On Thu, 28 May 2009, Nick Piggin wrote:
> Please humour a relative vfs/procfs newbie... Why do filesystems insist that
> a dentry must be !DCACHE_UNHASHED in order to work in /proc/pid/fd/? I can't
> find the code that checks this, and pipefs fds for example _seems_ to work in
> proc/pid with this patch applied...
I suspect this is more of an optimization: the old code tries not to
burden the global dentry cache with all the anonymous pipes, sockets,
etc. since they won't ever be looked up from the cache.
Thanks,
Miklos
>
>
> ---
> fs/anon_inodes.c | 16 ----------------
> fs/pipe.c | 18 ------------------
> net/socket.c | 19 -------------------
> 3 files changed, 53 deletions(-)
>
> Index: linux-2.6/fs/pipe.c
> ===================================================================
> --- linux-2.6.orig/fs/pipe.c
> +++ linux-2.6/fs/pipe.c
> @@ -873,17 +873,6 @@ void free_pipe_info(struct inode *inode)
> }
>
> static struct vfsmount *pipe_mnt __read_mostly;
> -static int pipefs_delete_dentry(struct dentry *dentry)
> -{
> - /*
> - * 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;
> -}
>
> /*
> * pipefs_dname() is called from d_path().
> @@ -895,7 +884,6 @@ static char *pipefs_dname(struct dentry
> }
>
> static const struct dentry_operations pipefs_dentry_operations = {
> - .d_delete = pipefs_delete_dentry,
> .d_dname = pipefs_dname,
> };
>
> @@ -955,12 +943,6 @@ struct file *create_write_pipe(int flags
> goto err_inode;
>
> dentry->d_op = &pipefs_dentry_operations;
> - /*
> - * We dont want to publish this dentry into global dentry hash table.
> - * We pretend dentry is already hashed, by unsetting DCACHE_UNHASHED
> - * This permits a working /proc/$pid/fd/XXX on pipes
> - */
> - dentry->d_flags &= ~DCACHE_UNHASHED;
> d_instantiate(dentry, inode);
>
> err = -ENFILE;
> Index: linux-2.6/net/socket.c
> ===================================================================
> --- linux-2.6.orig/net/socket.c
> +++ linux-2.6/net/socket.c
> @@ -307,18 +307,6 @@ static struct file_system_type sock_fs_t
> .kill_sb = kill_anon_super,
> };
>
> -static int sockfs_delete_dentry(struct dentry *dentry)
> -{
> - /*
> - * At creation time, we pretended this dentry was hashed
> - * (by clearing DCACHE_UNHASHED bit in d_flags)
> - * At delete time, we restore the truth : not hashed.
> - * (so that dput() can proceed correctly)
> - */
> - dentry->d_flags |= DCACHE_UNHASHED;
> - return 0;
> -}
> -
> /*
> * sockfs_dname() is called from d_path().
> */
> @@ -329,7 +317,6 @@ static char *sockfs_dname(struct dentry
> }
>
> static const struct dentry_operations sockfs_dentry_operations = {
> - .d_delete = sockfs_delete_dentry,
> .d_dname = sockfs_dname,
> };
>
> @@ -378,12 +365,6 @@ static int sock_attach_fd(struct socket
> return -ENOMEM;
>
> dentry->d_op = &sockfs_dentry_operations;
> - /*
> - * We dont want to push this dentry into global dentry hash table.
> - * We pretend dentry is already hashed, by unsetting DCACHE_UNHASHED
> - * This permits a working /proc/$pid/fd/XXX on sockets
> - */
> - dentry->d_flags &= ~DCACHE_UNHASHED;
> d_instantiate(dentry, SOCK_INODE(sock));
>
> sock->file = file;
> Index: linux-2.6/fs/anon_inodes.c
> ===================================================================
> --- linux-2.6.orig/fs/anon_inodes.c
> +++ linux-2.6/fs/anon_inodes.c
> @@ -33,24 +33,11 @@ static int anon_inodefs_get_sb(struct fi
> mnt);
> }
>
> -static int anon_inodefs_delete_dentry(struct dentry *dentry)
> -{
> - /*
> - * We faked vfs to believe the dentry was hashed when we created it.
> - * Now we restore the flag so that dput() will work correctly.
> - */
> - dentry->d_flags |= DCACHE_UNHASHED;
> - return 1;
> -}
> -
> static struct file_system_type anon_inode_fs_type = {
> .name = "anon_inodefs",
> .get_sb = anon_inodefs_get_sb,
> .kill_sb = kill_anon_super,
> };
> -static const struct dentry_operations anon_inodefs_dentry_operations = {
> - .d_delete = anon_inodefs_delete_dentry,
> -};
>
> /**
> * anon_inode_getfd - creates a new file instance by hooking it up to an
> @@ -106,9 +93,6 @@ int anon_inode_getfd(const char *name, c
> */
> atomic_inc(&anon_inode_inode->i_count);
>
> - dentry->d_op = &anon_inodefs_dentry_operations;
> - /* Do not publish this dentry inside the global dentry hash table */
> - dentry->d_flags &= ~DCACHE_UNHASHED;
> d_instantiate(dentry, anon_inode_inode);
>
> error = -ENFILE;
> --
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