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Date:   Tue, 21 Jul 2020 08:59:25 -0700
From:   "Darrick J. Wong" <darrick.wong@oracle.com>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Goldwyn Rodrigues <rgoldwyn@suse.de>,
        Dave Chinner <david@fromorbit.com>,
        Damien Le Moal <damien.lemoal@wdc.com>,
        Naohiro Aota <naohiro.aota@wdc.com>,
        Johannes Thumshirn <jth@kernel.org>,
        linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        cluster-devel@redhat.com, linux-ext4@vger.kernel.org,
        linux-xfs@vger.kernel.org,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        linux-man@vger.kernel.org
Subject: Re: RFC: iomap write invalidation
Message-ID: <20200721155925.GB3151642@magnolia>
References: <20200713074633.875946-1-hch@lst.de>
 <20200720215125.bfz7geaftocy4r5l@fiona>
 <20200721145313.GA9217@lst.de>
 <20200721150432.GH15516@casper.infradead.org>
 <20200721150615.GA10330@lst.de>
 <20200721151437.GI15516@casper.infradead.org>
 <20200721151616.GA11074@lst.de>
 <20200721152754.GD7597@magnolia>
 <20200721154132.GA11652@lst.de>
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On Tue, Jul 21, 2020 at 05:41:32PM +0200, Christoph Hellwig wrote:
> On Tue, Jul 21, 2020 at 08:27:54AM -0700, Darrick J. Wong wrote:
> > On Tue, Jul 21, 2020 at 05:16:16PM +0200, Christoph Hellwig wrote:
> > > On Tue, Jul 21, 2020 at 04:14:37PM +0100, Matthew Wilcox wrote:
> > > > On Tue, Jul 21, 2020 at 05:06:15PM +0200, Christoph Hellwig wrote:
> > > > > On Tue, Jul 21, 2020 at 04:04:32PM +0100, Matthew Wilcox wrote:
> > > > > > I thought you were going to respin this with EREMCHG changed to ENOTBLK?
> > > > > 
> > > > > Oh, true.  I'll do that ASAP.
> > > > 
> > > > Michael, could we add this to manpages?
> > > 
> > > Umm, no.  -ENOTBLK is internal - the file systems will retry using
> > > buffered I/O and the error shall never escape to userspace (or even the
> > > VFS for that matter).
> > 
> > It's worth dropping a comment somewhere that ENOTBLK is the desired
> > "fall back to buffered" errcode, seeing as Dave and I missed that in
> > XFS...
> 
> Sounds like a good idea, but what would a good place be?

In the comment that precedes iomap_dio_rw() for the iomap version,
and...

...ye $deity, the old direct-io.c file is a mess of wrappers.  Uh...  a
new comment preceding __blockdev_direct_IO?  Or blockdev_direct_IO?  Or
both?

Or I guess the direct_IO documentation in vfs.rst...?

``direct_IO``
	called by the generic read/write routines to perform direct_IO -
	that is IO requests which bypass the page cache and transfer
	data directly between the storage and the application's address
	space.  This function can return -ENOTBLK to signal that it is
	necessary to fallback to buffered IO.  Note that
	blockdev_direct_IO and variants can also return -ENOTBLK.

--D