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([27.4.92.188]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2b9887a4ba0sm37547795ad.32.2026.04.29.21.49.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Apr 2026 21:49:12 -0700 (PDT) From: "shaikh.kamal" To: Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , David Rientjes , Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-rt-devel@lists.linux.dev Cc: pbonzini@redhat.com, skhan@linuxfoundation.org, me@brighamcampbell.com, "shaikh.kamal" , syzbot+c3178b6b512446632bac@syzkaller.appspotmail.com Subject: [PATCH v2 1/1] mm/mmu_notifier: Add async OOM cleanup via call_srcu() Date: Thu, 30 Apr 2026 10:18:54 +0530 Message-ID: <20260430044854.11132-1-shaikhkamal2012@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit When an mm undergoes OOM kill, the OOM reaper unmaps memory while holding the mmap_lock. MMU notifier subscribers (notably KVM) need to be informed so they can tear down their secondary mappings. The current synchronous unregister path can deadlock on PREEMPT_RT because synchronize_srcu() is called from contexts that cannot safely sleep. This patch implements the asynchronous cleanup design proposed by Paolo Bonzini in v1 review: a new optional after_oom_unregister callback in struct mmu_notifier_ops, invoked after the SRCU grace period via call_srcu() so that no readers can still reference the subscription when cleanup runs. The flow is: 1. The OOM reaper calls mmu_notifier_oom_enter() from __oom_reap_task_mm(). 2. mmu_notifier_oom_enter() walks the subscription list and, for each subscriber that provides after_oom_unregister, detaches the subscription from the active list and schedules a call_srcu() callback. 3. The deferred callback invokes after_oom_unregister once the grace period has elapsed and all in-flight readers have finished. 4. Subsystems waiting to free structures referenced by the callback can call the new mmu_notifier_barrier() helper, which wraps srcu_barrier() to wait for all outstanding callbacks scheduled this way. after_oom_unregister is mutually exclusive with alloc_notifier because allocated notifiers can have additional outstanding references that the OOM path cannot safely drop. KVM is updated to provide after_oom_unregister, which clears mn_active_invalidate_count, and to detect via hlist_unhashed() in kvm_destroy_vm() when its subscription was already detached by the OOM path; in that case it calls mmu_notifier_barrier() and drops the mm reference rather than calling mmu_notifier_unregister(). Reported-by: syzbot+c3178b6b512446632bac@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=c3178b6b512446632bac Suggested-by: Paolo Bonzini Link: https://lore.kernel.org/all/20260209161527.31978-1-shaikhkamal2012@gmail.com/ Signed-off-by: shaikh.kamal --- include/linux/mmu_notifier.h | 10 +++ mm/mmu_notifier.c | 123 +++++++++++++++++++++++++++++++++++ mm/oom_kill.c | 3 + virt/kvm/kvm_main.c | 27 +++++++- 4 files changed, 162 insertions(+), 1 deletion(-) diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h index 07a2bbaf86e9..0ccd590f55d3 100644 --- a/include/linux/mmu_notifier.h +++ b/include/linux/mmu_notifier.h @@ -88,6 +88,14 @@ struct mmu_notifier_ops { void (*release)(struct mmu_notifier *subscription, struct mm_struct *mm); + /* + * Any mmu notifier that defines this is automatically unregistered + * when its mm is the subject of an OOM kill. after_oom_unregister() + * is invoked after all other outstanding callbacks have terminated. + */ + void (*after_oom_unregister)(struct mmu_notifier *subscription, + struct