From 134b450cb65b1986c78e0b292cff78c2f1882a2b Mon Sep 17 00:00:00 2001
From: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Thu, 24 Apr 2014 17:44:14 +0200
Subject: ext2fs: cache the superblock

Previously, the superblock was mmaped and a pointer stored in sblock
by map_hypermetadata.  This memory is backed by our disk pager.

This is rather unfortunate, as this means that whenever we read a
value from that location, we might generate a request our disk pager.
This amplifies the so-called thread-storm problem.

Rather than relying on a mmaped region of memory, just use the data
loaded by get_hypermetadata.

* ext2fs/hyper.c (get_hypermetadata): Do not free sblock.
(mapped_sblock): New variable.
(map_hypermetadata): Map the superblock to mapped_sblock instead.
(diskfs_set_hypermetadata): Copy superblock into mapped_superblock.
* ext2fs/ext2fs.h (get_hypermetadata, map_hypermetadata): Adjust
comments accordingly.
---
 ext2fs/ext2fs.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

(limited to 'ext2fs/ext2fs.h')

diff --git a/ext2fs/ext2fs.h b/ext2fs/ext2fs.h
index e01d1a59..3422af2f 100644
--- a/ext2fs/ext2fs.h
+++ b/ext2fs/ext2fs.h
@@ -297,10 +297,14 @@ unsigned log2_stat_blocks_per_fs_block;
 /* A handy page of page-aligned zeros.  */
 vm_address_t zeroblock;
 
-/* Get the superblock from the disk, & setup various global info from it.  */
+/* Get the superblock from the disk, point `sblock' to it, and setup
+   various global info from it.  */
 void get_hypermetadata ();
 
-/* Map `sblock' and `group_desc_image' pointers to disk cache.  */
+/* Map `group_desc_image' pointers to disk cache.  Also, establish a
+   non-exported mapping to the superblock that will be used by
+   diskfs_set_hypermetadata to update the superblock from the cache
+   `sblock' points to.  */
 void map_hypermetadata ();
 
 /* ---------------------------------------------------------------- */
-- 
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