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6d7d1a0dc7
linux/fs
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Linus Torvalds 6d7d1a0dc7 vfs: get rid of batshit-insane pointless dentry hash calculations 
For some odd historical reason, the final mixing round for the dentry
cache hash table lookup had an insane "xor with big constant" logic.  In
two places.

The big constant that is being xor'ed is GOLDEN_RATIO_PRIME, which is a
fairly random-looking number that is designed to be *multiplied* with so
that the bits get spread out over a whole long-word.

But xor'ing with it is insane.  It doesn't really even change the hash -
it really only shifts the hash around in the hash table.  To make
matters worse, the insane big constant is different on 32-bit and 64-bit
builds, even though the name hash bits we use are always 32-bit (and the
bits from the pointer we mix in effectively are too).

It's all total voodoo programming, in other words.

Now, some testing and analysis of the hash chains shows that the rest of
the hash function seems to be fairly good.  It does pick the right bits
of the parent dentry pointer, for example, and while it's generally a
bad idea to use an xor to mix down the upper bits (because if there is a
repeating pattern, the xor can cause "destructive interference"), it
seems to not have been a disaster.

For example, replacing the hash with the normal "hash_long()" code (that
uses the GOLDEN_RATIO_PRIME constant correctly, btw) actually just makes
the hash worse.  The hand-picked hash knew which bits of the pointer had
the highest entropy, and hash_long() ends up mixing bits less optimally
at least in some trivial tests.

So the hash function overall seems fine, it just has that really odd
"shift result around by a constant xor".

So get rid of the silly xor, and replace the down-mixing of the bits
with an add instead of an xor that tends to not have the same kind of
destructive interference issues.  Some stats on the resulting hash
chains shows that they look statistically identical before and after,
but the code is simpler and no longer makes you go "WTF?".

Also, the incoming hash really is just "unsigned int", not a long, and
there's no real point to worry about the high 26 bits of the dentry
pointer for the 64-bit case, because they are all going to be identical
anyway.

