linux-kernel - Re: [PATCH] nvmet: disable direct I/O when unavailable

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Message-ID: <1551111016.31902.149.camel@acm.org>
Date:   Mon, 25 Feb 2019 08:10:16 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Johannes Thumshirn <jthumshirn@...e.de>
Cc:     Max Gurtovoy <maxg@...lanox.com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Christoph Hellwig <hch@....de>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>
Subject: Re: [PATCH] nvmet: disable direct I/O when unavailable

On Mon, 2019-02-25 at 16:38 +0100, Johannes Thumshirn wrote:
> On Mon, Feb 25, 2019 at 06:49:04AM -0800, Bart Van Assche wrote:
> > On 2/25/19 1:37 AM, Johannes Thumshirn wrote:
> > > On 24/02/2019 11:54, Max Gurtovoy wrote:
> > > > What is the status of iSCSI/SRP targets in this case ?
> > > 
> > > iSCSI/SRP passes in the following:
> > > 
> > >   /*
> > >    * Use O_DSYNC by default instead of O_SYNC to forgo syncing
> > >    * of pure timestamp updates.
> > >    */
> > >   flags = O_RDWR | O_CREAT | O_LARGEFILE | O_DSYNC;
> > 
> > That code fragment comes from the LIO file backend. There is no requirement
> > for a SCSI target core file backend to use O_DSYNC. SCST allows users to
> > choose whether or not O_DSYNC should be used:
> 
> Yes, LIO file backend is the in-tree equivalent to NVMe's file backend. That's
> why I copied it here.
> 
> 
> > 
> > 	if (virt_dev->wt_flag && !virt_dev->nv_cache)
> > 		open_flags |= O_DSYNC;
> > 
> 
> OK. Do your open_flags include O_DIRECT per default as well?

I don't think it has ever been supported to pass O_DIRECT to filp_open().
As one can see in build_open_flags() O_DIRECT is ignored. The only way I
know of to submit direct I/O from kernel context is by setting the
IOCB_DIRECT flag.

Bart.