[PATCH 3/3] avoid extra path_get/path_put cycle in path_openat()

Al Viro
2024-08-22T00:41:51Z

Once we'd opened the file, nd->path and file->f_path have the
same contents. Rather than having both pinned and nd->path
dropped by terminate_walk(), let's have them share the
references from the moment when FMODE_OPENED is set and
clear nd->path just before the terminate_walk() in such case.

To do that, we
* add a variant of vfs_open() that does *not* do conditional
path_get() (vfs_open_borrow()); use it in do_open().
* don't grab f->f_path.mnt in finish_open() - only
f->f_path.dentry. Have atomic_open() drop the child dentry
in FMODE_OPENED case and return f->path.dentry without grabbing it.
* adjust vfs_tmpfile() for finish_open() change (it
is called from ->tmpfile() instances).
* make do_o_path() use vfs_open_borrow(), collapse path_put()
there with the conditional path_get() we would've get in vfs_open().
* in FMODE_OPENED case clear nd->path before calling
terminate_walk().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/internal.h | 1 +
fs/namei.c | 22 ++++++++++++++--------
fs/open.c | 19 ++++++++++++++++++-
3 files changed, 33 insertions(+), 9 deletions(-)

diff --git a/fs/internal.h b/fs/internal.h
index cdd73209eecb..11834829cc3f 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -194,6 +194,7 @@ int do_fchownat(int dfd, const char __user *filename, uid_t user, gid_t group,
int flag);
int chown_common(const struct path *path, uid_t user, gid_t group);
extern int vfs_open(const struct path *, struct file *);
+extern int vfs_open_borrow(const struct path *, struct file *);

/*
* inode.c
diff --git a/fs/namei.c b/fs/namei.c
index 5512cb10fa89..e02160460422 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3443,10 +3443,8 @@ static struct dentry *atomic_open(struct nameidata *nd, struct dentry *dentry,
d_lookup_done(dentry);
if (!error) {
if (file->f_mode & FMODE_OPENED) {
- if (unlikely(dentry != file->f_path.dentry)) {
- dput(dentry);
- dentry = dget(file->f_path.dentry);
- }
+ dput(dentry);
+ dentry = file->f_path.dentry;
} else if (WARN_ON(file->f_path.dentry == DENTRY_NOT_SET)) {
error = -EIO;
} else {
@@ -3724,7 +3722,7 @@ static int do_open(struct nameidata *nd,
}
error = may_open(idmap, &nd->path, acc_mode, open_flag);
if (!error && !(file->f_mode & FMODE_OPENED))
- error = vfs_open(&nd->path, file);
+ error = vfs_open_borrow(&nd->path, file);
if (!error)
error = security_file_post_open(file, op->acc_mode);
if (!error && do_truncate)
@@ -3777,8 +3775,10 @@ int vfs_tmpfile(struct mnt_idmap *idmap,
mode = vfs_prepare_mode(idmap, dir, mode, mode, mode);
error = dir->i_op->tmpfile(idmap, dir, file, mode);
dput(child);
- if (file->f_mode & FMODE_OPENED)
+ if (file->f_mode & FMODE_OPENED) {
+ mntget(file->f_path.mnt);
fsnotify_open(file);
+ }
if (error)
return error;
/* Don't check for other permissions, the inode was just created */
@@ -3857,8 +3857,9 @@ static int do_o_path(struct nameidata *nd, unsigned flags, struct file *file)
int error = path_lookupat(nd, flags, &path);
if (!error) {
audit_inode(nd->name, path.dentry, 0);
- error = vfs_open(&path, file);
- path_put(&path);
+ error = vfs_open_borrow(&path, file);
+ if (!(file->f_mode & FMODE_OPENED))
+ path_put(&path);
}
return error;
}
@@ -3884,6 +3885,11 @@ static struct file *path_openat(struct nameidata *nd,
;
if (!error)
error = do_open(nd, file, op);
+ if (file->f_mode & FMODE_OPENED) {
+ // borrowed into file->f_path, transfer it there
+ nd->path.mnt = NULL;
+ nd->path.dentry = NULL;
+ }
terminate_walk(nd);
}
if (likely(!error)) {
diff --git a/fs/open.c b/fs/open.c
index 0ec2e9a33856..f9988427fb97 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1046,7 +1046,7 @@ int finish_open(struct file *file, struct dentry *dentry,
file->f_path.dentry = dentry;
err = do_dentry_open(file, open);
if (file->f_mode & FMODE_OPENED)
- path_get(&file->f_path);
+ dget(&file->f_path.dentry);
return err;
}
EXPORT_SYMBOL(finish_open);
@@ -1102,6 +1102,23 @@ int vfs_open(const struct path *path, struct file *file)
return ret;
}

+int vfs_open_borrow(const struct path *path, struct file *file)
+{
+ int ret;
+
+ file->f_path = *path;
+ ret = do_dentry_open(file, NULL);
+ if (!ret) {
+ /*
+ * Once we return a file with FMODE_OPENED, __fput() will call
+ * fsnotify_close(), so we need fsnotify_open() here for
+ * symmetry.
+ */
+ fsnotify_open(file);
+ }
+ return ret;
+}
+
struct file *dentry_open(const struct path *path, int flags,
const struct cred *cred)
{
--
2.39.2