linux-ext4 - Re: RFC: iomap write invalidation

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Date:   Tue, 21 Jul 2020 09:03:42 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>,
        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 04:52:01PM +0100, Matthew Wilcox wrote:
> On Tue, Jul 21, 2020 at 05:42:40PM +0200, Christoph Hellwig wrote:
> > On Tue, Jul 21, 2020 at 04:31:36PM +0100, Matthew Wilcox wrote:
> > > > 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).
> > > 
> > > Ah, I made the mistake of believing the comments that I could see in
> > > your patch instead of reading the code.
> > > 
> > > Can I suggest deleting this comment:
> > > 
> > >         /*
> > >          * No fallback to buffered IO on errors for XFS, direct IO will either
> > >          * complete fully or fail.
> > >          */
> > > 
> > > and rewording this one:
> > > 
> > >                 /*
> > >                  * Allow a directio write to fall back to a buffered
> > >                  * write *only* in the case that we're doing a reflink
> > >                  * CoW.  In all other directio scenarios we do not
> > >                  * allow an operation to fall back to buffered mode.
> > >                  */
> > > 
> > > as part of your revised patchset?
> > 
> > That isn't actually true.  In current mainline we only fallback on
> > reflink RMW cases, but with this series we also fall back for
> > invalidation failures.
> 
> ... that's why I'm suggesting that you delete the first one and rewrite
> the second one.  Because they aren't true.

/*
 * We allow only three outcomes of a directio: (1) it succeeds
 * completely; (2) it fails with a negative error code; or (3) it
 * returns -ENOTBLK to signal that we should fall back to buffered IO.
 *
 * The third scenario should only happen if iomap refuses to perform the
 * direct IO, or the direct write request requires CoW but is not aligned
 * to the filesystem block size.
 */

?

--D