So also change the hashing to be done in the more natural 'unsigned int'
that is the real size of the actual hashed data anyway.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2012-03-19 16:19:53 -07:00
..
9p Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ericvh/v9fs 2012-01-10 15:09:01 -08:00
adfs vfs: switch ->show_options() to struct dentry * 2012-01-06 23:19:54 -05:00
affs affs: propagate umode_t 2012-01-03 22:55:04 -05:00
afs afs: Remote abort can cause BUG in rxrpc code 2012-03-16 17:01:41 -07:00
autofs4 autofs: work around unhappy compat problem on x86-64 2012-02-25 12:10:27 -08:00
befs vfs: fix the stupidity with i_dentry in inode destructors 2012-01-03 22:52:40 -05:00
bfs switch ->create() to umode_t 2012-01-03 22:54:53 -05:00
btrfs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs 2012-03-09 18:09:18 -08:00
cachefiles fs: move code out of buffer.c 2012-01-03 22:54:07 -05:00
ceph Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client 2012-02-02 15:47:33 -08:00
cifs CIFS: Do not kmalloc under the flocks spinlock 2012-03-06 21:50:15 -06:00
coda coda: switch coda_cnode_make() to sane API as well, clean coda_lookup() 2012-01-10 11:13:16 -05:00
configfs configfs: convert to umode_t 2012-01-03 22:54:57 -05:00
cramfs Merge branches 'vfsmount-guts', 'umode_t' and 'partitions' into Z 2012-01-06 23:15:54 -05:00
debugfs kernel-doc: fix new warnings in debugfs 2012-01-24 10:47:41 -08:00
devpts devpts: fix double-free on mount failure 2012-01-08 20:19:03 -05:00
dlm Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/teigland/linux-dlm 2012-01-10 14:55:55 -08:00
ecryptfs ecryptfs: fix printk format warning for size_t 2012-02-28 16:55:30 -08:00
efs vfs: fix the stupidity with i_dentry in inode destructors 2012-01-03 22:52:40 -05:00
exofs Merge branch 'for-linus' of git://git.open-osd.org/linux-open-osd 2012-01-09 12:51:01 -08:00
exportfs
ext2 ext2: protect inode changes in the SETVERSION and SETFLAGS ioctls 2012-01-11 13:39:02 +01:00
ext3 Merge branch 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs 2012-01-09 12:51:21 -08:00
ext4 Merge branch 'for_linus' into for_linus_merged 2012-01-10 11:54:07 -05:00
fat Merge branch 'usb-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb 2012-01-09 12:09:47 -08:00
freevxfs fs: propagate umode_t, misc bits 2012-01-03 22:55:10 -05:00
fscache
fuse Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse 2012-01-12 12:39:21 -08:00
gfs2 GFS2: Read resource groups on mount 2012-02-28 09:52:39 +00:00
hfs vfs: switch ->show_options() to struct dentry * 2012-01-06 23:19:54 -05:00
hfsplus hfsplus: creation of hidden dir on mount can fail 2012-01-10 17:48:52 -05:00
hostfs vfs: switch ->show_options() to struct dentry * 2012-01-06 23:19:54 -05:00
hpfs switch ->mknod() to umode_t 2012-01-03 22:54:54 -05:00
hppfs vfs: for usbfs, etc. internal vfsmounts ->mnt_sb->s_root == ->mnt_root 2012-01-03 22:52:41 -05:00
hugetlbfs mm: compaction: introduce sync-light migration for use by compaction 2012-01-12 20:13:09 -08:00
isofs isofs: inode leak on mount failure 2012-01-09 10:48:11 -05:00
jbd jbd: Issue cache flush after checkpointing 2012-01-11 13:36:57 +01:00
jbd2 Merge branch 'for_linus' into for_linus_merged 2012-01-10 11:54:07 -05:00
jffs2 jffs2: do not initialize variable unnecessarily 2012-01-11 09:53:51 +00:00
jfs Merge branch 'pm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm 2012-01-08 13:10:57 -08:00
lockd module_param: make bool parameters really bool (drivers & misc) 2012-01-13 09:32:20 +10:30
logfs mtd: fix merge conflict resolution breakage 2012-02-01 11:10:24 -08:00
minix Merge branch 'for-linus2' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2012-01-08 12:19:57 -08:00
ncpfs vfs: switch ->show_options() to struct dentry * 2012-01-06 23:19:54 -05:00
nfs NFSv4: fix server_scope memory leak 2012-02-17 17:34:03 -05:00
nfs_common
nfsd Merge branch 'for-3.3' of git://linux-nfs.org/~bfields/linux 2012-01-14 12:26:41 -08:00
nilfs2 nilfs2: fix NULL pointer dereference in nilfs_load_super_block() 2012-03-16 17:14:44 -07:00
nls NLS: raname "maxlen" to "maxout" in UTF conversion routines 2011-11-26 19:58:47 -08:00
notify fsnotify: don't BUG in fsnotify_destroy_mark() 2012-01-14 18:01:42 -08:00
ntfs NTFS: Correct two spelling errors "dealocate" to "deallocate" in mft.c. 2012-02-24 09:17:09 +00:00
ocfs2 ocfs2: deal with wraparounds of i_nlink in ocfs2_rename() 2012-02-13 20:45:39 -05:00
omfs omfs: propagate umode_t 2012-01-03 22:55:01 -05:00
openpromfs vfs: fix the stupidity with i_dentry in inode destructors 2012-01-03 22:52:40 -05:00
proc Fix race in process_vm_rw_core 2012-02-02 12:55:17 -08:00
pstore pstore: gracefully handle NULL pstore_info functions 2011-11-18 13:49:00 -08:00
qnx4 qnx4: don't leak ->BitMap on late failure exits 2012-01-19 13:54:36 -05:00
quota quota: Fix deadlock with suspend and quotas 2012-02-13 20:45:39 -05:00
ramfs pohmelfs: propagate umode_t 2012-01-03 22:55:07 -05:00
reiserfs reiserfs: don't lock root inode searching 2012-01-10 16:30:54 -08:00
romfs MTD pull for 3.3 2012-01-10 13:45:22 -08:00
squashfs Merge git://git.kernel.org/pub/scm/linux/kernel/git/pkl/squashfs-next 2012-01-13 10:34:57 -08:00
