From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.subspace.kernel.org (Postfix) with ESMTP id B559C606A31C for ; Wed, 11 Nov 2026 07:48:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=XpB76X8F Received: by kanga.kvack.org (Postfix) id 9B46E6B05B4; Wed, 11 Nov 2026 02:48:57 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 98C796B05B8; Wed, 11 Nov 2026 02:48:57 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 87C036B05C0; Wed, 11 Nov 2026 02:48:57 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id 43E216B05B4 for ; Wed, 11 Nov 2026 02:48:57 -0500 (EST) Received: from smtpin17.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay02.hostedemail.com (Postfix) with ESMTP id 93E801201B8 for ; Wed, 11 Nov 2026 07:48:56 +0000 (UTC) X-FDA: 85422061872.17.EE28ECF Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf06.hostedemail.com (Postfix) with ESMTP id C0932180002 for ; Wed, 11 Nov 2026 07:48:54 +0000 (UTC) Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=XpB76X8F; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf06.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1794383334; 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=RYAYBlC4Aq7hCU7wUZTzDQfIXhN+TRHtBXMHO3E6CkQ=; b=zToBxflFtJRc481Xv9XI8w8H4pzwkfbJpMkqWJ1YG52cSDEMjb7g0h/HrbaYU8xFf91TIT /IO594vfX6NCO/c8z/q+hif76rhDhPw8LJAhpq3n4Y0jL+zHdJWw0R7KllxyicA6sj0gJj 8ZH1x6pacDgE3xQK1qV//MIfrZtAZDQ= ARC-Authentication-Results: i=1; imf06.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=XpB76X8F; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf06.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1794383334; b=QtGLRJGIdUZ5ERWB1WosBNkmpPi3KCbBuc2TnKY0YxYUz93Qpnj36GDrGf1WjOYug4eGma DakmQ6epbY5Bv4AVI8HGv0ZJ7sYoVeJTVlv3wK/EEQ4khmiQR/S0qtqnxtiK7DCBEQINmU skkchb3Qxj1NisoKJQI38qithFS8hTQ= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 05E3E42AE8; Wed, 11 Nov 2026 07:48:54 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 676311F00A05; Wed, 11 Nov 2026 07:48:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794383333; bh=RYAYBlC4Aq7hCU7wUZTzDQfIXhN+TRHtBXMHO3E6CkQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=XpB76X8FNZtM3NH0/YB4l8soqMgeYC5BSvJP9fOhtXpXdAsNCyUggfq84YF/fWp37 CvpvWp7blGPMvAEQawhyGihbKkzKBahgYGhk5NGuJJzCSIeQT8bde4UX22Irm1zsMP IvldPSR71P9ilJYG+dE/G+H+WdXviPa6ylCr+yxuXP48OLnKgdwZ3SdKfnecmIQX32 PnRwSJ+oK2zEAshNLlQwt19Fvj8Dh5oqm30m9mHl3H9tqQNHXs7/nqYKFSqdKvc/2i 17+7LA7xXkotGy5OmHz5qYYmLM/it+3JMETAIt60M+AH4Lch2t8IEFQW2pl3GnAAYQ 14QGytf9zl19A== Message-ID: <3e5ceb38-d04c-4cd4-a783-5d0edcedf5dd@kernel.org> Date: Wed, 11 Nov 2026 08:48:45 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: use memory barriers for non PMD-sized folios to sync To: Baolin Wang , akpm@linux-foundation.org, ljs@kernel.org Cc: riel@surriel.com, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, harry@kernel.org, jannh@google.com, lance.yang@linux.dev, qi.zheng@linux.dev, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <09736bc2e01bec0ca7436733078c82c7e0cc6627.1794376686.git.baolin.wang@linux.alibaba.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <09736bc2e01bec0ca7436733078c82c7e0cc6627.1794376686.git.baolin.wang@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: C0932180002 X-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Stat-Signature: 14e8nd37b7sw4auqd93dabojcb5sxsfg X-HE-Tag: 1794383334-735463 X-HE-Meta: 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 rgt2Rl1Z WwR6TXCo8BQpuHTMlMaAu9RfX0fLXJ3ZSKx+2TdoVXh0i19WxY3xlUq1wBWxku7knlWKHbOQtVryU+RjXJrvo4OX8RFm7/1gwwb64pSTmZjk1+cKLCsIQAbwx+KKA4ZvrjTn62qA4M40bpZ/+eiWqFtRD7T3IgWAIGpunDK7NJWNqo21m5BoU8w1Mlb/IwXRj3Afu3zBOHUTcDEiv1oyz46fzad/71ShO/CFWIXXY/ldKJczimt9u5EvtaiK56kyDQFEE8epZzp5ZRIe0XQVD5wewrVU+rT6EUxcqQZtelH3UqJtKDP9Gt5nLMw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 