linux-ext4 - Re: RFC: iomap write invalidation

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Date:   Tue, 21 Jul 2020 08:59:25 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Dave Chinner <david@...morbit.com>,
        Damien Le Moal <damien.lemoal@....com>,
        Naohiro Aota <naohiro.aota@....com>,
        Johannes Thumshirn <jth@...nel.org>,
        linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        cluster-devel@...hat.com, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man@...r.kernel.org
Subject: Re: RFC: iomap write invalidation

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