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commit a3ccff628fcd9d9866bac415758be8d55d6cf13f
Author: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Thu Apr 24 17:44:14 2014 +0200
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.
diff --git a/ext2fs/hyper.c b/ext2fs/hyper.c
index 5bcc2ab..5f288bf 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);
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