shrink struct dentry

struct dentry is one of the most critical structures in the kernel. So it's
sad to see it going neglected.

With CONFIG_PROFILING turned on (which is probably the common case at least
for distros and kernel developers), sizeof(struct dcache) == 208 here
(64-bit). This gives 19 objects per slab.

I packed d_mounted into a hole, and took another 4 bytes off the inline
name length to take the padding out from the end of the structure. This
shinks it to 200 bytes. I could have gone the other way and increased the
length to 40, but I'm aiming for a magic number, read on...

I then got rid of the d_cookie pointer. This shrinks it to 192 bytes. Rant:
why was this ever a good idea? The cookie system should increase its hash
size or use a tree or something if lookups are a problem. Also the "fast
dcookie lookups" in oprofile should be moved into the dcookie code -- how
can oprofile possibly care about the dcookie_mutex? It gets dropped after
get_dcookie() returns so it can't be providing any sort of protection.

At 192 bytes, 21 objects fit into a 4K page, saving about 3MB on my system
with ~140 000 entries allocated. 192 is also a multiple of 64, so we get
nice cacheline alignment on 64 and 32 byte line systems -- any given dentry
will now require 3 cachelines to touch all fields wheras previously it
would require 4.

I know the inline name size was chosen quite carefully, however with the
reduction in cacheline footprint, it should actually be just about as fast
to do a name lookup for a 36 character name as it was before the patch (and
faster for other sizes). The memory footprint savings for names which are
<= 32 or > 36 bytes long should more than make up for the memory cost for
33-36 byte names.

Performance is a feature...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 19 insertions, 9 deletions

diff --git a/fs/dcookies.c b/fs/dcookies.c
index 855d4b1d619a..180e9fec4ad8 100644
--- a/fs/dcookies.c
+++ b/fs/dcookies.c
@@ -93,10 +93,15 @@ static struct dcookie_struct *alloc_dcookie(struct path *path)
 {
         struct dcookie_struct *dcs = kmem_cache_alloc(dcookie_cache,
                                                         GFP_KERNEL);
+        struct dentry *d;
         if (!dcs)
                 return NULL;
 
-        path->dentry->d_cookie = dcs;
+        d = path->dentry;
+        spin_lock(&d->d_lock);
+        d->d_flags |= DCACHE_COOKIE;
+        spin_unlock(&d->d_lock);
+
         dcs->path = *path;
         path_get(path);
         hash_dcookie(dcs);
@@ -119,14 +124,14 @@ int get_dcookie(struct path *path, unsigned long *cookie)
                 goto out;
         }
 
-        dcs = path->dentry->d_cookie;
-
-        if (!dcs)
+        if (path->dentry->d_flags & DCACHE_COOKIE) {
+                dcs = find_dcookie((unsigned long)path->dentry);
+        } else {
                 dcs = alloc_dcookie(path);
-
-        if (!dcs) {
-                err = -ENOMEM;
-                goto out;
+                if (!dcs) {
+                        err = -ENOMEM;
+                        goto out;
+                }
         }
 
         *cookie = dcookie_value(dcs);
@@ -251,7 +256,12 @@ out_kmem:
 
 static void free_dcookie(struct dcookie_struct * dcs)
 {
-        dcs->path.dentry->d_cookie = NULL;
+        struct dentry *d = dcs->path.dentry;
+
+        spin_lock(&d->d_lock);
+        d->d_flags &= ~DCACHE_COOKIE;
+        spin_unlock(&d->d_lock);
+
         path_put(&dcs->path);
         kmem_cache_free(dcookie_cache, dcs);
 }