From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0CD1635677C; Thu, 24 Sep 2026 09:31:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790242284; cv=none; b=iMqmHgEGKoqRZ9yB7kQHVxJIpQH+wfdnc6lbk8oqW9ZUQpU8I5H5oPzQTCJvxxRXFshr9beQPGj3B+XFWpmN7MEpcJ+8kXUqS3uLAejv3fWn8MBmywBPchU9qktYcQ1WrHwiTAwT5rLYefFiWEs8jArfv2Bczy8tSdumpo/tx84= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790242284; c=relaxed/simple; bh=3016/YcUrafmSJfnVIIyNUFVP7OY0QoPQKi4VrtHhdc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=KOjqat7oLnA/Q7Pk/LTrUZ6P7Q8tcaWDnVhGh5eoAfJuFXjLWpT+Zm3lOdWNLAFMKltKStOi9sGdMrpR0GoOWGYR0pGq6hlgM+UmWqaEnAEJq0xUYp4qF6IKlNtMk65r4GVkDvQoeq6+9vXeJvYhRmnXGqnbNxGZymRYASU2+74= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EZmbNWDr; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="EZmbNWDr" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6B9CC1F000FF; Thu, 24 Sep 2026 09:31:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790242282; bh=NeTuCFcemX/adNl5XbtKYCBGxqR8ivj2ssA68DygpBo=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=EZmbNWDrJtzCFAJtM9thEalDnEZjbzv/YFJ6VRB0CFMs6afx6LFyjqWeB9uvZu9Qj x81U9J6SOwj+0cLIr/MZhV0koClWIOxcD5o/aYGCmtyDTOVlrSVvKX6ZfcZBj68e6K vYdqUfgrFK//iQz1ODBsg0qkf5SSSUOk+wALZMA2yrtTuYJTmKd2zxd3tMMblpobmo OPdWy5LyVA52bNPo6ohdMZWzXomoavKccUxrgflAMfYVBRrOQO6rK1bHxObbSADG3d kdoP8cjsGmQto1i2Tuxf28J9IBuOzTTBvd4L9nigmzb2llrZgIVXCPzl22BDUyAmMY BBCYQ4gHnz+VQ== Date: Thu, 24 Sep 2026 10:31:15 +0100 From: "Lorenzo Stoakes (ARM)" To: Mikhail Gavrilov Cc: Andrew Morton , David Hildenbrand , Dave Hansen , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Vishal Moola , Ingo Molnar , Lu Baolu , Jason Gunthorpe , Steven Rostedt , x86@kernel.org, linux-mm@kvack.org, regressions@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm: don't schedule deferred kernel page table freeing while booting Message-ID: References: <20260924092307.22813-1-mikhail.v.gavrilov@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260924092307.22813-1-mikhail.v.gavrilov@gmail.com> I know I said go ahead so it's my fault not yours, but in general please don't respin so fast :P I made a suggestion on the other thread which this has now raced. See below! On Thu, Sep 24, 2026 at 02:23:07PM +0500, Mikhail Gavrilov wrote: > Booting with a boot-time function tracer and a filter, for example > > ftrace=function ftrace_filter=pud_free_pmd_page > > panics on 7.3-rc4 as soon as the tracer starts: > > [ 23.531178] Starting tracer 'function' > [ 23.675800] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000038: 0000 [#1] SMP KASAN NOPTI > [ 23.819917] KASAN: null-ptr-deref in range [0x00000000000001c0-0x00000000000001c7] > [ 23.964025] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.3.0-rc4-fe2ec83746e5-with-fixes-v2+ #195 PREEMPT(undef) > [ 24.252248] RIP: 0010:__queue_work+0xab/0xf00 > [ 25.981629] Call Trace: > [ 26.125727] > [ 26.413912] ? pagetable_free_kernel+0x20/0x120 > [ 26.990283] queue_work_on+0x97/0xf0 > [ 27.134382] __cpa_collapse_large_pages+0x501/0x6f0 > [ 27.566662] cpa_flush+0x394/0x620 > [ 27.998953] change_page_attr_set_clr+0x321/0x4a0 > [ 29.151729] set_memory_rox+0xa2/0xf0 > [ 29.584018] create_trampoline+0x431/0x6f0 > ... > [ 44.343347] Kernel panic - not syncing: Attempted to kill the idle task! > > The boot-time tracer is started from early_trace_init(), which runs > before workqueue_init_early(). Making its trampoline read-only splits a > large page, and CPA collapses it again right away. The split table has > been a kernel page table since commit 9e4a3ec3411b > ("x86/mm/pat: Allocate split page tables as kernel page tables"), so the > collapse frees it through pagetable_free_kernel(), which queues