+ radix-tree-fix-comment-about-exceptional-bits.patch added to -mm tree

+ radix-tree-fix-comment-about-exceptional-bits.patch added to -mm tree

[akpm]

The patch titled
     Subject: radix-tree: fix comment about "exceptional" bits
has been added to the -mm tree.  Its filename is
     radix-tree-fix-comment-about-exceptional-bits.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/radix-tree-fix-comment-about-exceptional-bits.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/radix-tree-fix-comment-about-exceptional-bits.patch

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------------------------------------------------------
From: Ross Zwisler <ross.zwisler@linux.intel.com>
Subject: radix-tree: fix comment about "exceptional" bits

The bottom two bits of radix tree entries are reserved for special use by
the radix tree code itself.  A comment detailing their usage was added by:

commit 3bcadd6fa6c4 ("radix-tree: free up the bottom bit of exceptional
entries for reuse")

This comment states that if the bottom two bits are '11', this means that
this is a locked exceptional entry.

It turns out that this bit combination was never actually used.  Radix tree
locking for DAX was indeed implemented, but it actually used the third LSB:

  /* We use lowest available exceptional entry bit for locking */
  #define RADIX_DAX_ENTRY_LOCK (1 << RADIX_TREE_EXCEPTIONAL_SHIFT)

This locking code was also made specific to the DAX code instead of being
generally implemented in radix-tree.h.

So, fix the comment.

Link: http://lkml.kernel.org/r/1468997731-2155-1-git-send-email-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/radix-tree.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN include/linux/radix-tree.h~radix-tree-fix-comment-about-exceptional-bits include/linux/radix-tree.h
--- a/include/linux/radix-tree.h~radix-tree-fix-comment-about-exceptional-bits
+++ a/include/linux/radix-tree.h
@@ -35,7 +35,7 @@
  * 00 - data pointer
  * 01 - internal entry
  * 10 - exceptional entry
- * 11 - locked exceptional entry
+ * 11 - this bit combination is currently unused/reserved
  *
  * The internal entry may be a pointer to the next level in the tree, a
  * sibling entry, or an indicator that the entry in this slot has been moved
_

Patches currently in -mm which might be from ross.zwisler@linux.intel.com are

dax-some-small-updates-to-daxtxt-documentation.patch
dax-remote-unused-fault-wrappers.patch
radix-tree-fix-comment-about-exceptional-bits.patch