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 07:43:46 -0500
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     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  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
userspace process must be immediately informed if it in unable to
invalidate the page cache and complete the direct I/O. Currently, the
iomap code treats this as a writeback error and continues with the
direct I/O and the userspace process comes to know only during file
closure.

If such a change is incorporated, are the current userspace applications
prepared for it?


-- 
Goldwyn