[PATCH] Btrfs: improve inode hash function/inode lookup

[PATCH] Btrfs: improve inode hash function/inode lookup

[Filipe David Borba Manana]

Currently the hash value used for adding an inode to the VFS's inode
hash table consists of the plain inode number, which is a 64 bits
integer. This results in hash table buckets (hlist_head lists) with
too many elements for at least 2 important scenarios:

1) When we have many subvolumes. Each subvolume has its own btree
   where its files and directories are added to, and each has its
   own objectid (inode number) namespace. This means that if we have
   N subvolumes, and all have inode number X associated to a file or
   directory, the corresponding inodes all map to the same hash table
   entry, resulting in a bucket (hlist_head list) with N elements;

2) On 32 bits machines. Th VFS hash values are unsigned longs, which
   are 32 bits wide on 32 bits machines, and the inode (objectid)
   numbers are 64 bits unsigned integers. We simply cast the inode
   numbers to hash values, which means that for all inodes with the
   same 32 bits lower half, the same hash bucket is used for all of
   them. For example, all inodes with a number (objectid) between
   0x0000_0000_ffff_ffff and 0xffff_ffff_ffff_ffff will end up in
   the same hash table bucket.

This change ensures the inode's hash value depends both on the
objectid (inode number) and its subvolume's (btree root) objectid.
For 32 bits machines, this change gives better entropy by making
the hash value depend on both the upper and lower 32 bits of the
64 bits hash previously computed.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
---
 fs/btrfs/btrfs_inode.h |   20 ++++++++++++++++++++
 fs/btrfs/disk-io.c     |    2 +-
 fs/btrfs/inode.c       |    6 ++++--
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index 71f074e..fbe745e 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -19,6 +19,7 @@
 #ifndef __BTRFS_I__
 #define __BTRFS_I__
 
+#include <linux/hash.h>
 #include "extent_map.h"
 #include "extent_io.h"
 #include "ordered-data.h"
@@ -179,6 +180,25 @@ static inline struct btrfs_inode *BTRFS_I(struct inode *inode)
 	return container_of(inode, struct btrfs_inode, vfs_inode);
 }
 
+static inline unsigned long inode_hash(u64 objectid,
+				       const struct btrfs_root *root)
+{
+	u64 h = objectid ^ (root->objectid * GOLDEN_RATIO_PRIME);
+
+#if BITS_PER_LONG == 32
+	h = (h >> 32) ^ (h & 0xffffffff);
+#endif
+
+	return (unsigned long)h;
+}
+
+static inline void btrfs_insert_inode_hash(struct inode *inode)
+{
+	unsigned long h = inode_hash(inode->i_ino, BTRFS_I(inode)->root);
+
+	__insert_inode_hash(inode, h);
+}
+
 static inline u64 btrfs_ino(struct inode *inode)
 {
 	u64 ino = BTRFS_I(inode)->location.objectid;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index ade6c0e..d205bdd 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2294,7 +2294,7 @@ int open_ctree(struct super_block *sb,
 	       sizeof(struct btrfs_key));
 	set_bit(BTRFS_INODE_DUMMY,
 		&BTRFS_I(fs_info->btree_inode)->runtime_flags);
-	insert_inode_hash(fs_info->btree_inode);
+	btrfs_insert_inode_hash(fs_info->btree_inode);
 
 	spin_lock_init(&fs_info->block_group_cache_lock);
 	fs_info->block_group_cache_tree = RB_ROOT;
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 13b470c..3c76bf4 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4837,10 +4837,12 @@ static struct inode *btrfs_iget_locked(struct super_block *s,
 {
 	struct inode *inode;
 	struct btrfs_iget_args args;
+	unsigned long hashval = inode_hash(objectid, root);
+
 	args.ino = objectid;
 	args.root = root;
 
-	inode = iget5_locked(s, objectid, btrfs_find_actor,
+	inode = iget5_locked(s, hashval, btrfs_find_actor,
 			     btrfs_init_locked_inode,
 			     (void *)&args);
 	return inode;
@@ -5460,7 +5462,7 @@ static struct inode *btrfs_new_inode(struct btrfs_trans_handle *trans,
 				BTRFS_INODE_NODATASUM;
 	}
 
