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Date:   Tue, 7 Jul 2020 09:30:56 -0500
From:   Goldwyn Rodrigues <rgoldwyn@suse.de>
To:     "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Christoph Hellwig <hch@lst.de>, linux-fsdevel@vger.kernel.org,
        linux-btrfs@vger.kernel.org, fdmanana@gmail.com, dsterba@suse.cz,
        david@fromorbit.com, cluster-devel@redhat.com,
        linux-ext4@vger.kernel.org, linux-xfs@vger.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
Message-ID: <20200707143056.t7zf3xqvocty64td@fiona>
References: <20200629192353.20841-1-rgoldwyn@suse.de>
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On  7:01 07/07, Darrick J. Wong wrote:
> On Tue, Jul 07, 2020 at 08:49:52AM -0500, Goldwyn Rodrigues wrote:
> > On 13:57 07/07, 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@suse.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@oracle.com>
> > > > > > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.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.  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
> > 
> > That indeed would make reads faster. How about if the pages are dirty
> > during DIO reads?
> > Should a direct I/O read be responsible for making sure that the dirty
> > pages are written back. Technically direct I/O reads is that we are
> > reading from the device.
> 
> The filemap_write_and_wait_range should persist that data, right?

Right. filemap_write_and_wait_range() would not make sense for writes
though.

> 
> > > 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.
> > 
> > Yes, that makes sense.
> > If this is implemented, what would be the difference between O_DIRECT
> > and O_DSYNC, if any?
> 
> Presumably a direct write would proceed as it does today if there's no
> pagecache at all?
> 

Yes, correct. Just that it would leave pages in the cache instead of
invalidating it after DIO write is complete.

-- 
Goldwyn