From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-f199.google.com (mail-pg1-f199.google.com [209.85.215.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CFAB5328B4B for ; Fri, 7 Aug 2026 19:11:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786129867; cv=none; b=W9VpdoqnjNIPxKmuwKjA5P7mNh2DCMR/KqGzvf72Ycm5P/XdZweO0/dg7zGkHXJ3mKoRyroSJ3GiFFgcbm+LxszibWb+ZTuMDHqLFR5kZPyMtP1LC20s2eoYWbHHRdVq0ALymft9Q9QxZLxINzpbFVfUcaxCtgtDLgfmHLeOA8w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786129867; c=relaxed/simple; bh=8+7e+SOvMIMJWj8aydFmApQxqF94aJTOOymoIZpuYME=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=KLAr3YnY8kH8m2ZIG9pcs1kUkXcd8TVXQzcy5u+An5ZVcm1xSo7dgZUKUWo9GRPfSfIsH3OB9UH8goDsy2KiZ1g+AtT9HwrBFf3/BP+h/XpFsdegY8IhPmaBw/5i7fw12Cxqsp1gmy9QspTaQrdukRxlzvsUYpMVBRhUE2ZBVQQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=byFmlSvj; arc=none smtp.client-ip=209.85.215.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="byFmlSvj" Received: by mail-pg1-f199.google.com with SMTP id 41be03b00d2f7-cb4bd11ddf8so3973610a12.3 for ; Fri, 07 Aug 2026 12:11:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1786129865; x=1786734665; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:from:to:cc:subject:date:message-id:reply-to :content-type; bh=pEoaFtbBw4uM/HC2SCgHUUoy2AlMce3ARxmiATtbCDs=; b=byFmlSvjUT05leHDx07ETS17/AficWgibdyPMqT4mENRD71q3kZ0B9p6AOro8jO/rj ZKG3Qrio+elINiZD3idZEDW9FRdcVmSbU9xflagDd1uVFJPrW6Ol2kiOojld4hLelO0p QAt0XyrI+iXGcQOgNdycRu58dn35/dv5ww8gh8qJap+vqN2MgggypeXPJh1LHaehQWNq tHryK83XjmmYxqqDTKoz0Lbv719O1PXCnm/dYnxI7lQNtaBlPEiox5eNRRBo9yoSpDJS 052RL8WmIJsnUkR7xzxj0p46wMp/qyGUAK3MzHmwJOa78yDl5Xsx72s08yhfpv1rJm4i r3RA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786129865; x=1786734665; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=pEoaFtbBw4uM/HC2SCgHUUoy2AlMce3ARxmiATtbCDs=; b=dSTTW0qdGxGbSeKU7Sbet7/drQcN6xkapk70VjVKFJaVr82e1IJrVhKNKxW18Q8A3M 96LUTqFlZ/f1OX3M9Qs2A9vf57Pq6RBZubAQQ49d8OG+j0//ly+JdHVZeHyCaYYeXCKN 5RbzAQCulpESV54BW3BQZu1bLlniH1iNpt0FzP+XUV+MYGBq9aCOi3gqwF9tS/+UVdha lHevRg+SmLvCjuHWx0wLCVa91zIWj4r4Gwk81E8sPx1HH+C+mhboyFsyP9pek+ov/6Gh LW91a8tyoDqQ0to4RER6pBJ5uOv/ucTQ9rFeF8JHRSND7wj14G4r6aQKRCUvLCRU3XxY 5CLA== X-Forwarded-Encrypted: i=1; AHgh+RrtMBukOCPCLvDYtm+d3uivcEDz5ojfOQiQBw4rU3dxpNwrQyFXNpjkDH6XXLNH6qxvN0k=@vger.kernel.org X-Gm-Message-State: AOJu0YyWuMXEbIzEXdKqqWj/hRkGH2YaMi0ln7X21R85ic3UlhEygtDR DbYRDBNIyK+vQSYibiBFmXZXVw5mjiSEGsF4HQbD6cwdak3Gf/PUD2LZwQwQw0voBsEsLhG6/N4 vjtpdIw== X-Received: from pgmc25.prod.google.com ([2002:a63:1c59:0:b0:cbd:2000:e6f4]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a21:3398:b0:3c1:c6e:9360 with SMTP id adf61e73a8af0-3cbce974faemr2952166637.32.1786129864862; Fri, 07 Aug 2026 12:11:04 -0700 (PDT) Date: Fri, 7 Aug 2026 12:11:04 -0700 In-Reply-To: <6d8ac2f9487e5a00745be5fadac3f83679cae99f.camel@intel.