-	insert_inode_hash(inode);
+	btrfs_insert_inode_hash(inode);
 	inode_tree_add(inode);
 
 	trace_btrfs_inode_new(inode);
-- 
1.7.9.5

[PATCH v2] Btrfs: improve inode hash function/inode lookup

[Filipe David Borba Manana]

Currently the hash value used for adding an inode to the VFS's inode
hash table consists of the plain inode number, which is a 64 bits
integer. This results in hash table buckets (hlist_head lists) with
too many elements for at least 2 important scenarios:

1) When we have many subvolumes. Each subvolume has its own btree
   where its files and directories are added to, and each has its
   own objectid (inode number) namespace. This means that if we have
   N subvolumes, and all have inode number X associated to a file or
   directory, the corresponding inodes all map to the same hash table
   entry, resulting in a bucket (hlist_head list) with N elements;

2) On 32 bits machines. Th VFS hash values are unsigned longs, which
   are 32 bits wide on 32 bits machines, and the inode (objectid)
   numbers are 64 bits unsigned integers. We simply cast the inode
   numbers to hash values, which means that for all inodes with the
   same 32 bits lower half, the same hash bucket is used for all of
   them. For example, all inodes with a number (objectid) between
   0x0000_0000_ffff_ffff and 0xffff_ffff_ffff_ffff will end up in
   the same hash table bucket.

This change ensures the inode's hash value depends both on the
objectid (inode number) and its subvolume's (btree root) objectid.
For 32 bits machines, this change gives better entropy by making
the hash value depend on both the upper and lower 32 bits of the
64 bits hash previously computed.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
---

V2: Renamed function inode_hash() to btrfs_inode_hash().

 fs/btrfs/btrfs_inode.h |   20 ++++++++++++++++++++
 fs/btrfs/disk-io.c     |    2 +-
 fs/btrfs/inode.c       |    6 ++++--
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index 71f074e..ac0b39d 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -19,6 +19,7 @@
 #ifndef __BTRFS_I__
 #define __BTRFS_I__
 
+#include <linux/hash.h>
 #include "extent_map.h"
 #include "extent_io.h"
 #include "ordered-data.h"
@@ -179,6 +180,25 @@ static inline struct btrfs_inode *BTRFS_I(struct inode *inode)
 	return container_of(inode, struct btrfs_inode, vfs_inode);
 }
 
+static inline unsigned long btrfs_inode_hash(u64 objectid,
+					     const struct btrfs_root *root)
+{
+	u64 h = objectid ^ (root->objectid * GOLDEN_RATIO_PRIME);
+
+#if BITS_PER_LONG == 32
+	h = (h >> 32) ^ (h & 0xffffffff);
+#endif
+
+	return (unsigned long)h;
+}
+
+static inline void btrfs_insert_inode_hash(struct inode *inode)
+{
+	unsigned long h = btrfs_inode_hash(inode->i_ino, BTRFS_I(inode)->root);
+
+	__insert_inode_hash(inode, h);
+}
+
 static inline u64 btrfs_ino(struct inode *inode)
 {
 	u64 ino = BTRFS_I(inode)->location.objectid;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index ade6c0e..d205bdd 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2294,7 +2294,7 @@ int open_ctree(struct super_block *sb,
 	       sizeof(struct btrfs_key));
 	set_bit(BTRFS_INODE_DUMMY,
 		&BTRFS_I(fs_info->btree_inode)->runtime_flags);
-	insert_inode_hash(fs_info->btree_inode);
+	btrfs_insert_inode_hash(fs_info->btree_inode);
 
 	spin_lock_init(&fs_info->block_group_cache_lock);
 	fs_info->block_group_cache_tree = RB_ROOT;
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 13b470c..cc3de9d 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4837,10 +4837,12 @@ static struct inode *btrfs_iget_locked(struct super_block *s,
 {
 	struct inode *inode;
 	struct btrfs_iget_args args;
+	unsigned long hashval = btrfs_inode_hash(objectid, root);
+
 	args.ino = objectid;
 	args.root = root;
 
