Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756851AbdLPOtT (ORCPT ); Sat, 16 Dec 2017 09:49:19 -0500 Received: from mail-wm0-f66.google.com ([74.125.82.66]:33773 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753035AbdLPOtN (ORCPT ); Sat, 16 Dec 2017 09:49:13 -0500 X-Google-Smtp-Source: ACJfBos1CnDlf80WSATilbRpX7TAeOweQXSCWzb9jivQ3z8U8DOZ2k2kvA2AcEjCUGGeAy+HAb8g5w== Subject: Re: Detecting RWF_NOWAIT support To: Goldwyn Rodrigues , linux-kernel , linux-xfs@vger.kernel.org References: <4ae3426d-8104-c243-72e4-671e89401b23@suse.de> From: Avi Kivity Organization: ScyllaDB Message-ID: <0ae3b9d3-0e57-5562-5a8d-62496a261521@scylladb.com> Date: Sat, 16 Dec 2017 16:49:08 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: <4ae3426d-8104-c243-72e4-671e89401b23@suse.de> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1445 Lines: 35 On 12/14/2017 09:15 PM, Goldwyn Rodrigues wrote: > > On 12/14/2017 11:38 AM, Avi Kivity wrote: >> I'm looking to add support for RWF_NOWAIT within a linux-aio iocb. >> Naturally, I need to detect at runtime whether the kernel support >> RWF_NOWAIT or not. >> >> >> The only method I could find was to issue an I/O with RWF_NOWAIT set, >> and look for errors. This is somewhat less than perfect: >> >>  - from the error, I can't tell whether RWF_NOWAIT was the problem, or >> something else. If I enable a number of new features, I have to run >> through all combinations to figure out which ones are supported and >> which are not. > Here is the return codes for RWF_NOWAIT > EINVAL - not supported (older kernel) > EOPNOTSUPP - not supported > EAGAIN - supported but could not complete because I/O will be delayed Which of these are returned from io_submit() and which are returned in the iocb? > 0 - supported and I/O completed (success). > >>  - RWF_NOWAIT support is per-filesystem, so I can't just remember not to >> enable RWF_NOWAIT globally, I have to track it per file. > Yes, the support is per filesystem. So, the application must know if the > filesystem supports it, possibly by performing a small I/O. So the application must know about filesystem mount points, and be prepared to create a file and try to write it (in case the filesystem is empty) or alter its behavior during runtime depending on the errors it sees.