The patch titled Subject: mm: mmap_lock: replace get_memcg_path_buf() with on-stack buffer has been added to the -mm mm-unstable branch. Its filename is mm-mmap_lock-replace-get_memcg_path_buf-with-on-stack-buffer.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days

------------------------------------------------------ From: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Subject: mm: mmap_lock: replace get_memcg_path_buf() with on-stack buffer Date: Fri, 21 Jun 2024 10:08:41 +0900

Commit 2b5067a8143e ("mm: mmap_lock: add tracepoints around lock acquisition") introduced TRACE_MMAP_LOCK_EVENT() macro using preempt_disable() in order to let get_mm_memcg_path() return a percpu buffer exclusively used by normal, softirq, irq and NMI contexts respectively.

Commit 832b50725373 ("mm: mmap_lock: use local locks instead of disabling preemption") replaced preempt_disable() with local_lock(&memcg_paths.lock) based on an argument that preempt_disable() has to be avoided because get_mm_memcg_path() might sleep if PREEMPT_RT=y.

But syzbot started reporting

inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage.

and

inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.

messages, for local_lock() does not disable IRQ.

We could replace local_lock() with local_lock_irqsave() in order to suppress these messages. But this patch instead replaces percpu buffers with on-stack buffer, for the size of each buffer returned by get_memcg_path_buf() is only 256 bytes which is tolerable for allocating from current thread's kernel stack memory.

Link: https://lkml.kernel.org/r/ef22d289-eadb-4ed9-863b-fbc922b33d8d@I-love.SAKURA... Reported-by: syzbot syzbot+40905bca570ae6784745@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=40905bca570ae6784745 Fixes: 832b50725373 ("mm: mmap_lock: use local locks instead of disabling preemption") Signed-off-by: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Reviewed-by: Axel Rasmussen axelrasmussen@google.com Cc: Nicolas Saenz Julienne nsaenzju@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/mmap_lock.c | 175 +++++------------------------------------------ 1 file changed, 20 insertions(+), 155 deletions(-)

--- a/mm/mmap_lock.c~mm-mmap_lock-replace-get_memcg_path_buf-with-on-stack-buffer +++ a/mm/mmap_lock.c @@ -19,14 +19,7 @@ EXPORT_TRACEPOINT_SYMBOL(mmap_lock_relea

#ifdef CONFIG_MEMCG

-/* - * Our various events all share the same buffer (because we don't want or need - * to allocate a set of buffers *per event type*), so we need to protect against - * concurrent _reg() and _unreg() calls, and count how many _reg() calls have - * been made. - */ -static DEFINE_MUTEX(reg_lock); -static int reg_refcount; /* Protected by reg_lock. */ +static atomic_t reg_refcount;

/* * Size of the buffer for memcg path names. Ignoring stack trace support, @@ -34,136 +27,22 @@ static int reg_refcount; /* Protected by */ #define MEMCG_PATH_BUF_SIZE MAX_FILTER_STR_VAL

-/* - * How many contexts our trace events might be called in: normal, softirq, irq, - * and NMI. - */ -#define CONTEXT_COUNT 4 - -struct memcg_path { - local_lock_t lock; - char __rcu *buf; - local_t buf_idx; -}; -static DEFINE_PER_CPU(struct memcg_path, memcg_paths) = { - .lock = INIT_LOCAL_LOCK(lock), - .buf_idx = LOCAL_INIT(0), -}; - -static char **tmp_bufs; - -/* Called with reg_lock held. */ -static void free_memcg_path_bufs(void) -{ - struct memcg_path *memcg_path; - int cpu; - char **old = tmp_bufs; - - for_each_possible_cpu(cpu) { - memcg_path = per_cpu_ptr(&memcg_paths, cpu); - *(old++) = rcu_dereference_protected(memcg_path->buf, - lockdep_is_held(&reg_lock)); - rcu_assign_pointer(memcg_path->buf, NULL); - } - - /* Wait for inflight memcg_path_buf users to finish. */ - synchronize_rcu(); - - old = tmp_bufs; - for_each_possible_cpu(cpu) { - kfree(*(old++)); - } - - kfree(tmp_bufs); - tmp_bufs = NULL; -} - int trace_mmap_lock_reg(void) { - int cpu; - char *new; - - mutex_lock(&reg_lock); - - /* If the refcount is going 0->1, proceed with allocating buffers. */ - if (reg_refcount++) - goto out; - - tmp_bufs = kmalloc_array(num_possible_cpus(), sizeof(*tmp_bufs), - GFP_KERNEL); - if (tmp_bufs == NULL) - goto out_fail; - - for_each_possible_cpu(cpu) { - new = kmalloc(MEMCG_PATH_BUF_SIZE * CONTEXT_COUNT, GFP_KERNEL); - if (new == NULL) - goto out_fail_free; - rcu_assign_pointer(per_cpu_ptr(&memcg_paths, cpu)->buf, new); - /* Don't need to wait for inflights, they'd have gotten NULL. */ - } - -out: - mutex_unlock(&reg_lock); + atomic_inc(&reg_refcount); return 0; - -out_fail_free: - free_memcg_path_bufs(); -out_fail: - /* Since we failed, undo the earlier ref increment. */ - --reg_refcount; - - mutex_unlock(&reg_lock); - return -ENOMEM; }

