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.

2 files changed, 17 insertions, 10 deletions

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 ();
 
 /* ---------------------------------------------------------------- */
diff --git a/ext2fs/hyper.c b/ext2fs/hyper.c
index 5bcc2abe..5f288bf1 100644
--- a/ext2fs/hyper.c
+++ b/ext2fs/hyper.c
@@ -61,7 +61,9 @@ get_hypermetadata (void)
   error_t err;
   size_t read = 0;
 
-  assert (! sblock);
+  if (sblock != NULL)
+    munmap (sblock, SBLOCK_SIZE);
+
   err = store_read (store, SBLOCK_OFFS >> store->log2_block_size,
 		    SBLOCK_SIZE, (void **)&sblock, &read);
   if (err || read != SBLOCK_SIZE)
@@ -161,19 +163,19 @@ get_hypermetadata (void)
       zeroblock = (vm_address_t) mmap (0, block_size, PROT_READ, MAP_ANON, 0, 0);
       assert (zeroblock != (vm_address_t) MAP_FAILED);
     }
-
-  munmap (sblock, SBLOCK_SIZE);
-  sblock = NULL;
 }
 
+static struct ext2_super_block *mapped_sblock;
+
 void
 map_hypermetadata (void)
 {
-  sblock = (struct ext2_super_block *) boffs_ptr (SBLOCK_OFFS);
+  mapped_sblock = (struct ext2_super_block *) boffs_ptr (SBLOCK_OFFS);
 
   /* Cache a convenient pointer to the block group descriptors for allocation.
      These are stored in the filesystem blocks following the superblock.  */
-  group_desc_image = (struct ext2_group_desc *) bptr (bptr_block (sblock) + 1);
+  group_desc_image =
+    (struct ext2_group_desc *) bptr (bptr_block (mapped_sblock) + 1);
 }
 
 error_t
@@ -196,8 +198,9 @@ diskfs_set_hypermetadata (int wait, int clean)
  if (sblock_dirty)
    {
      sblock_dirty = 0;
-     disk_cache_block_ref_ptr (sblock);
-     record_global_poke (sblock);
+     memcpy (mapped_sblock, sblock, SBLOCK_SIZE);
+     disk_cache_block_ref_ptr (mapped_sblock);
+     record_global_poke (mapped_sblock);
    }
 
   sync_global (wait);