSG Jens Axboe

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