When the audit subsystem is enabled, it can do a lot of get_fs_pwd() calls to get references to fs->pwd and then releasing those references back with path_put() later. That may cause a lot of spinlock contention on a single pwd's dentry lock because of the constant changes to the reference count when there are many processes on the same working directory actively doing open/close system calls. This can cause noticeable performance regresssion when compared with the case where the audit subsystem is turned off especially on systems with a lot of CPUs which is becoming more common these days.
A simple and elegant solution to avoid this kind of performance regression is to add a common pool of extra fs->pwd references inside the fs_struct. When a caller needs a pwd reference, it can borrow one from pool, if available, to avoid an explicit path_get(). When it is time to release the reference, it can put it back into the common pool if fs->pwd isn't changed before without doing a path_put(). We still need to acquire the fs's spinlock, but fs_struct is more distributed and it is less common to have many tasks sharing a single fs_struct. A new set of get_fs_pwd_pool/put_fs_pwd_pool() APIs are introduced with this patch to enable other subsystems to acquire and release a pwd reference from the common pool without doing unnecessary path_get/path_put(). Besides fs/fs_struct.c, the copy_mnt_ns() function of fs/namespace.c is also modified to properly handle the extra pwd references, if available. Signed-off-by: Waiman Long <[email protected]> --- fs/fs_struct.c | 26 +++++++++++++++++++++----- fs/namespace.c | 8 ++++++++ include/linux/fs_struct.h | 30 +++++++++++++++++++++++++++++- 3 files changed, 58 insertions(+), 6 deletions(-) diff --git a/fs/fs_struct.c b/fs/fs_struct.c index b8c46c5a38a0..621fe1677913 100644 --- a/fs/fs_struct.c +++ b/fs/fs_struct.c @@ -32,15 +32,19 @@ void set_fs_root(struct fs_struct *fs, const struct path *path) void set_fs_pwd(struct fs_struct *fs, const struct path *path) { struct path old_pwd; + int count; path_get(path); write_seqlock(&fs->seq); old_pwd = fs->pwd; fs->pwd = *path; + count = fs->pwd_refs + 1; + fs->pwd_refs = 0; write_sequnlock(&fs->seq); if (old_pwd.dentry) - path_put(&old_pwd); + while (count--) + path_put(&old_pwd); } static inline int replace_path(struct path *p, const struct path *old, const struct path *new) @@ -62,10 +66,15 @@ void chroot_fs_refs(const struct path *old_root, const struct path *new_root) task_lock(p); fs = p->fs; if (fs) { - int hits = 0; + int hits; + write_seqlock(&fs->seq); + hits = replace_path(&fs->pwd, old_root, new_root); + if (hits && fs->pwd_refs) { + count += fs->pwd_refs; + fs->pwd_refs = 0; + } hits += replace_path(&fs->root, old_root, new_root); - hits += replace_path(&fs->pwd, old_root, new_root); while (hits--) { count++; path_get(new_root); @@ -81,8 +90,11 @@ void chroot_fs_refs(const struct path *old_root, const struct path *new_root) void free_fs_struct(struct fs_struct *fs) { + int count = fs->pwd_refs + 1; + path_put(&fs->root); - path_put(&fs->pwd); + while (count--) + path_put(&fs->pwd); kmem_cache_free(fs_cachep, fs); } @@ -110,6 +122,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old) if (fs) { fs->users = 1; fs->in_exec = 0; + fs->pwd_refs = 0; seqlock_init(&fs->seq); fs->umask = old->umask; @@ -117,7 +130,10 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old) fs->root = old->root; path_get(&fs->root); fs->pwd = old->pwd; - path_get(&fs->pwd); + if (old->pwd_refs) + old->pwd_refs--; + else + path_get(&fs->pwd); read_sequnlock_excl(&old->seq); } return fs; diff --git a/fs/namespace.c b/fs/namespace.c index c58674a20cad..a2323ba84d76 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -4135,6 +4135,14 @@ struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns, * as belonging to new namespace. We have already acquired a private * fs_struct, so tsk->fs->lock is not needed. */ + if (new_fs) + WARN_ON_ONCE(new_fs->users != 1); + + /* Release the extra pwd references of new_fs, if present. */ + while (new_fs && new_fs->pwd_refs) { + path_put(&new_fs->pwd); + new_fs->pwd_refs--; + } p = old; q = new; while (p) { diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h index 0070764b790a..093648e65c20 100644 --- a/include/linux/fs_struct.h +++ b/include/linux/fs_struct.h @@ -8,10 +8,11 @@ #include <linux/seqlock.h> struct fs_struct { - int users; seqlock_t seq; + int users; int umask; int in_exec; + int pwd_refs; /* A pool of extra pwd references */ struct path root, pwd; } __randomize_layout; @@ -40,6 +41,33 @@ static inline void get_fs_pwd(struct fs_struct *fs, struct path *pwd) read_sequnlock_excl(&fs->seq); } +/* Acquire a pwd reference from the pwd_refs pool, if available */ +static inline void get_fs_pwd_pool(struct fs_struct *fs, struct path *pwd) +{ + read_seqlock_excl(&fs->seq); + *pwd = fs->pwd; + if (fs->pwd_refs) + fs->pwd_refs--; + else + path_get(pwd); + read_sequnlock_excl(&fs->seq); +} + +/* Release a pwd reference back to the pwd_refs pool, if appropriate */ +static inline void put_fs_pwd_pool(struct fs_struct *fs, struct path *pwd) +{ + bool put = false; + + read_seqlock_excl(&fs->seq); + if ((fs->pwd.dentry == pwd->dentry) && (fs->pwd.mnt == pwd->mnt)) + fs->pwd_refs++; + else + put = true; + read_sequnlock_excl(&fs->seq); + if (put) + path_put(pwd); +} + extern bool current_chrooted(void); static inline int current_umask(void) -- 2.52.0