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260806214050.78058-1-seanjc@google.com> <20260806214050.78058-2-seanjc@google.com> <6d8ac2f9487e5a00745be5fadac3f83679cae99f.camel@intel.com> Message-ID: Subject: Re: [PATCH 1/4] KVM: x86/mmu: Reload MMU on *every* page pre-fault attempt/iteration From: Sean Christopherson To: Rick P Edgecombe Cc: "pbonzini@redhat.com" , "sashiko-bot@kernel.org" , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Kai Huang , Yan Y Zhao Content-Type: text/plain; charset="us-ascii" On Fri, Aug 07, 2026, Rick P Edgecombe wrote: > On Thu, 2026-08-06 at 14:40 -0700, Sean Christopherson wrote: > > Reload the MMU (which is a nop if the MMU doesn't need to be reloaded) on > > every attempt to pre-fault a guest page, i.e. when the page fault path > > signals that the caller should retry. If the synchronize_srcu_expedited() > > in kvm_invalidate_memslot() completes before kvm_vcpu_pre_fault_memory() > > grabs SRCU, but kvm_mmu_reload() in the pre-fault path completes before > > kvm_invalidate_memslot() triggers x86's "fast zap all", then the pre-fault > > task will reach kvm_tdp_page_prefault() with an invalid root. > > > > Attempting to fault-in memory with an invalid root ultimately puts > > kvm_tdp_page_prefault() into an infinite (breakable) retry loop, which > > manifests most obviously as a hang in the pre_fault_memory_test selftest, > > but also eventually causes RCU (SRCU?) to complain. > > > > INFO: rcu_tasks detected stalls on tasks: > > 000000000cda47bd: .. nvcsw: 6/6 holdout: 1 idle_cpu: -1/25 > > task:pre_fault_memor state:R running task stack:12696 > > pid:95588 tgid:95588 ppid:95584 task_flags:0x400000 flags:0x00080801 > > Call Trace: > > > > lock_release+0x4e/0x320 > > __get_user_pages+0x546/0xcd0 > > up_read+0x1b/0x30 > > get_user_pages_unlocked+0xee/0x350 > > hva_to_pfn+0xd3/0x3d0 [kvm] > > lock_release+0x4e/0x320 > > xa_load+0x5c/0x170 > > xa_load+0x14c/0x170 > > __kvm_faultin_pfn+0xd9/0x130 [kvm] > > lock_acquire+0x65/0x2b0 > > lock_release+0x4e/0x320 > > kvm_mmu_faultin_pfn+0x1e1/0x690 [kvm] > > gup_fast_fallback+0x63e/0xdf0 > > kvm_tdp_page_fault+0xeb/0x140 [kvm] > > kvm_mmu_do_page_fault+0x12e/0x200 [kvm] > > kvm_arch_vcpu_pre_fault_memory+0x16e/0x200 [kvm] > > kvm_vcpu_pre_fault_memory+0xc1/0x1f0 [kvm] > > kvm_vcpu_pre_fault_memory+0x116/0x1f0 [kvm] > > kvm_vcpu_ioctl+0x3a4/0x6b0 [kvm] > > clockevents_program_event+0x5d/0x170 > > __se_sys_ioctl+0x6d/0xb0 > > entry_SYSCALL_64_after_hwframe+0x4b/0x53 > > do_syscall_64+0x10a/0x480 > > __irq_exit_rcu+0x8e/0x140 > > entry_SYSCALL_64_after_hwframe+0x4b/0x53 > > > > > > Fixes: 6e01b7601dfe ("KVM: x86: Implement kvm_arch_vcpu_pre_fault_memory()") > > Signed-off-by: Sean Christopherson > > > Reviewed-by: Rick Edgecombe > > But, did you hit this somehow with an unmodified test and kernel? The log makes > me think it was hit in a normal test run. (which I failed to reproduce) Yes and yes. I often test by running all selftests in parallel. My guess is that PREEMPT_LAZY plus oversubscribed CPUs allowed the timing condition to be hit. It wasn't anywhere near 100% reproducible on my end either, maybe 1 out of every 10 runs? 20 runs? (of the entire suite of selftests). P.S. I was very proud of myself: because the failure was so flaky, I used perf and bpftrace to debug the issue instead of my usual hack-the-kernel approach. :-)