From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 8C5FA37BE9D for ; Wed, 14 Jan 2026 11:55:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768391717; cv=none; b=Ej8Odh83IQbX1JKm3bhpXNKtaQy4/4CRYXIFrpudiKSbDsOdcY2sInflGk7JUu8F5i/SsdXWmHwpOkRPB9XqddiuUvd/7KYEEE3b+INgrROeD8ZWdDUCpQERc9NNaeLnauzHHQBOl0UBXCHVPI0U2XaT4pKeKLynEN3qGdrGk5U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768391717; c=relaxed/simple; bh=TGLnAOZEnluJL7qVfeusuM/e5JiEfUlkxwku89gsD1I=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=txAL3UMzIljxA0aQ95KeyfWVD9hzCzNonnXDZVzm1ycTJnlxlVC3F+4dMPkfTXed/BD42ggMa6FrNNLruLUObZz6Hf01CNai0933y4e1L8QjWFPCEgvdAiJHlbn1djOYdLkoABXmmgSBtdiqHwGWmehlZRr+Ask30RTt9NCeQpA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=p9UG6VOW; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="p9UG6VOW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6A0FBC4CEF7; Wed, 14 Jan 2026 11:55:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768391717; bh=TGLnAOZEnluJL7qVfeusuM/e5JiEfUlkxwku89gsD1I=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=p9UG6VOW3ganQU6RMIS1RtGAMfEa3dZfN228V/Yuxko1x39f7bWpdV7IwRu8hi3cx h5Ca0y/VhAHHbsWiF+Vb93qKAy/rpcp5DAbGuWgPTbqROtIYOBRbFL0hTYNUTfXK8R 15LPWrGCppUzmw4u3VtCGjMPwmQTA03SEsurFG9zDbyZwZCXEUYX8ItCBUw6LFWBHY GohlUayuu9PQeRlwG9JtOUTDMKhIB7YzNqotdy4mawHOrYZyV0piLB105CGTdsBeS0 5IfjvwJkGDmRaBmOw9MOMFSbYjmp3IEOzBBb3skQh7Q/Zdc8gvu5KbP4UPb+rFyEEh OICNoNBTydgcw== Message-ID: <97413239-2bdc-456f-8511-65fb9f1e301c@kernel.org> Date: Wed, 14 Jan 2026 12:55:11 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/8] Introduce a huge-page pre-zeroing mechanism To: Li Zhe Cc: akpm@linux-foundation.org, ankur.a.arora@oracle.com, fvdl@google.com, joao.m.martins@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, mjguzik@gmail.com, muchun.song@linux.dev, osalvador@suse.de, raghavendra.kt@amd.com References: <3d8398f1-0130-4d3b-ac54-d23877811747@kernel.org> <20260114113635.97621-1-lizhe.67@bytedance.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/14/26 12:36, Li Zhe wrote: > On Wed, 14 Jan 2026 11:41:48 +0100, david@kernel.org wrote: > >> On 1/13/26 13:41, Li Zhe wrote: >>> On Tue, 13 Jan 2026 11:15:29 +0100, david@kernel.org wrote: >>> >>>> On 1/13/26 07:37, Li Zhe wrote: >>>>> On Mon, 12 Jan 2026 20:52:12 +0100, david@kernel.org wrote: >>>>> >>>>>>> As for concern (4), I believe it is orthogonal to this patchset, and >>>>>>> the cover letter already contains a performance comparison that >>>>>>> demonstrates the additional benefit. >>>>>>> >>>>>>>> I did see some comments in [1] about QEMU supporting user-mode >>>>>>>> parallel zero-page operations; I'm just not sure what the current >>>>>>>> state of that support looks like, or what the corresponding benchmark >>>>>>>> numbers are. >>>>>>> >>>>>>> As noted above, QEMU already employs a parallel page-touch mechanism, >>>>>>> yet the elapsed time remains noticeable. I am not deeply familiar with >>>>>>> QEMU; please correct me if I am mistaken. >>>>>> >>>>>> I implemented some part of the parallel preallocation support in QEMU. >>>>>> >>>>>> With QEMU, you can specify the number of threads and even specify the >>>>>> NUMA-placement of these threads. So you can pretty much fine-tune that >>>>>> for an environment. >>>>>> >>>>>> You still pre-zero all hugetlb pages at VM startup time, just in >>>>>> parallel though. So you pay some price at APP startup time. >>>>> >>>>> Hi David, >>>>> >>>>> Thank you for the comprehensive explanation. >>>>> >>>>> You are absolutely correct: QEMU's parallel preallocation is performed >>>>> only during VM start-up. We submitted this patch series mainly >>>>> because we observed that, even with the existing parallel mechanism, >>>>> launching large-size VMs still incurs prohibitive delays. (Bringing up >>>>> a 2 TB VM still requires more than 40 seconds for zeroing) >>>>> >>>>>> If you know that you will run such a VM (or something else) later, you >>>>>> could pre-zero the memory from user space by using a hugetlb-backed file >>>>>> and supplying that to QEMU as memory backend for the VM. Then, you can >>>>>> start your VM without any pre-zeroing. >>>>>> >>>>>> I guess that approach should work universally. Of course, there are >>>>>> limitations, as you would have to know how much memory an app needs, and >>>>>> have a way to supply that memory in form of a file to that app. >>>>> >>>>> Regarding user-space pre-zeroing, I agree that it is feasible once the >>>>> VM's memory footprint is known. We evaluated this approach internally; >>>>> however, in production environments, it is almost impossible to predict >>>>> the exact amount of memory a VM will require. >>>> >>>> Of course, you could preallocate to the expected maximum and then >>>> truncate the file to the size you need :) >>> >>> The solution you described seems similar to delegating hugepage >>> management to a userspace daemon. I haven't explored this approach >>> before, but it appears quite complex. Beyond ensuring secure memory >>> isolation between VMs, we would also need to handle scenarios where >>> the management daemon or the QEMU process crashes, which implies >>> implementing robust recovery and memory reclamation mechanisms. >> >> Yes, but I don't think that's particularly complicated. You have to >> remove the backing file, yes. >> >>> Do >>> you happen to have any documentation or references regarding >>> userspace hugepage management that I could look into? >> >> Not really any documentation. I pretty much only know how QEMU+libvirt >> ends up using it :) >> >>> Compared to >>> the userspace approach, I wonder if implementing hugepage >>> pre-zeroing directly within the kernel would be a simpler and more >>> direct way to accelerate VM creation. >> >> I mean, yes. I don't particularly enjoy user-space having to poll for >> pre-zeroing of pages ... it feels like an odd interface for something >> that is supposed to be simple. >> >> I do understand the reasoning that "zeroing must be charged to >> somebody", and that using a kthread is a bit suboptimal as well. > > My previous explanation may have caused misunderstanding. This > patchset merely exports an interface that allows users to initiate > and halt page zeroing on demand; the CPU cost is borne by the user, > and no kernel thread is introduced. You said "I wonder if implementing hugepage pre-zeroing directly within the kernel would be a simpler and more direct way to accelerate VM creation". And I agree. But to make that fly (no user space polling interface), I was wondering whether we could do it like "init_on_free" and let whoever frees a hugetlb folio just reinitialize it with 0. No kernel thread, no user space thread involved. -- Cheers David