linux-kernel - Re: [PATCH] Fix bad data from non-direct-io read after direct-io write

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Date:	Fri, 26 Oct 2007 18:10:42 -0400
From:	Karl Schendel <kschendel@...allegro.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Zach Brown <zach.brown@...cle.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Leonid Ananiev <leonid.i.ananiev@...ux.intel.com>
Subject: Re: [PATCH] Fix bad data from non-direct-io read after direct-io
 write

Linus Torvalds wrote:

> .... So we may
> actually end up doing some IO, but then returning the "wrong" error code
> from the invalidate. Hmm?
>

A point.  In an all-seeing, all-caring universe, it would be the read
hitting the cached page that couldn't be invalidated that would get
the error, not the write.  I can't get too worked up over that, though.

In any case, as you say, if the write worked it should report as such.
Perhaps this equivalent but slightly cleaned-up patch instead?

Karl

--- linux-2.6.23.1-base/mm/filemap.c	2007-10-12 12:43:44.000000000 -0400
+++ linux-2.6.23.1/mm/filemap.c	2007-10-26 18:00:00.000000000 -0400
@@ -2194,21 +2194,17 @@ generic_file_direct_IO(int rw, struct ki
 	}

 	retval = mapping->a_ops->direct_IO(rw, iocb, iov, offset, nr_segs);
-	if (retval)
-		goto out;

 	/*
 	 * Finally, try again to invalidate clean pages which might have been
-	 * faulted in by get_user_pages() if the source of the write was an
-	 * mmap()ed region of the file we're writing.  That's a pretty crazy
-	 * thing to do, so we don't support it 100%.  If this invalidation
-	 * fails and we have -EIOCBQUEUED we ignore the failure.
+	 * cached by non-direct readahead, or faulted in by get_user_pages()
+	 * if the source of the write was an mmap'ed region of the file
+	 * we're writing.  Either one is a pretty crazy thing to do,
+	 * so we don't support it 100%.  If this invalidation
+	 * fails, tough, the write still worked...
 	 */
-	if (rw == WRITE && mapping->nrpages) {
-		int err = invalidate_inode_pages2_range(mapping,
-					      offset >> PAGE_CACHE_SHIFT, end);
-		if (err && retval >= 0)
-			retval = err;
+	if (retval >= 0 && rw == WRITE && mapping->nrpages) {
+		invalidate_inode_pages2_range(mapping, offset >> PAGE_CACHE_SHIFT, end);
 	}
 out:
 	return retval;
-
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