linux-ext4 - Re: [PATCH 0/8 v2] Non-blocking AIO

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Message-ID: <9b64c78e-c984-cf29-8f79-c48332a4c450@scylladb.com>
Date:   Mon, 6 Mar 2017 17:29:41 +0200
From:   Avi Kivity <avi@...lladb.com>
To:     Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>
Cc:     Goldwyn Rodrigues <rgoldwyn@...e.de>, jack@...e.com,
        hch@...radead.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH 0/8 v2] Non-blocking AIO



On 03/06/2017 05:19 PM, Jens Axboe wrote:
> On 03/06/2017 01:25 AM, Jan Kara wrote:
>> On Sun 05-03-17 16:56:21, Avi Kivity wrote:
>>>> The goal of the patch series is to return -EAGAIN/-EWOULDBLOCK if
>>>> any of these conditions are met. This way userspace can push most
>>>> of the write()s to the kernel to the best of its ability to complete
>>>> and if it returns -EAGAIN, can defer it to another thread.
>>>>
>>> Is it not possible to push the iocb to a workqueue?  This will allow
>>> existing userspace to work with the new functionality, unchanged. Any
>>> userspace implementation would have to do the same thing, so it's not like
>>> we're saving anything by pushing it there.
>> That is not easy because until IO is fully submitted, you need some parts
>> of the context of the process which submits the IO (e.g. memory mappings,
>> but possibly also other credentials). So you would need to somehow transfer
>> this information to the workqueue.
> Outside of technical challenges, the API also needs to return EAGAIN or
> start blocking at some point. We can't expose a direct connection to
> queue work like that, and let any user potentially create millions of
> pending work items (and IOs).

You wouldn't expect more concurrent events than the maxevents parameter 
that was supplied to io_setup syscall; it should have reserved any 
resources needed.

>   That's why the current API is safe, even
> though it does suck that it block seemingly randomly for users.