On Wed, Nov 20, 2024 at 12:20:34PM +0100, Mateusz Guzik wrote:
> When utilized it dodges strlen() in vfs_readlink(), giving about 1.5%
> speed up when issuing readlink on /initrd.img on ext4.
>
> Filesystems opt in by calling inode_set_cached_link() when creating an
> inode.
>
> The size is stored in a new union utilizing the same space as i_devices,
> thus avoiding growing the struct or taking up any more space.
>
> Churn-wise the current readlink_copy() helper is patched to accept the
> size instead of calculating it.
>
> Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
> ---
> fs/namei.c | 34 +++++++++++++++++++---------------
> fs/proc/namespaces.c | 2 +-
> include/linux/fs.h | 15 +++++++++++++--
> security/apparmor/apparmorfs.c | 2 +-
> 4 files changed, 34 insertions(+), 19 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 9d30c7aa9aa6..e56c29a22d26 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -5272,19 +5272,16 @@ SYSCALL_DEFINE2(rename, const char __user *, oldname, const char __user *, newna
> getname(newname), 0);
> }
>
> -int readlink_copy(char __user *buffer, int buflen, const char *link)
> +int readlink_copy(char __user *buffer, int buflen, const char *link, int linklen)
> {
> - int len = PTR_ERR(link);
> - if (IS_ERR(link))
> - goto out;
> + int copylen;
>
> - len = strlen(link);
> - if (len > (unsigned) buflen)
> - len = buflen;
> - if (copy_to_user(buffer, link, len))
> - len = -EFAULT;
> -out:
> - return len;
> + copylen = linklen;
> + if (unlikely(copylen > (unsigned) buflen))
> + copylen = buflen;
> + if (copy_to_user(buffer, link, copylen))
> + copylen = -EFAULT;
> + return copylen;
> }
>
> /**
> @@ -5304,6 +5301,9 @@ int vfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
> const char *link;
> int res;
>
> + if (inode->i_opflags & IOP_CACHED_LINK)
> + return readlink_copy(buffer, buflen, inode->i_link, inode->i_linklen);
> +
> if (unlikely(!(inode->i_opflags & IOP_DEFAULT_READLINK))) {
> if (unlikely(inode->i_op->readlink))
> return inode->i_op->readlink(dentry, buffer, buflen);
> @@ -5322,7 +5322,7 @@ int vfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
> if (IS_ERR(link))
> return PTR_ERR(link);
> }
> - res = readlink_copy(buffer, buflen, link);
> + res = readlink_copy(buffer, buflen, link, strlen(link));
> do_delayed_call(&done);
> return res;
> }
> @@ -5391,10 +5391,14 @@ EXPORT_SYMBOL(page_put_link);
>
> int page_readlink(struct dentry *dentry, char __user *buffer, int buflen)
> {
> + const char *link;
> + int res;
> +
> DEFINE_DELAYED_CALL(done);
> - int res = readlink_copy(buffer, buflen,
> - page_get_link(dentry, d_inode(dentry),
> - &done));
> + link = page_get_link(dentry, d_inode(dentry), &done);
> + res = PTR_ERR(link);
> + if (!IS_ERR(link))
> + res = readlink_copy(buffer, buflen, link, strlen(link));
> do_delayed_call(&done);
> return res;
> }
> diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
> index 8e159fc78c0a..c610224faf10 100644
> --- a/fs/proc/namespaces.c
> +++ b/fs/proc/namespaces.c
> @@ -83,7 +83,7 @@ static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int bufl
> if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
> res = ns_get_name(name, sizeof(name), task, ns_ops);
> if (res >= 0)
> - res = readlink_copy(buffer, buflen, name);
> + res = readlink_copy(buffer, buflen, name, strlen(name));
> }
> put_task_struct(task);
> return res;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 7e29433c5ecc..2cc98de5af43 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -626,6 +626,7 @@ is_uncached_acl(struct posix_acl *acl)
> #define IOP_XATTR 0x0008
> #define IOP_DEFAULT_READLINK 0x0010
> #define IOP_MGTIME 0x0020
> +#define IOP_CACHED_LINK 0x0040
>
> /*
> * Keep mostly read-only and often accessed (especially for
> @@ -723,7 +724,10 @@ struct inode {
> };
> struct file_lock_context *i_flctx;
> struct address_space i_data;
> - struct list_head i_devices;
> + union {
> + struct list_head i_devices;
> + int i_linklen;
> + };
I think that i_devices should be moved into the union as it's really
only used with i_cdev but it's not that easily done because list_head
needs to be initialized. I roughly envisioned something like:
union {
struct {
struct cdev *i_cdev;
struct list_head i_devices;
};
struct {
char *i_link;
unsigned int i_link_len;
};
struct pipe_inode_info *i_pipe;
unsigned i_dir_seq;
};
But it's not important enough imho.