sysfs sysfs: Complain bitterly about attempts to remove files from nonexistent directories. 2012-01-24 12:12:32 -08:00
sysv vfs: prefer ->dentry->d_sb to ->mnt->mnt_sb 2012-01-06 23:16:53 -05:00
ubifs UBIFS: fix non-debug configuration build 2012-01-15 13:46:02 +02:00
udf udf: Fix deadlock in udf_release_file() 2012-03-10 16:05:38 -05:00
ufs vfs: switch ->show_options() to struct dentry * 2012-01-06 23:19:54 -05:00
xfs xfs: make inode quota check more general 2012-02-21 10:12:43 -06:00
aio.c aio: fix the "too late munmap()" race 2012-03-09 18:59:59 -08:00
anon_inodes.c
attr.c switch is_sxid() to umode_t 2012-01-03 22:55:11 -05:00
bad_inode.c switch ->mknod() to umode_t 2012-01-03 22:54:54 -05:00
binfmt_aout.c aout: move setup_arg_pages() prior to reading/mapping the binary 2012-03-05 13:51:32 -08:00
binfmt_elf_fdpic.c
binfmt_elf.c regset: Prevent null pointer reference on readonly regsets 2012-03-02 11:38:15 -08:00
binfmt_em86.c
binfmt_flat.c
binfmt_misc.c vfs: prefer ->dentry->d_sb to ->mnt->mnt_sb 2012-01-06 23:16:53 -05:00
binfmt_script.c
binfmt_som.c
bio-integrity.c fs: add export.h to files using EXPORT_SYMBOL/THIS_MODULE macros 2011-10-31 19:30:31 -04:00
bio.c bio: don't overflow in bio_get_nr_vecs() 2012-02-08 22:07:18 +01:00
block_dev.c block: Fix NULL pointer dereference in sd_revalidate_disk 2012-03-02 10:38:33 +01:00
buffer.c fs: move code out of buffer.c 2012-01-03 22:54:07 -05:00
char_dev.c char_dev.c: fix up some whitespace errors 2011-12-13 11:18:17 -08:00
compat_binfmt_elf.c
compat_ioctl.c Merge branch 'v4l_for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media 2012-01-15 12:49:56 -08:00
compat.c vfs: fix compat_sys_stat() handling of overflows in st_nlink 2012-02-13 20:45:39 -05:00
dcache.c vfs: get rid of batshit-insane pointless dentry hash calculations 2012-03-19 16:19:53 -07:00
dcookies.c
direct-io.c Restore direct_io / truncate locking API 2012-02-23 15:56:21 -08:00
drop_caches.c
eventfd.c
eventpoll.c Don't limit non-nested epoll paths 2012-03-18 12:25:04 -07:00
exec.c coredump_wait: don't call complete_vfork_done() 2012-03-05 15:49:42 -08:00
fcntl.c
fhandle.c vfs: prefer ->dentry->d_sb to ->mnt->mnt_sb 2012-01-06 23:16:53 -05:00
fifo.c
file_table.c vfs: prevent remount read-only if pending removes 2012-01-06 23:20:13 -05:00
file.c
filesystems.c vfs: convert fs_supers to hlist 2012-01-03 22:52:39 -05:00
fs_struct.c
fs-writeback.c writeback: fix NULL bdi->dev in trace writeback_single_inode 2012-02-01 16:53:40 +08:00
generic_acl.c
inode.c restore smp_mb() in unlock_new_inode() 2012-03-10 17:07:28 -05:00
internal.h vfs: protect remounting superblock read-only 2012-01-06 23:20:12 -05:00
ioctl.c vfs: fix up ENOIOCTLCMD error handling 2012-01-05 15:40:12 -08:00
ioprio.c block: strip out locking optimization in put_io_context() 2012-02-07 07:51:30 +01:00
Kconfig Merge branch 'for-linus' of git://git.open-osd.org/linux-open-osd 2012-01-09 12:51:01 -08:00
Kconfig.binfmt fs: binfmt_elf: create Kconfig variable for PIE randomization 2012-01-10 16:30:51 -08:00
libfs.c fs: move code out of buffer.c 2012-01-03 22:54:07 -05:00
locks.c vfs: fix handling of lock allocation failure in lease-break case 2011-12-26 10:25:26 -08:00
Makefile Merge branches 'vfsmount-guts', 'umode_t' and 'partitions' into Z 2012-01-06 23:15:54 -05:00
mbcache.c
mount.h vfs: keep list of mounts for each superblock 2012-01-06 23:20:12 -05:00
mpage.c fs: remove unneeded plug in mpage_readpages() 2012-01-12 09:19:54 +01:00
namei.c vfs: fix return value from do_last() 2012-03-10 17:05:30 -05:00
namespace.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial 2012-01-08 13:21:22 -08:00
no-block.c
open.c switch security_path_chmod() to struct path * 2012-01-06 23:16:53 -05:00
pipe.c pipe: fail cleanly when root tries F_SETPIPE_SZ with big size 2012-01-12 20:13:04 -08:00
pnode.c vfs: switch pnode.h macros to struct mount * 2012-01-03 22:57:11 -05:00
pnode.h vfs: switch pnode.h macros to struct mount * 2012-01-03 22:57:11 -05:00
posix_acl.c vfs: pass all mask flags check_acl and posix_acl_permission 2011-10-28 14:58:54 +02:00
proc_namespace.c vfs: switch ->show_options() to struct dentry * 2012-01-06 23:19:54 -05:00
read_write.c Cross Memory Attach 2011-10-31 17:30:44 -07:00
read_write.h
readdir.c
select.c sys_poll: fix incorrect type for 'timeout' parameter 2012-02-21 17:24:20 -08:00
seq_file.c constify seq_file stuff 2012-01-03 22:52:40 -05:00
signalfd.c epoll: ep_unregister_pollwait() can use the freed pwq->whead 2012-02-24 11:42:50 -08:00
splice.c fs: move code out of buffer.c 2012-01-03 22:54:07 -05:00
stack.c filesystems: add set_nlink() 2011-11-02 12:53:43 +01:00
stat.c readlinkat: ensure we return ENOENT for the empty pathname for normal lookups 2011-11-02 12:53:42 +01:00
statfs.c vfs: new helper - vfs_ustat() 2012-01-03 22:53:07 -05:00
super.c vfs: Provide function to get superblock and wait for it to thaw 2012-02-13 20:45:38 -05:00
sync.c fs: move code out of buffer.c 2012-01-03 22:54:07 -05:00
timerfd.c
utimes.c
xattr_acl.c
xattr.c vfs: mnt_drop_write_file() 2012-01-03 22:52:40 -05:00