mm_struct *mm); + /* * clear_flush_young is called after the VM is * test-and-clearing the young/accessed bitflag in the @@ -375,6 +383,8 @@ mmu_interval_check_retry(struct mmu_interval_notifier *interval_sub, extern void __mmu_notifier_subscriptions_destroy(struct mm_struct *mm); extern void __mmu_notifier_release(struct mm_struct *mm); +void mmu_notifier_oom_enter(struct mm_struct *mm); +extern void mmu_notifier_barrier(void); extern int __mmu_notifier_clear_flush_young(struct mm_struct *mm, unsigned long start, unsigned long end); diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c index a6cdf3674bdc..b8fa58fe6b7d 100644 --- a/mm/mmu_notifier.c +++ b/mm/mmu_notifier.c @@ -49,6 +49,37 @@ struct mmu_notifier_subscriptions { struct hlist_head deferred_list; }; +/* + * Callback structure for asynchronous OOM cleanup. + * Used with call_srcu() to defer after_oom_unregister callbacks + * until after SRCU grace period completes. + */ +struct mmu_notifier_oom_callback { + struct rcu_head rcu; + struct mmu_notifier *subscription; + struct mm_struct *mm; +}; + +/* + * Callback function invoked after SRCU grace period. + * Safely calls after_oom_unregister once all readers have finished. + */ +static void mmu_notifier_oom_callback_fn(struct rcu_head *rcu) +{ + struct mmu_notifier_oom_callback *cb = + container_of(rcu, struct mmu_notifier_oom_callback, rcu); + + /* Safe - all SRCU readers have finished */ + cb->subscription->ops->after_oom_unregister(cb->subscription, cb->mm); + + /* Release mm reference taken when callback was scheduled */ + WARN_ON_ONCE(atomic_read(&cb->mm->mm_count) <= 0); + mmdrop(cb->mm); + + /* Free callback structure */ + kfree(cb); +} + /* * This is a collision-retry read-side/write-side 'lock', a lot like a * seqcount, however this allows multiple write-sides to hold it at @@ -359,6 +390,85 @@ void __mmu_notifier_release(struct mm_struct *mm) mn_hlist_release(subscriptions, mm); } +void mmu_notifier_oom_enter(struct mm_struct *mm) +{ + struct mmu_notifier_subscriptions *subscriptions = + mm->notifier_subscriptions; + struct mmu_notifier *subscription; + struct hlist_node *tmp; + HLIST_HEAD(oom_list); + int id; + + if (!subscriptions) + return; + + id = srcu_read_lock(&srcu); + + /* + * Prevent further calls to the MMU notifier, except for + * release and after_oom_unregister. + */ + spin_lock(&subscriptions->lock); + hlist_for_each_entry_safe(subscription, tmp, + &subscriptions->list, hlist) { + if (!subscription->ops->after_oom_unregister) + continue; + + /* + * after_oom_unregister and alloc_notifier are incompatible, + * because there could be other references to allocated + * notifiers. + */ + if (WARN_ON(subscription->ops->alloc_notifier)) + continue; + + hlist_del_init_rcu(&subscription->hlist); + hlist_add_head(&subscription->hlist, &oom_list); + } + spin_unlock(&subscriptions->lock); + hlist_for_each_entry(subscription, &oom_list, hlist) + if (subscription->ops->release) + subscription->ops->release(subscription, mm); + + srcu_read_unlock(&srcu, id); + + if (hlist_empty(&oom_list)) + return; + + hlist_for_each_entry_safe(subscription, tmp, + &oom_list, hlist) { + struct mmu_notifier_oom_callback *cb; + /* + * Remove from stack-based oom_list and reset hlist to unhashed state. + * This sets subscription->hlist.pprev = NULL, so future callers of + * mmu_notifier_unregister() (e.g. kvm_destroy_vm) will see + * hlist_unhashed() == true and take the safe path, avoiding + * use-after-free on the stack-allocated oom_list head. + */ + hlist_del_init(&subscription->hlist); + + /* + * GFP_ATOMIC