linux-ext4 - Re: always fall back to buffered I/O after invalidation failures, was: Re: [PATCH 2/6] iomap: IOMAP_DIO_RWF_NO_STALE_PAGECACHE return if page invalidation fails

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Date:   Tue, 7 Jul 2020 15:00:30 +0200
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Christoph Hellwig <hch@....de>, linux-fsdevel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, fdmanana@...il.com, dsterba@...e.cz,
        david@...morbit.com, darrick.wong@...cle.com,
        cluster-devel@...hat.com, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: always fall back to buffered I/O after invalidation failures,
 was: Re: [PATCH 2/6] iomap: IOMAP_DIO_RWF_NO_STALE_PAGECACHE return
 if page invalidation fails

On Tue, Jul 07, 2020 at 01:57:05PM +0100, Matthew Wilcox wrote:
> On Tue, Jul 07, 2020 at 07:43:46AM -0500, Goldwyn Rodrigues wrote:
> > On  9:53 01/07, Christoph Hellwig wrote:
> > > On Mon, Jun 29, 2020 at 02:23:49PM -0500, Goldwyn Rodrigues wrote:
> > > > From: Goldwyn Rodrigues <rgoldwyn@...e.com>
> > > > 
> > > > For direct I/O, add the flag IOMAP_DIO_RWF_NO_STALE_PAGECACHE to indicate
> > > > that if the page invalidation fails, return back control to the
> > > > filesystem so it may fallback to buffered mode.
> > > > 
> > > > Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com>
> > > > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@...e.com>
> > > 
> > > I'd like to start a discussion of this shouldn't really be the
> > > default behavior.  If we have page cache that can't be invalidated it
> > > actually makes a whole lot of sense to not do direct I/O, avoid the
> > > warnings, etc.
> > > 
> > > Adding all the relevant lists.
> > 
> > Since no one responded so far, let me see if I can stir the cauldron :)
> > 
> > What error should be returned in case of such an error? I think the
> 
> Christoph's message is ambiguous.  I don't know if he means "fail the
> I/O with an error" or "satisfy the I/O through the page cache".  I'm
> strongly in favour of the latter.

Same here.  Sorry if my previous mail was unclear.

> Indeed, I'm in favour of not invalidating
> the page cache at all for direct I/O.  For reads, I think the page cache
> should be used to satisfy any portion of the read which is currently
> cached.  For writes, I think we should write into the page cache pages
> which currently exist, and then force those pages to be written back,
> but left in cache.

Something like that, yes.