linux-kernel - Re: RFC: hold i_rwsem until aio completes

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Message-ID: <20200203174421.GB20001@lst.de>
Date:   Mon, 3 Feb 2020 18:44:21 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: RFC: hold i_rwsem until aio completes

On Thu, Jan 16, 2020 at 03:00:04PM +0100, Jan Kara wrote:
> I'd like to note that using i_dio_count has also one advantage you didn't
> mention. For AIO case, if you need to hold i_rwsem in exclusive mode,
> holding the i_rwsem just for submission part is a significant performance
> advantage (shorter lock hold times allow for higher IO parallelism). I
> guess this could be mitigated by downgrading the lock to shared mode
> once the IO is submitted. But there will be still some degradation visible
> for the cases of mixed exclusive and shared acquisitions because shared
> holders will be blocking exclusive ones for longer time.
> 
> This may be especially painful for filesystems that don't implement DIO
> overwrites with i_rwsem in shared mode...

True.  Fortunately there are patches for ext4 out to move over to that
scheme.  gfs2 will need a little more attention, but that also for other
reasons.