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 884CDC79F9E for ; Tue, 8 Sep 2026 12:33:14 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 77DA76B0092; Tue, 8 Sep 2026 08:33:13 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 72E566B0093; Tue, 8 Sep 2026 08:33:13 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 643FF6B0095; Tue, 8 Sep 2026 08:33:13 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 3426E6B0092 for ; Tue, 8 Sep 2026 08:33:13 -0400 (EDT) Received: from smtpin16.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay10.hostedemail.com (Postfix) with ESMTP id B9328C04C0 for ; Tue, 8 Sep 2026 12:33:12 +0000 (UTC) X-FDA: 85190535024.16.550F3A5 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf30.hostedemail.com (Postfix) with ESMTP id F111E8000B for ; Tue, 8 Sep 2026 12:33:10 +0000 (UTC) Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=aLtpz7A5; spf=pass (imf30.hostedemail.com: domain of ljs@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1788870791; 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=qrlRHDKkKksSse2z7V875Db2PCZ3B/h/JK+Hse17ruk=; b=UEnOnei0qrasZPZ2FRMAJFUYPuLZH2XBQ3FsJkXrVU6hP5M+MJdANhfunRoN8sowfs2Jok KvBHOKJS0ReZSc8B4Yp/Can4Jqkv2/5j6sWkRDMa2ABpUOps4VCk03ASWRgjcJT9lcTBGc htQ4mkyORiaqkz0LfUuV7kY6dAibDfo= ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1788870791; b=stmMHbk+gI3FG4Dgz/jdReNEiUZN8/bxe5sEA/og9Qbf1y3F5+6V8VB7njQpHPEKqkUbsF LU4Re9NdPev449QcfsPqk0LJX56QksJ/4lAcoFYTrFg9Wa8MjgoKtTx54LUlHpmpwyHtht gEQvlpAp5D21o0s2f/AtvBsoio34vHE= ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=aLtpz7A5; spf=pass (imf30.hostedemail.com: domain of ljs@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 97086601DB; Tue, 8 Sep 2026 12:33:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2BC691F00A3A; Tue, 8 Sep 2026 12:32:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788870790; bh=qrlRHDKkKksSse2z7V875Db2PCZ3B/h/JK+Hse17ruk=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=aLtpz7A5VymVqyE5xIUldqu4FEfYkW3C/kRkUKk7eS5Fd938lNBcUqjWIc4HUU61D DVipoD/pdYsc76ZExc8Qvd07r0+EVBvsmN0INrIpBqY43rMzTPkA1y8XItRLueMgmx 3YOfOCnlih3OLyVjTsPc67cCII18Lx0cRR9hCnypeOHaV41VQzUaFqL6rwG0rnaocV dq/wVeanPRwRB8LUnw28+2OE1zJ6OXYKiYMDTUC/ChepsXs8Tbov72fkpOZpN5UuX4 BnK+GneNUBzkfBv1WkAj2ZiQktcmuToqdhISbN/sVHSCyc1BsNQXmhbGQ6rk9lwLrr h4poq5HhB3GXw== From: "Lorenzo Stoakes (ARM)" Date: Tue, 08 Sep 2026 13:32:10 +0100 Subject: [PATCH v2 01/12] mm/huge_memory: zap deposited page tables after an RCU grace period MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260908-rcu-pagetable-freeing-v2-1-1f60b64e878e@kernel.org> References: <20260908-rcu-pagetable-freeing-v2-0-1f60b64e878e@kernel.org> In-Reply-To: <20260908-rcu-pagetable-freeing-v2-0-1f60b64e878e@kernel.org> To: Andrew Morton , David Hildenbrand , Zi Yan , Baolin Wang , "Liam R. 