linux-ext4 - Re: nowait aio return value

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Date:   Wed, 26 Jul 2017 16:19:34 -0500
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     Jason Baron <jbaron@...mai.com>
Cc:     jack@...e.cz, axboe@...nel.dk, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: nowait aio return value

Hi Jason,

On 07/26/2017 01:45 PM, Jason Baron wrote:
> Hi,
> 
> In testing nowait aio on ext4, I found that when appending to a file
> the return value is EAGAIN/EWOULDBLOCK, because as mentioned in the
> commit this will potentially trigger an allocation. However, the EAGAIN,
> seems somewhat misleading to me, in that if I continuously try the
> write, it will never succeed.
> 
> The relevant commit is:
> 728fbc0 ext4: nowait aio support
> 
> As you can see there, failure to get the inode lock is treated as
> EAGAIN, which seems more appropriate to me, as its very likely
> to succeed on subsequent calls.
> 
> Perhaps, it could be switched to -EINVAL, or something else?
> 

Thanks for testing this.

I would suggest read it as -EWOULDBLOCK. The idea is to pass on
IOCB_NOWAIT when we don't want the I/O process to wait, and if it does
return -EWOULDBLOCK. If it returns EWOULDBLOCK in case of allocation,
you may want to allocate your file space before performing the I/O.

I would return EINVAL in case the parameters passed are incorrect.

-- 
Goldwyn