linux-kernel - Re: [PATCH v11 07/21] dax,ext2: Replace XIP read and write with DAX I/O

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Date:	Thu, 16 Oct 2014 18:33:31 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 07/21] dax,ext2: Replace XIP read and write with DAX
 I/O

On Thu, Oct 16, 2014 at 03:51:12PM -0400, Matthew Wilcox wrote:
> On Thu, Oct 16, 2014 at 11:50:27AM +0200, Mathieu Desnoyers wrote:
> > > +			if (rw == WRITE) {
> > > +				if (!buffer_mapped(bh)) {
> > > +					retval = -EIO;
> > > +					/* FIXME: fall back to buffered I/O */
> > 
> > Fallback on buffered I/O would void guarantee about having data stored
> > into persistent memory after write returns. Not sure we actually want
> > that.
> 
> Yeah, I think that comment is just stale.  I can't see a way in which
> buffered I/O would succeed after DAX I/O falis.

On further consideration, I think the whole thing is just foolish.
I don't see how get_block(create == 1) can return success *and* a buffer
that is !mapped.

So I did this nice simplification:

-                       if (rw == WRITE) {
-                               if (!buffer_mapped(bh)) {
-                                       retval = -EIO;
-                                       /* FIXME: fall back to buffered I/O */
-                                       break;
-                               }
-                               hole = false;
-                       } else {
-                               hole = !buffer_written(bh);
-                       }
+                       hole = (rw != WRITE) && !buffer_written(bh);

(compile-tested only; I'm going to run all the changes through xfstests
next week when I'm back home before sending out a v12).
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