Messages in this thread Patch in this message | | | Date | Wed, 01 Nov 2006 14:54:56 +0100 | From | Eric Dumazet <> | Subject | Re: [RFC, PATCH] dont insert sockets/pipes dentries into dentry_hashtable. |
| |
David Miller a écrit : > It turns out that while procfs uses a different "mechanism", those > procfs symlinks do point to the real socket dentry, so when you > readlink() on it you do d_path() on the real socket dentry. > > Al Viro just suggested a way around this to me: > > 1) Just mark the dentry HASHED by hand in the dentry flags, but don't > actually hash it. > > 2) Create a special dentry->d_deleted method for sockets that returns > 0 and clears by hand the HASHED flag bit in the dentry (see what > dput() does when this happens). > > It's an abuse but it will work. >
Thank you David and Al for the feedback.
Here is a new version of the patch with your suggestions included.
If necessary, I could split this patch in 3 elementary patches. I chose to sent it as one patch for ease of discussion.
[RFC, PATCH] dont insert sockets/pipes dentries into dentry_hashtable.
We currently insert sockets/pipes dentries into the global dentry hashtable. This is *useless* because there is currently no way these entries can be used for a lookup(). (/proc/xxx/fd/xxx uses a different mechanism)
Machines with a lot of sockets/pipes might suffer from longer chains in dentry hashtable.
Since dentries an unhashed dentry means __dpath() adds a " (deleted)", the trick for socket/pipe dentries is to :
- Right after d_alloc(), pretend they are hashed by clearing the DCACHE_UNHASHED bit. __dpath() & friends work as intended.
- Call d_instantiate() instead of d_add() : dentry is not inserted in hash table.
- Once dput() must clear the dentry, setting again DCACHE_UNHASHED bit inside the custom d_delete() function provided by socket/pipe code.
[patch 1/3] Small optimization to bypass RCU freeing in d_free() for dentries that were never hashed (like sockets and pipes). Such dentries dont have to wait a RCU grace period.
[patch 2/3] Change socket code to use d_instantiate() instead of d_add()
[patch 3/3] Change pipe code to use d_instantiate() instead of d_add()
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com> --- linux-2.6.19-rc4/fs/dcache.c 2006-11-01 12:38:23.000000000 +0100 +++ linux-2.6.19-rc4-ed/fs/dcache.c 2006-11-01 13:22:44.000000000 +0100 @@ -68,15 +68,19 @@ .age_limit = 45, }; -static void d_callback(struct rcu_head *head) +static void __d_free(struct dentry *dentry) { - struct dentry * dentry = container_of(head, struct dentry, d_u.d_rcu); - if (dname_external(dentry)) kfree(dentry->d_name.name); kmem_cache_free(dentry_cache, dentry); } +static void d_callback(struct rcu_head *head) +{ + struct dentry * dentry = container_of(head, struct dentry, d_u.d_rcu); + __d_free(dentry); +} + /* * no dcache_lock, please. The caller must decrement dentry_stat.nr_dentry * inside dcache_lock. @@ -85,7 +89,11 @@ { if (dentry->d_op && dentry->d_op->d_release) dentry->d_op->d_release(dentry); - call_rcu(&dentry->d_u.d_rcu, d_callback); + /* if dentry was never inserted into hash, immediate free is OK */ + if (dentry->d_hash.pprev == NULL) + __d_free(dentry); + else + call_rcu(&dentry->d_u.d_rcu, d_callback); } /* --- linux-2.6.19-rc4/net/socket.c 2006-11-01 12:40:27.000000000 +0100 +++ linux-2.6.19-rc4-ed/net/socket.c 2006-11-01 14:33:18.000000000 +0100 @@ -306,7 +306,14 @@ static int sockfs_delete_dentry(struct dentry *dentry) { - return 1; + /* + * At creation time, we pretended this dentry was hashed + * (by clearing DCACHE_UNHASHED bit in d_flags) + * At delete time, we restore the truth : not hashed. + * (so that dput() can proceed correctly) + */ + dentry->d_flags |= DCACHE_UNHASHED; + return 0; } static struct dentry_operations sockfs_dentry_operations = { .d_delete = sockfs_delete_dentry, @@ -354,14 +361,20 @@ this.len = sprintf(name, "[%lu]", SOCK_INODE(sock)->i_ino); this.name = name; - this.hash = SOCK_INODE(sock)->i_ino; + this.hash = 0; file->f_dentry = d_alloc(sock_mnt->mnt_sb->s_root, &this); if (unlikely(!file->f_dentry)) return -ENOMEM; file->f_dentry->d_op = &sockfs_dentry_operations; - d_add(file->f_dentry, SOCK_INODE(sock)); + /* + * We dont want to push this dentry into global dentry hash table. + * We pretend dentry is already hashed, by unsetting DCACHE_UNHASHED + * This hack permits a working /proc/$pid/fd/XXX on sockets + */ + file->f_dentry->d_flags &= ~DCACHE_UNHASHED; + d_instantiate(file->f_dentry, SOCK_INODE(sock)); file->f_vfsmnt = mntget(sock_mnt); file->f_mapping = file->f_dentry->d_inode->i_mapping; --- linux-2.6.19-rc4/fs/pipe.c 2006-11-01 12:56:05.000000000 +0100 +++ linux-2.6.19-rc4-ed/fs/pipe.c 2006-11-01 14:33:18.000000000 +0100 @@ -830,7 +830,14 @@ static struct vfsmount *pipe_mnt __read_mostly; static int pipefs_delete_dentry(struct dentry *dentry) { - return 1; + /* + * At creation time, we pretended this dentry was hashed + * (by clearing DCACHE_UNHASHED bit in d_flags) + * At delete time, we restore the truth : not hashed. + * (so that dput() can proceed correctly) + */ + dentry->d_flags |= DCACHE_UNHASHED; + return 0; } static struct dentry_operations pipefs_dentry_operations = { @@ -891,17 +898,22 @@ if (!inode) goto err_file; - sprintf(name, "[%lu]", inode->i_ino); + this.len = sprintf(name, "[%lu]", inode->i_ino); this.name = name; - this.len = strlen(name); - this.hash = inode->i_ino; /* will go */ + this.hash = 0; err = -ENOMEM; dentry = d_alloc(pipe_mnt->mnt_sb->s_root, &this); if (!dentry) goto err_inode; dentry->d_op = &pipefs_dentry_operations; - d_add(dentry, inode); + /* + * We dont want to push this dentry into global dentry hash table. + * We pretend dentry is already hashed, by unsetting DCACHE_UNHASHED + * This hack permits a working /proc/$pid/fd/XXX on pipes + */ + dentry->d_flags &= ~DCACHE_UNHASHED; + d_instantiate(dentry, inode); f->f_vfsmnt = mntget(pipe_mnt); f->f_dentry = dentry; f->f_mapping = inode->i_mapping; | |