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Peter Anvin" , Arnd Bergmann , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jason Gunthorpe , John Hubbard , Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-openrisc@vger.kernel.org, linux-sh@vger.kernel.org, linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-snps-arc@lists.infradead.org, linux-arch@vger.kernel.org, sparclinux@vger.kernel.org, linux-alpha@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-um@lists.infradead.org, Hugh Dickins , Qi Zheng , "Lorenzo Stoakes (ARM)" X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=1907; i=ljs@kernel.org; h=from:subject:message-id; bh=QsmXD4bhTqKAKkUCvIrmgq1QZ07I5df5Am7bAMtZYGk=; b=owGbwMvMwCV2fu7ZrsZH9SKMp9WSGLIWMEQz2MotteNZL1imp//16Cr13zP57DKuWi88+/7V3 rv7jPrnd5SyMIhxMciKKbI8/yK+P0gkbF7nBX83mDmsTGBDuDgFYCIdVYwMTyUVDice2bv+1N8t ptvOq5Z+sY5uu1n26MCPewUH//vl7WD4n9LEE+pReenVX+nclMY/0peKpct75+X5nPI4xSKj2PW GFQA= X-Developer-Key: i=ljs@kernel.org; a=openpgp; fpr=E7F417BF5214569E89D04F46CF9DCD8A81E27F14 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: F111E8000B X-Stat-Signature: bqtrziyokhpegdq5d7wnktbaty8f6mer X-Rspam-User: X-HE-Tag: 1788870790-807610 X-HE-Meta: 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 FKedvsX3 wb35bnrpphqKHGnmOeOH8dNYaxDXOVHjEB4W7WO/k9mIGUHI0DAfiDHoj4quyBIjR3Hq10FA0yi4NQkDEg3aZXvZhPK+ubMdwjRKndcWawCTpmkyVA/bbhUPjeWsIIPWf+lYrC8uVGUXl7GCpI28drf09LWOcl39ZxNKYCXBJ4arTmeZNYqpOFxacZJkJ9iMSQwxkL+FpAI8PcTmGrJpDCB/vJHPe6Xlpg3qMAfaPcaQQ7oDQX525/fYku1lSwaUiHWVm/q1CxUrMsLsy72sJcX7kxFQ0sisNmKpLUJlz4AUHTTpvIxaUA54i3Z93X5XJFAWOXRJKjQhlxVa/nFUA9VKAH8sRHaLBxmPyzZiKL3To9VSsEQ2rPy/lRAFHZ+FEWVctSiy7a5CjHpFt0f8b2Rcw679OZlLNHpMxoamaPLcJ8S7VSIlvkgL8LX61yFGuQemSbQ47xUseBZ7D00FFL3bkNFTpSqz4qt2MJRBMnftg+PY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: When an anonymous mapping is collapsed for THP, a PTE page table is 'deposited' with the installed PMD entry. This is done in order that a split can be performed without needing to allocate additional memory. The freeing occurs in zap_deposited_table() and is done directly without any delay via pte_free(). This is currently not a problem as existing page table walks are protected by the mmap or anon rmap lock. However this becomes problematic in a future where RCU-only page table walkers exist, as there is nothing to prevent a page table walker that started the walk prior to collapse having its PTE table freed underneath it. Commit 13cf577e6b66 ("mm/pgtable: add pte_free_defer() for pgtable as page") already provides us the mechanism by which to solve this - pte_free_defer(). Therefore, as a prerequisite to a future commit which will permit fully RCU page table walks, update zap_deposited_table() to use pte_free_defer() rather than pte_free(). Note that the IPI sync in collapse_huge_page() is still required to ensure refcount correctness against a GUP-fast operation. This is because GUP-fast might increment refcount, but __collapse_huge_page_isolate() determines whether it is safe to proceed by checking folio_ref_count() against folio_expected_ref_count(), so the two must be mutually excluded. Signed-off-by: Lorenzo Stoakes (ARM) --- mm/huge_memory.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 54494c3fa983..505f7b62ff28 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2476,7 +2476,7 @@ static inline void zap_deposited_table(struct mm_struct *mm, pmd_t *pmd) pgtable_t pgtable; pgtable = pgtable_trans_huge_withdraw(mm, pmd); - pte_free(mm, pgtable); + pte_free_defer(mm, pgtable); mm_dec_nr_ptes(mm); } -- 2.55.0