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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768387312; bh=bnSwEejEd+dUBVdPcR47MVQZG1UBBrNB0/uQd3LWVro=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=J7U48QQyCuy6Sg0jort3YyrgS4UhK10c7UN3FNmOyIi2+LGDqT2MbRMhZ/iryGQs0 mNLJ8DClURwJ9l6hVL98wnZ4VFGulO+ePvBKh1FYKasmJrYiv87KAv9tHBi1sQt4SI IcP8f9q9A8ztPw+A3g1EndeigJriodZAvgvfrLv847f44O7iC82FnaummysSIivUAj EEOpQcZPBldgE/a1+9toYUN5TmhAxcComXCJo+LjlRxjtD6BdvXp56l/3llJs7p7x5 RfuVm9GVa/x13huGB0p5kPb3hwPdH9zfTbp611w4lyzqtpC0BqltSW+g0ouvH2EJ/K IAYhK1ekh1OCA== Message-ID: <3d8398f1-0130-4d3b-ac54-d23877811747@kernel.org> Date: Wed, 14 Jan 2026 11:41:48 +0100 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: <7963534f-cce8-4330-8a67-3f31bd6b2166@kernel.org> <20260113124147.48460-1-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 X-Rspam-User: X-Stat-Signature: pu6wrnae8omsmm9qkef3fabrxqm5xpcc X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: B297A8000A X-HE-Tag: 1768387313-475521 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. Here is a thought: with "init_on_free", we charge zeroing of pages to whoever frees a page. Can't we have a hugetlb mode where we zero hugetlb folios as they are getting freed back to the hugetlb allcoator? IOW, we charge it to whoever puts the last reference. just a thought, maybe it was discussed before ... -- Cheers David