work on > system_percpu_wq - still NULL at that point. That commit is correct in > itself; it only lets CPA reach pagetable_free_kernel() before the > workqueue that function relies on exists. > > Keep putting the table on the list, but don't schedule the work while > the system is still booting. The next kernel page table freed after > boot schedules it, and the work then frees the early table too, after > the same IOMMU flush as any other. If no kernel page table is freed > after boot, the ones freed during boot stay on the list. > > Fixes: 9e4a3ec3411b ("x86/mm/pat: Allocate split page tables as kernel page tables") > Suggested-by: David Hildenbrand (Arm) > Cc: stable@vger.kernel.org > Signed-off-by: Mikhail Gavrilov > Link: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@gmail.com > --- > v2: > - Keep the table on the list and only skip scheduling the work while > booting, instead of freeing it directly (David Hildenbrand) > - Say that 9e4a3ec3411b is correct in itself and only exposes the > problem (Lorenzo Stoakes) > v1: https://lore.kernel.org/20260924064321.23787-1-mikhail.v.gavrilov@gmail.com > > Tested on a Ryzen 9 7950X with a Radeon RX 7900 XTX, lockdep and KASAN > enabled, on 7.3-rc4 (fe2ec83746e5) with the same unrelated local > changes as noted for v1, booting with > > ftrace=function ftrace_filter=pud_free_pmd_page,pagetable_free_kernel,kernel_pgtable_work_func > > The boot that panicked without the fix completes. The table freed > while the tracer installs itself does not show up in the trace, since > the tracer is not live yet at that point, but the first kernel page > table freed after boot does: systemd-modules-load freeing one from > __cpa_collapse_large_pages() schedules the work, and > kernel_pgtable_work_func() runs 0.8 ms later and drains the list. From > then on every pagetable_free_kernel() in the trace (660 entries, none > lost) is followed by a work run within a few milliseconds. So on this > box the early tables wait until the first module is loaded, and no > separate drain is needed. > > mm/pgtable-generic.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c > index b91b1a98029c..f7f504f57914 100644 > --- a/mm/pgtable-generic.c > +++ b/mm/pgtable-generic.c > @@ -444,6 +444,12 @@ void pagetable_free_kernel(struct ptdesc *pt) > list_add(&pt->pt_list, &kernel_pgtable_work.list); > spin_unlock(&kernel_pgtable_work.lock); > > - schedule_work(&kernel_pgtable_work.work); > + /* > + * The workqueue may not exist yet while the system is booting. > + * The next kernel page table freed after boot schedules the work, > + * which then frees this one as well. > + */ > + if (system_state != SYSTEM_BOOTING) > + schedule_work(&kernel_pgtable_work.work); > } > #endif > -- > 2.55.0 > Maybe we want to ensure the drain? Like below: diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c index f3754cefb19e..67f286169632 100644 --- a/mm/pgtable-generic.c +++ b/mm/pgtable-generic.c @@ -457,12 +457,29 @@ static void kernel_pgtable_work_func(struct work_struct *work) __pagetable_free(pt); } +static void schedule_kernel_pgtable_free(void) +{ + schedule_work(&kernel_pgtable_work.work); +} + void pagetable_free_kernel(struct ptdesc *pt) { spin_lock(&kernel_pgtable_work.lock); list_add(&pt->pt_list, &kernel_pgtable_work.list); spin_unlock(&kernel_pgtable_work.lock); - schedule_work(&kernel_pgtable_work.work); + /* No workqueues exist yet. */ + if (system_state != SYSTEM_BOOTING) + schedule_kernel_pgtable_free(); } + +static int kernel_pgtable_drain_early(void) +{ + /* Drain any early kernel page table frees. */ + schedule_kernel_pgtable_free(); + return 0; +} + +core_initcall(kernel_pgtable_drain_early); + #endif -- Cheers, Lorenzo