void trace_mmap_lock_unreg(void) { - mutex_lock(&reg_lock); - - /* If the refcount is going 1->0, proceed with freeing buffers. */ - if (--reg_refcount) - goto out; - - free_memcg_path_bufs(); - -out: - mutex_unlock(&reg_lock); -} - -static inline char *get_memcg_path_buf(void) -{ - struct memcg_path *memcg_path = this_cpu_ptr(&memcg_paths); - char *buf; - int idx; - - rcu_read_lock(); - buf = rcu_dereference(memcg_path->buf); - if (buf == NULL) { - rcu_read_unlock(); - return NULL; - } - idx = local_add_return(MEMCG_PATH_BUF_SIZE, &memcg_path->buf_idx) - - MEMCG_PATH_BUF_SIZE; - return &buf[idx]; + atomic_dec(&reg_refcount); }

-static inline void put_memcg_path_buf(void) -{ - local_sub(MEMCG_PATH_BUF_SIZE, &this_cpu_ptr(&memcg_paths)->buf_idx); - rcu_read_unlock(); -} - -#define TRACE_MMAP_LOCK_EVENT(type, mm, ...) \ - do { \ - const char *memcg_path; \ - local_lock(&memcg_paths.lock); \ - memcg_path = get_mm_memcg_path(mm); \ - trace_mmap_lock_##type(mm, \ - memcg_path != NULL ? memcg_path : "", \ - ##__VA_ARGS__); \ - if (likely(memcg_path != NULL)) \ - put_memcg_path_buf(); \ - local_unlock(&memcg_paths.lock); \ +#define TRACE_MMAP_LOCK_EVENT(type, mm, ...) \ + do { \ + char buf[MEMCG_PATH_BUF_SIZE]; \ + get_mm_memcg_path(mm, buf, sizeof(buf)); \ + trace_mmap_lock_##type(mm, buf, ##__VA_ARGS__); \ } while (0)

#else /* !CONFIG_MEMCG */ @@ -185,37 +64,23 @@ void trace_mmap_lock_unreg(void) #ifdef CONFIG_TRACING #ifdef CONFIG_MEMCG /* - * Write the given mm_struct's memcg path to a percpu buffer, and return a - * pointer to it. If the path cannot be determined, or no buffer was available - * (because the trace event is being unregistered), NULL is returned. - * - * Note: buffers are allocated per-cpu to avoid locking, so preemption must be - * disabled by the caller before calling us, and re-enabled only after the - * caller is done with the pointer. - * - * The caller must call put_memcg_path_buf() once the buffer is no longer - * needed. This must be done while preemption is still disabled. + * Write the given mm_struct's memcg path to a buffer. If the path cannot be + * determined or the trace event is being unregistered, empty string is written. */ -static const char *get_mm_memcg_path(struct mm_struct *mm) +static void get_mm_memcg_path(struct mm_struct *mm, char *buf, size_t buflen) { - char *buf = NULL; - struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm); + struct mem_cgroup *memcg;

+ buf[0] = '\0'; + /* No need to get path if no trace event is registered. */ + if (!atomic_read(&reg_refcount)) + return; + memcg = get_mem_cgroup_from_mm(mm); if (memcg == NULL) - goto out; - if (unlikely(memcg->css.cgroup == NULL)) - goto out_put; - - buf = get_memcg_path_buf(); - if (buf == NULL) - goto out_put; - - cgroup_path(memcg->css.cgroup, buf, MEMCG_PATH_BUF_SIZE); - -out_put: + return; + if (memcg->css.cgroup) + cgroup_path(memcg->css.cgroup, buf, buflen); css_put(&memcg->css); -out: - return buf; }

#endif /* CONFIG_MEMCG */ _

Patches currently in -mm which might be from penguin-kernel@I-love.SAKURA.ne.jp are

mm-mmap_lock-replace-get_memcg_path_buf-with-on-stack-buffer.patch