failure is exceedingly rare. We cannot sleep + * here (would reintroduce the deadlock this patch fixes) + * and cannot call after_oom_unregister synchronously + * without first waiting for SRCU readers. The subscriber + * will not receive after_oom_unregister but cleanup will + * eventually happen via the unregister path. + */ + cb = kmalloc(sizeof(*cb), GFP_ATOMIC); + if (!cb) + continue; + + cb->subscription = subscription; + cb->mm = mm; + mmgrab(mm); + + /* Schedule callback - returns immediately */ + call_srcu(&srcu, &cb->rcu, mmu_notifier_oom_callback_fn); + } + +} + /* * If no young bitflag is supported by the hardware, ->clear_flush_young can * unmap the address and return 1 or 0 depending if the mapping previously @@ -1096,3 +1206,16 @@ void mmu_notifier_synchronize(void) synchronize_srcu(&srcu); } EXPORT_SYMBOL_GPL(mmu_notifier_synchronize); + +/** + * mmu_notifier_barrier - Wait for all pending MMU notifier callbacks + * + * Waits for all call_srcu() callbacks scheduled by mmu_notifier_oom_enter() + * to complete. Used by subsystems during cleanup to prevent use-after-free + * when destroying structures accessed by the callbacks. + */ +void mmu_notifier_barrier(void) +{ + srcu_barrier(&srcu); +} +EXPORT_SYMBOL_GPL(mmu_notifier_barrier); diff --git a/mm/oom_kill.c b/mm/oom_kill.c index 5c6c95c169ee..029e041afc57 100644 --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -519,6 +519,9 @@ static bool __oom_reap_task_mm(struct mm_struct *mm) bool ret = true; MA_STATE(mas, &mm->mm_mt, ULONG_MAX, ULONG_MAX); + /* Notify MMU notifiers about the OOM event */ + mmu_notifier_oom_enter(mm); + /* * Tell all users of get_user/copy_from_user etc... that the content * is no longer stable. No barriers really needed because unmapping diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 1bc1da66b4b0..a2df83d3b413 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -885,6 +885,24 @@ static void kvm_mmu_notifier_release(struct mmu_notifier *mn, srcu_read_unlock(&kvm->srcu, idx); } +static void kvm_mmu_notifier_after_oom_unregister(struct mmu_notifier *mn, + struct mm_struct *mm) +{ + struct kvm *kvm; + + kvm = mmu_notifier_to_kvm(mn); + + /* + * At this point the unregister has completed and all other callbacks + * have terminated. Clean up any unbalanced invalidation counts. + */ + WARN_ON(rcuwait_active(&kvm->mn_memslots_update_rcuwait)); + if (kvm->mn_active_invalidate_count) + kvm->mn_active_invalidate_count = 0; + else + WARN_ON(kvm->mmu_invalidate_in_progress); +} + static const struct mmu_notifier_ops kvm_mmu_notifier_ops = { .invalidate_range_start = kvm_mmu_notifier_invalidate_range_start, .invalidate_range_end = kvm_mmu_notifier_invalidate_range_end, @@ -892,6 +910,7 @@ static const struct mmu_notifier_ops kvm_mmu_notifier_ops = { .clear_young = kvm_mmu_notifier_clear_young, .test_young = kvm_mmu_notifier_test_young, .release = kvm_mmu_notifier_release, + .after_oom_unregister = kvm_mmu_notifier_after_oom_unregister, }; static int kvm_init_mmu_notifier(struct kvm *kvm) @@ -1280,7 +1299,13 @@ static void kvm_destroy_vm(struct kvm *kvm) kvm->buses[i] = NULL; } kvm_coalesced_mmio_free(kvm); - mmu_notifier_unregister(&kvm->mmu_notifier, kvm->mm); + if (hlist_unhashed(&kvm->mmu_notifier.hlist)) { + /* Subscription removed by OOM. Wait for async callback. */ + mmu_notifier_barrier(); + mmdrop(kvm->mm); + } else { + mmu_notifier_unregister(&kvm->mmu_notifier, kvm->mm); + } /* * At this point, pending calls to invalidate_range_start() * have completed but no more MMU notifiers will run, so -- 2.43.0