linux-kernel - [PATCH v2 RESEND] mm: silently skip readahead for DAX inodes

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Message-Id: <20160825164232.8989-1-ross.zwisler@linux.intel.com>
Date:   Thu, 25 Aug 2016 10:42:32 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     linux-kernel@...r.kernel.org
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jan Kara <jack@...e.com>, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, Jeff Moyer <jmoyer@...hat.com>,
        stable@...r.kernel.org
Subject: [PATCH v2 RESEND] mm: silently skip readahead for DAX inodes

For DAX inodes we need to be careful to never have page cache pages in the
mapping->page_tree.  This radix tree should be composed only of DAX
exceptional entries and zero pages.

ltp's readahead02 test was triggering a warning because we were trying to
insert a DAX exceptional entry but found that a page cache page had
already been inserted into the tree.  This page was being inserted into the
radix tree in response to a readahead(2) call.

Readahead doesn't make sense for DAX inodes, but we don't want it to report
a failure either.  Instead, we just return success and don't do any work.

Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
Reported-by: Jeff Moyer <jmoyer@...hat.com>
Cc: <stable@...r.kernel.org>    [4.5+]
---

Changes from v1:
 - Added a comment so readers don't have to go putzing around in the git
   tree to understand why we're doing what we're doing. :)  (akpm)
 - Resending, adding stable@...r.kernel.org.  Thank you, akpm, for the
   catch.

---
 mm/readahead.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/readahead.c b/mm/readahead.c
index 65ec288..c8a955b 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -8,6 +8,7 @@
  */
 
 #include <linux/kernel.h>
+#include <linux/dax.h>
 #include <linux/gfp.h>
 #include <linux/export.h>
 #include <linux/blkdev.h>
@@ -544,6 +545,14 @@ do_readahead(struct address_space *mapping, struct file *filp,
 	if (!mapping || !mapping->a_ops)
 		return -EINVAL;
 
+	/*
+	 * Readahead doesn't make sense for DAX inodes, but we don't want it
+	 * to report a failure either.  Instead, we just return success and
+	 * don't do any work.
+	 */
+	if (dax_mapping(mapping))
+		return 0;
+
 	return force_page_cache_readahead(mapping, filp, index, nr);
 }
 
-- 
2.9.0