From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 016DCCD343F for ; Wed, 13 May 2026 02:12:15 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 59F666B0005; Tue, 12 May 2026 22:12:15 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 54F366B008A; Tue, 12 May 2026 22:12:15 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 466016B008C; Tue, 12 May 2026 22:12:15 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 35B426B0005 for ; Tue, 12 May 2026 22:12:15 -0400 (EDT) Received: from smtpin16.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay06.hostedemail.com (Postfix) with ESMTP id B615B1C0522 for ; Wed, 13 May 2026 02:12:14 +0000 (UTC) X-FDA: 84760771788.16.2AA6748 Received: from out-171.mta0.migadu.com (out-171.mta0.migadu.com [91.218.175.171]) by imf14.hostedemail.com (Postfix) with ESMTP id F245B100013 for ; Wed, 13 May 2026 02:12:12 +0000 (UTC) Authentication-Results: imf14.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="FvqK/RE1"; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf14.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.171 as permitted sender) smtp.mailfrom=lance.yang@linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1778638333; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=cdjJWSLheRQ+3GuD9xg1u/bkib2oHeCrXtM/HsiT9hA=; b=TG3HE9oPjDkGtPNwYP7lz4pUGE0eCntwV+yQYtX+n+ftzSuhbtFi0mV0HNOC38v/y/H1PT V/kqThRf/S38I1il12dc0+WV1pXFBlahMqdZHKDgNSdgfgg8cpKJ7Ux0WikkWJF5KISTiW ULExXEqXTLRmNBozHi96ROeMAqkRLEY= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1778638333; a=rsa-sha256; cv=none; b=jdBqW+RuwshVhjRiip9aksUM3pavZTeeOr0WWf07FfxMKkFXhdODn/ig4l4FGz5mkqHVok EcFlqsZgxoPelK1H6TJZd9T27/SO6RrnC3DfXwWogtHeoY3JpPcYPkJA8eKs0Uqpa0qir/ 8NbW+Fx9Bs1Iu1mIqvOKOVul+dXaZHc= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="FvqK/RE1"; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf14.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.171 as permitted sender) smtp.mailfrom=lance.yang@linux.dev X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1778638329; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=cdjJWSLheRQ+3GuD9xg1u/bkib2oHeCrXtM/HsiT9hA=; b=FvqK/RE1HsPP0f+Olp2dZTiI5rOPjZzpmfjMhV4TCxIgrlljmH7uX3cIBO75/yLdaj1TkK fwZTR2ctao8DGHXSY/4uBJ9orQKkWJgVytD8f91uGBoXacu3/XNJWE62Bp8Gsz7UYuybaH 03SZrVpWg5okbuI8VjHVv+E+iz1cRzs= From: Lance Yang To: shy828301@gmail.com, jannh@google.com Cc: lance.yang@linux.dev, rppt@kernel.org, akpm@linux-foundation.org, arnd@arndb.de, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, ardb@kernel.org, sethjenkins@google.com Subject: Re: [PATCH] mm: make zeropage read-only Date: Wed, 13 May 2026 10:11:48 +0800 Message-Id: <20260513021148.17588-1-lance.yang@linux.dev> In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: F245B100013 X-Stat-Signature: atcj6dq1sfqcgpfwffnrbadqrucjyqtc X-Rspam-User: X-HE-Tag: 1778638332-937269 X-HE-Meta: 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 Di8EBNLI iQhKz6izAhymCButS/2LM+5Sf5TE5AJAX9Reg6rj5QF/xvgyhUORA9Ow7QOIaCJd+LTXDnKfO8Y+ZpGhjrdKoyozLn4S4RtegoMQACutYjkQ1qNMl4lzU2t6Ua8B6e4ifYitB1WO8x8+FCRQbq+Tm5StDzNRMvqxhRGruUFR4ozhBMHfYxF85Fhhczy0n59183SHYEBrvyeErvOTCBgQyjfNNe6pvdnHsjdNg Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, May 12, 2026 at 10:14:01AM -0700, Yang Shi wrote: >On Tue, May 12, 2026 at 6:48 AM Jann Horn wrote: >> >> On Tue, May 12, 2026 at 4:31 AM Lance Yang wrote: >> > Would it makes sense to apply a similar treatment to huge_zero_folio >> > as well? >> > >> > with CONFIG_PERSISTENT_HUGE_ZERO_FOLIO=y, it is allocated at boot and >> > never freed, so it should never be written after initialization either :) >> >> Oh, neat, I didn't realize that that feature exists. >> >> I guess there are two aspects of making the huge zero folio RO that >> could be problematic: >> >> 1. If the huge zero folio comes from the page allocator, making it >> read-only might require splitting a huge PUD, which could have >> performance implications. >> 2. I vaguely remember arm64 has rules about how PUD/PMD entries in the >> linear mapping can't be split at runtime at all depending on hardware >> capabilities, meaning the entire linear mapping may need to be mapped >> without any huge PUD/PMD entries - IDK if thp_shrinker_init() runs >> early enough to be excepted from that. See can_set_direct_map() and >> force_pte_mapping() in arch/arm64/. > >Yes. First of all, this relies rodata mode. If rodata=on (used to be >called full), the linear mapping may be mapped by PUD/PMD if the >hardware can support BBML2_NOABORT, otherwise it is mapped at PTE >level all the time. But how huge zero folio is mapped in linear >mapping should not matter, you just need to change the linear mapping >permission to RO anyway. > >If the rodata mode is off or noalias (used to be called on), the >linear mapping may be mapped by PUD/PMD, but basically changing linear >mapping permission is not expected by kernel. Ah, right. So for huge_zero_folio the hard part is not just making the backing memory read-only, but also whether we can change the linear mapping permission for that range. That depends on the arm64 rodata mode / direct-map setup. Thanks Jann and Yang for the explanations! Lance >Thanks, >Yang > >> >> So making the huge zero folio RO in the linear map would probably >> require adding a new config flag, connecting that to >> ARCH_HAS_SET_DIRECT_MAP, and changing one or two places in arm64 >> memory management. >> >