From patchwork Thu Oct 27 22:40:11 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Beau Belgrave X-Patchwork-Id: 13022922 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 31FADFA3744 for ; Thu, 27 Oct 2022 22:40:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234184AbiJ0WkT (ORCPT ); Thu, 27 Oct 2022 18:40:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47312 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235629AbiJ0WkR (ORCPT ); Thu, 27 Oct 2022 18:40:17 -0400 Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id DB6F65B119; Thu, 27 Oct 2022 15:40:15 -0700 (PDT) Received: from W11-BEAU-MD.localdomain (unknown [76.135.50.127]) by linux.microsoft.com (Postfix) with ESMTPSA id 1CE2E210DD4C; Thu, 27 Oct 2022 15:40:15 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 1CE2E210DD4C DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1666910415; bh=f2uIrdDboNxBVlV0TX5bvVI0+LO4ZU7b2wcvL6RBD5E=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=G7Ew7TwvX2FgHZ0Cr/bVEvCqhx7ci+yepweA5NuYFQ/Q2d7v4jBncbB7+DEDxeEmo 24WdcYJ3+TGe0nOW4rj5zYI1hrlzz2TcTv4E4WCEGgGL6lGmLAqUMixdTuEw7Q+jVs V2HBcWOmrn0wdO6dOMY2meYJAeCa+LIuIa9DsjEY= From: Beau Belgrave To: rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, dcook@linux.microsoft.com, alanau@linux.microsoft.com Cc: linux-trace-devel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [RFC PATCH 2/2] tracing/user_events: Fixup enable faults asyncly Date: Thu, 27 Oct 2022 15:40:11 -0700 Message-Id: <20221027224011.2075-3-beaub@linux.microsoft.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20221027224011.2075-1-beaub@linux.microsoft.com> References: <20221027224011.2075-1-beaub@linux.microsoft.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-trace-devel@vger.kernel.org When events are enabled within the various tracing facilities, such as ftrace/perf, the event_mutex is held. As events are enabled pages are accessed. We do not want page faults to occur under this lock. Instead queue the fault to a workqueue to be handled in a process context safe way without the lock. The enable address is disabled while the async fault-in occurs. This ensures that we don't attempt fault-in more than is necessary. Once the page has been faulted in, the address write is attempted again. If the page couldn't fault-in, then we wait until the next time the event is enabled to prevent any potential infinite loops. Signed-off-by: Beau Belgrave --- kernel/trace/trace_events_user.c | 125 ++++++++++++++++++++++++++++++- 1 file changed, 121 insertions(+), 4 deletions(-) diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c index 633f24c2a1ac..f1eb8101e053 100644 --- a/kernel/trace/trace_events_user.c +++ b/kernel/trace/trace_events_user.c @@ -81,11 +81,22 @@ struct user_event_enabler { struct list_head link; struct mm_struct *mm; struct file *file; + refcount_t refcnt; unsigned long enable_addr; unsigned int enable_bit: 5, - __reserved: 27; + __reserved: 26, + disabled: 1; }; +/* Used for asynchronous faulting in of pages */ +struct user_event_enabler_fault { + struct work_struct work; + struct user_event_enabler *enabler; + struct user_event *event; +}; + +static struct kmem_cache *fault_cache; + /* * Stores per-event properties, as users register events * within a file a user_event might be created if it does not @@ -236,6 +247,19 @@ static void user_event_enabler_destroy(struct user_event_enabler *enabler) kfree(enabler); } +static __always_inline struct user_event_enabler +*user_event_enabler_get(struct user_event_enabler *enabler) +{ + refcount_inc(&enabler->refcnt); + return enabler; +} + +static void user_event_enabler_put(struct user_event_enabler *enabler) +{ + if (refcount_dec_and_test(&enabler->refcnt)) + user_event_enabler_destroy(enabler); +} + static