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 777E64908B0 for ; Mon, 7 Sep 2026 15:29:47 +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=1788794988; cv=none; b=LYNhIXRGggl2b9924mfGHOoislzLMaEgG7kLLQVf543WKkerhfYA5F2EEWNknQjfVGc4yasBH11tWIpyBoyuA5g4gYeJ0Z+NZEdLgFBjtoDMEvh9jJdiJsLTweyOVpbYnLEsOaZm3wLkm7feS0VehYWkMGvvW1ARoCaeCFg/Mrw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788794988; c=relaxed/simple; bh=VNNJaMv7+hXXos69y/SfKUSWLDxluqHo/2Ekexo2LrE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=D0s0/qS1AvwtU3fu5U3ZLEw4PCooY9pNVQrH9f9JRsdC1vCPcJFzkG8pjs3TiJr2uLJr0rjARto8TkxjxQfA3rOm3vHb2dXoEFuRK7dHZiI6/w89OmLbnOmxDj7yxwxH25yrCDmM4J1Y7juh/RuTSldLDHRAt6TlkYRoA45mC+M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hVL1qCoE; 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="hVL1qCoE" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 93FC41F00A3A; Mon, 7 Sep 2026 15:29:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788794987; bh=9SJTzceowrNohQipk3v/6QfqCF0/MRLk5RUcto72/EU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hVL1qCoESBzyncNyjZOjHebgaRnwQq7OTLas7qK5ytnPnGT/qVnxaJ8epup1Q3Gnv 3fZX01jlfiPJ8xBm8egrhGjOfsoBy6lwtuUjXreRoArmc78LQGYhgHobBdJ4/YJBrl 88oaQeAmO1iosbVzMtlohN2iPpfueXA873ipSpOYerzdG7DTRR15+OHWjaDCDBz7lE UHO5/LG7wcW8TivdO2WWQzflga8qsaBF/kT3ZIZ6NqVWme8+uvxZtG22o+Xxxtf4fk 4rIHo7Kw7SC1kQhcu+DXKFWKfR+7s5udZgXs0KgtmTycvUOn5Sztcy/nehHBUGvPXB 5/wzFQCynHLjA== Message-ID: <47a6817e-e98e-4a00-8e2c-6a66d15ddb37@kernel.org> Date: Mon, 7 Sep 2026 17:29:42 +0200 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] mm/hugetlb: fix overbroad MMU notifiers for unshared PMDs To: Andrew Morton , Li Zhe Cc: muchun.song@linux.dev, osalvador@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, aiqi.i7@bytedance.com References: <20260831091023.66581-1-lizhe.67@bytedance.com> <20260831173220.72ab28190a44a305dacf4d04@linux-foundation.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/1/26 02:32, Andrew Morton wrote: > On Mon, 31 Aug 2026 17:10:23 +0800 "Li Zhe" wrote: > >> Hugetlb currently expands MMU notifier ranges to PUD boundaries whenever >> PMD sharing is possible. That is only needed when huge_pmd_unshare() >> actually detaches a shared PMD page table, because clearing the PUD >> invalidates the whole PUD-sized virtual address range. >> >> For hugetlbfs hole punch and MADV_DONTNEED, a shared mapping can pass >> the "PMD sharing is possible" range test in function >> adjust_range_if_pmd_sharing_possible() even when the PMD table covering >> the target 2M page is not shared. KVM then receives a 1G invalidation for >> a 2M operation and zaps unrelated secondary mappings, so the guest has to >> fault them back in. >> >> Fix this by using the existing cheap "sharing possible" test only as a >> gate, then inspect the candidate PMD tables under the locks held by the >> hugetlb unmap paths. The notifier is expanded only for PUDs whose PMD >> table is actually shared, while the other callers keep the existing >> conservative expansion. > > Thanks. > >> On a Redis-in-VM workload that punches cold 2M hugetlb pages, this >> patch improves P99 QPS stability while punching pages, reducing the QPS >> degradation ratio from 7.09% to 1.45%. > > So a modest performance improvement? Is that worth the complexity, though? -- Cheers, David