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]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6A89EC48BF6 for ; Mon, 4 Mar 2024 16:54:18 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id D1A356B007E; Mon, 4 Mar 2024 11:54:17 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id CA3A16B0080; Mon, 4 Mar 2024 11:54:17 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B441D6B0082; Mon, 4 Mar 2024 11:54:17 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id 9D19D6B007E for ; Mon, 4 Mar 2024 11:54:17 -0500 (EST) Received: from smtpin08.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 438051A0C9B for ; Mon, 4 Mar 2024 16:54:17 +0000 (UTC) X-FDA: 81859954554.08.258A57D Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf05.hostedemail.com (Postfix) with ESMTP id 2BCFB100020 for ; Mon, 4 Mar 2024 16:54:14 +0000 (UTC) Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=BVvT+Ek+; dmarc=none; spf=none (imf05.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1709571255; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding:in-reply-to: references:dkim-signature; bh=G0cnapOzxtcXhcdaDnzcGInd2LbIz3QiReDnd4uwgo8=; b=Tgb2xKRtwzRvO+hCShNDYK+hd1jx1Gmu0dRR+q0NygxWLT03+g/NoBfhXsi9R7l5TystId DOCszHH8eBaqD1Ggn1z/0Z0HP5lkkUgaNCfzGyp+Y5wbMQmUsMqNhRKdpZ93HpT3/ODoLk tCQ0dsY1rBsi4XDv/DD9VEmI3eY4Y0E= ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=BVvT+Ek+; dmarc=none; spf=none (imf05.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1709571255; a=rsa-sha256; cv=none; b=qmHljQmwt8yrxRsIoaj4nhN6WwI7it8IqRtvvb++YOfou3BpT2H2XB2cgDAIVhUGbqixNc vnmQotkdVNjsB+VlOLMKToW2VbqOsEMF41jmi4dL0rYUPhy7ecuCpaHJipB3x0QR/s0BLg 4fypudg6iEqvz81EvQHHl0we6j1RhEg= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=Content-Type:MIME-Version:Message-ID: Subject:To:From:Date:Sender:Reply-To:Cc:Content-Transfer-Encoding:Content-ID: Content-Description:In-Reply-To:References; bh=G0cnapOzxtcXhcdaDnzcGInd2LbIz3QiReDnd4uwgo8=; b=BVvT+Ek+7O24JXT2ZSd2psAgog Z1UchLQtIrnmg9bGAlVzZiY2t1GpYi4wE+aXjwHurU7awF3JyIZVd5jmUv0G667JaCPblbm4md7RH sd+F1mfynxXIy3mJOg5L5NF3eCMPMhfRKKDIqh606elyonPo79RIIkn4h2SKqQ3jNVLgOLJvy6MCe nfRBlK8kXIeQlCA0LYBZ0KnKKrnv45IcYGP8sRGZUzyX6GdQ2/UqCn91dSDAVKD4iT2pPJ1f//2a7 HIrEtSZXGiInkElC1ToPDKsmTS8A5NK6a+ymo5CFpaduq6z/ybrL1SWwVcAKb4y59UbBiMW/nfT31 fV0i0lNA==; Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1rhBZt-00000001wBn-0Enj for linux-mm@kvack.org; Mon, 04 Mar 2024 16:54:13 +0000 Date: Mon, 4 Mar 2024 16:54:12 +0000 From: Matthew Wilcox To: linux-mm@kvack.org Subject: Can the huge zero page be partially mapped? Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspam-User: X-Stat-Signature: u47e6q4wu7xh4ardn8cqt8xosdm8p5q4 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 2BCFB100020 X-HE-Tag: 1709571254-863911 X-HE-Meta: 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 DKFqweEy HU8Re8supJV+OtakooxTUgtHrp+rcBLt1oLln1hm7yEx4pPSDUMjTnw9O/ePub0ej4gQDt940OovDbIgQ/ZHiltB4LNh+TxCISJsBeBUvyMoXtXOXd76q6a9mmzinEVY5LtTT3NyzfNkuBBqVgjxizYJCzOTHcw0bAgi+eLVfpidemIKIpYQ1NhMzpA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.001571, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: I looked at the definition of is_huge_zero_page(): static inline bool is_huge_zero_page(struct page *page) { return READ_ONCE(huge_zero_page) == page; } That made me raise my eyebrows a bit because it will return false for tail pages of the HZP (that was at least unexpected for me). Then we have this beauty: void free_page_and_swap_cache(struct page *page) { struct folio *folio = page_folio(page); free_swap_cache(folio); if (!is_huge_zero_page(page)) folio_put(folio); } So if we can call free_page_and_swap_cache() with a tail of the HZP we can absolutely screw up its refcounting. Now, we have VM_BUGs to catch the refcount going below 0, and I haven't seen them being hit, so I _presume_ it doesn't happen, but maybe somebody inventive could come up with a way of putting a HZP tail into a page table ...?