void user_event_enabler_remove(struct file *file, struct user_event *user) { @@ -249,13 +273,93 @@ static void user_event_enabler_remove(struct file *file, if (enabler->file != file) continue; + enabler->disabled = 0; list_del(&enabler->link); - user_event_enabler_destroy(enabler); + user_event_enabler_put(enabler); } mutex_unlock(&event_mutex); } +static void user_event_enabler_write(struct user_event_enabler *enabler, + struct user_event *user); + +static void user_event_enabler_fault_fixup(struct work_struct *work) +{ + struct user_event_enabler_fault *fault = container_of( + work, struct user_event_enabler_fault, work); + struct user_event_enabler *enabler = fault->enabler; + struct user_event *user = fault->event; + struct mm_struct *mm = enabler->mm; + unsigned long uaddr = enabler->enable_addr; + bool unlocked = false; + int ret; + + might_sleep(); + + mmap_read_lock(mm); + + ret = fixup_user_fault(mm, uaddr, FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE, + &unlocked); + + mmap_read_unlock(mm); + + if (ret) + pr_warn("user_events: Fixup fault failed with %d " + "for mm: 0x%pK offset: 0x%llx event: %s\n", ret, mm, + (unsigned long long)uaddr, EVENT_NAME(user)); + + /* Prevent state changes from racing */ + mutex_lock(&event_mutex); + + /* + * If we managed to get the page, re-issue the write. We do not + * want to get into a possible infinite loop, which is why we only + * attempt again directly if the page came in. If we couldn't get + * the page here, then we will try again the next time the event is + * enabled/disabled. + */ + enabler->disabled = 0; + + if (!ret) + user_event_enabler_write(enabler, user); + + mutex_unlock(&event_mutex); + + refcount_dec(&user->refcnt); + user_event_enabler_put(enabler); + kmem_cache_free(fault_cache, fault); +} + +static bool user_event_enabler_queue_fault(struct user_event_enabler *enabler, + struct user_event *user) +{ + struct user_event_enabler_fault *fault; + + fault = kmem_cache_zalloc(fault_cache, GFP_NOWAIT | __GFP_NOWARN); + + if (!fault) + return false; + + INIT_WORK(&fault->work, user_event_enabler_fault_fixup); + fault->enabler = user_event_enabler_get(enabler); + fault->event = user; + + refcount_inc(&user->refcnt); + enabler->disabled = 1; + + if (!schedule_work(&fault->work)) { + enabler->disabled = 0; + refcount_dec(&user->refcnt); + user_event_enabler_put(enabler); + kmem_cache_free(fault_cache, fault); + + return false; + } + + return true; +} + static void user_event_enabler_write(struct user_event_enabler *enabler, struct user_event *user) { @@ -266,6 +370,11 @@ static void user_event_enabler_write(struct user_event_enabler *enabler, void *kaddr; int ret; + lockdep_assert_held(&event_mutex); + + if (unlikely(enabler->disabled)) + return; + mmap_read_lock(mm); ret = pin_user_pages_remote(mm, uaddr, 1, FOLL_WRITE | FOLL_NOFAULT, @@ -273,8 +382,10 @@ static void user_event_enabler_write(struct user_event_enabler *enabler, mmap_read_unlock(mm); - if (ret <= 0) { - pr_warn("user_events: Enable write failed\n"); + if (unlikely(ret <= 0)) { + if (!user_event_enabler_queue_fault(enabler, user)) + pr_warn("user_events: Unable to queue fault handler\n"); + return; } @@ -321,6 +432,7 @@ static struct user_event_enabler enabler->file = file; enabler->enable_addr = (unsigned long)reg->enable_addr; enabler->enable_bit = reg->enable_bit; + refcount_set(&enabler->refcnt, 1); /* Prevents state changes from racing with new enablers */ mutex_lock(&event_mutex); @@ -1902,6 +2014,11 @@ static int __init trace_events_user_init(void) { int ret; + fault_cache = KMEM_CACHE(user_event_enabler_fault, 0); + + if (!fault_cache) + return -ENOMEM; + init_group = user_event_group_create(&init_user_ns); if (!init_group)