-	inode = iget5_locked(s, objectid, btrfs_find_actor,
+	inode = iget5_locked(s, hashval, btrfs_find_actor,
 			     btrfs_init_locked_inode,
 			     (void *)&args);
 	return inode;
@@ -5460,7 +5462,7 @@ static struct inode *btrfs_new_inode(struct btrfs_trans_handle *trans,
 				BTRFS_INODE_NODATASUM;
 	}
 
-	insert_inode_hash(inode);
+	btrfs_insert_inode_hash(inode);
 	inode_tree_add(inode);
 
 	trace_btrfs_inode_new(inode);
-- 
1.7.9.5

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Re: [PATCH v2] Btrfs: improve inode hash function/inode lookup

[David Sterba]

On Sun, Oct 06, 2013 at 10:22:33PM +0100, Filipe David Borba Manana wrote:
> 2) On 32 bits machines. Th VFS hash values are unsigned longs, which
>    are 32 bits wide on 32 bits machines, and the inode (objectid)
>    numbers are 64 bits unsigned integers. We simply cast the inode
>    numbers to hash values, which means that for all inodes with the
>    same 32 bits lower half, the same hash bucket is used for all of
>    them. For example, all inodes with a number (objectid) between
>    0x0000_0000_ffff_ffff and 0xffff_ffff_ffff_ffff will end up in
>    the same hash table bucket.

Well, inode number that does not fit into 32 bits on a 32 bit machine
causes other problems. And subvolume ids that do not fit into 32 bits
cannot be stored in radix tree.

It would be safer to refuse creating/accessing inode/subvolume with
nubmer that does not fit into 32bits.

david

Re: [PATCH v2] Btrfs: improve inode hash function/inode lookup

[Filipe David Manana]

On Fri, Oct 11, 2013 at 12:42 PM, David Sterba <dsterba@suse.cz> wrote:
> On Sun, Oct 06, 2013 at 10:22:33PM +0100, Filipe David Borba Manana wrote:
>> 2) On 32 bits machines. Th VFS hash values are unsigned longs, which
>>    are 32 bits wide on 32 bits machines, and the inode (objectid)
>>    numbers are 64 bits unsigned integers. We simply cast the inode
>>    numbers to hash values, which means that for all inodes with the
>>    same 32 bits lower half, the same hash bucket is used for all of
>>    them. For example, all inodes with a number (objectid) between
>>    0x0000_0000_ffff_ffff and 0xffff_ffff_ffff_ffff will end up in
>>    the same hash table bucket.
>
> Well, inode number that does not fit into 32 bits on a 32 bit machine
> causes other problems. And subvolume ids that do not fit into 32 bits
> cannot be stored in radix tree.
>
> It would be safer to refuse creating/accessing inode/subvolume with
> nubmer that does not fit into 32bits.

Good point David. However it would be a rather radical behaviour change imho.
I think it should be a separate change if there are no objections
against such change.

>
> david



-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."

Re: [PATCH v2] Btrfs: improve inode hash function/inode lookup

[David Sterba]

On Fri, Oct 11, 2013 at 06:20:16PM +0100, Filipe David Manana wrote:
> Good point David. However it would be a rather radical behaviour change imho.

A radical step to prevent this practictly impossible but nasty bug to
happen :)

> I think it should be a separate change if there are no objections
> against such change.

It is a separate change that makes your patch obsolete.

david

Re: [PATCH v2] Btrfs: improve inode hash function/inode lookup

[Filipe David Manana]

On Tue, Oct 22, 2013 at 12:21 AM, David Sterba <dsterba@suse.cz> wrote:
> On Fri, Oct 11, 2013 at 06:20:16PM +0100, Filipe David Manana wrote:
>> Good point David. However it would be a rather radical behaviour change imho.
>
> A radical step to prevent this practictly impossible but nasty bug to
> happen :)
>
>> I think it should be a separate change if there are no objections
>> against such change.
>
> It is a separate change that makes your patch obsolete.

Except the part of making the hash value depend on both object id and
root id, no?

>
> david



-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."

Re: [PATCH v2] Btrfs: improve inode hash function/inode lookup

[David Sterba]

On Tue, Oct 22, 2013 at 12:27:21AM +0100, Filipe David Manana wrote:
> > It is a separate change that makes your patch obsolete.
> 
> Except the part of making the hash value depend on both object id and
> root id, no?

I've read the patch again, yes, case 1 makes sense.

david