11/11/26 07:06, Baolin Wang wrote: > Commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped folios") > uses PMD lock via sync_with_folio_pmd_zap() to synchronize when a cleared > PMD is found while unmapping a folio. > > However, for non PMD-sized folios, the cleared PMD can only come from > PTE table reclaim, which clears the PMD only after the rmaps for the > table have been removed. This is plain message passing, and taking > the PMD lock is more than needed, like I did in the previous patch[1]. > > Without barriers, the walker can observe the cleared PMD but a stale > mapcount on weakly ordered architectures: > > CPU 0 (unmap folio) CPU 1 (MADV_DONTNEED, full PMD) > ================================ ================================ > folio_lock() > lock(ptl) > clear PTEs > remove rmaps /* W mapcount */ > [mapcount load satisfied early, > folio still mapped] > pmd_clear() /* W pmd */ > unlock(ptl) > pmdp_get_lockless() -> none /* R pmd */ > skip the PMD > folio_mapped() -> true /* R mapcount, stale */ > unmapping assumed to have failed > > Pairing an smp_wmb() before the PMD clear with an smp_rmb() after the > PMD read forbids this outcome: > > CPU 0 (unmap folio) CPU 1 (MADV_DONTNEED, full PMD) > ================================ ================================ > remove rmaps /* W mapcount */ > smp_wmb() > pmd_clear() /* W pmd */ > pmdp_get_lockless() -> none /* R pmd */ > smp_rmb() > folio_mapped() /* R mapcount */ > > If R pmd observes W pmd, R mapcount must observe W mapcount, so > folio_mapped() no longer reports the removed mappings. > > No changes for PMD-sized folios unmapping. > > [1] https://lore.kernel.org/all/542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com/ > Signed-off-by: Baolin Wang > --- > mm/internal.h | 26 ++++++++++++++++++++++---- > mm/memory.c | 12 ++++++++++-- > mm/page_vma_mapped.c | 2 +- > 3 files changed, 33 insertions(+), 7 deletions(-) > > diff --git a/mm/internal.h b/mm/internal.h > index cd3675c044e0..a277bf4841f8 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -592,6 +592,7 @@ void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte); > * sync_with_folio_pmd_zap - sync with concurrent zapping of folios > * @mm: The mm_struct. > * @pmdp: Pointer to the pmd that was found to be pmd_none(). > + * @nr_pages: The number of pages in the folio. > * > * When we find a pmd_none() while unmapping a folio without holding the PTL, > * zap_huge_pmd() may have cleared the PMD but not yet updated the folio > @@ -600,13 +601,30 @@ void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte); > * against later mapcount reads. Skipping the PMD without synchronization > * could make folio unmapping code assume that unmapping failed. > * > - * Wait for concurrent zapping to complete by grabbing the PTL. > + * Wait for concurrent zapping to complete by grabbing the PTL for PMD-sized > + * folios, otherwise smp_rmb() is sufficient. > */ > -static inline void sync_with_folio_pmd_zap(struct mm_struct *mm, pmd_t *pmdp) > +static inline void sync_with_folio_pmd_zap(struct mm_struct *mm, pmd_t *pmdp, > + unsigned int nr_pages) > { > - spinlock_t *ptl = pmd_lock(mm, pmdp); > + if (nr_pages >= HPAGE_PMD_NR) { > + spinlock_t *ptl = pmd_lock(mm, pmdp); > > - spin_unlock(ptl); > + spin_unlock(ptl); > + return; > + } > + > + /* > + * For non PMD-sized folios zapping, concurrent zap_empty_pte_table() > + * may clear the PMD after the rmaps for the table have been removed, > + * but later mapcount reads could still be observed before the PMD > + * read, making the folio appear to be still mapped after unmapping. > + * Order the PMD read before later mapcount reads, so that observing > + * the cleared PMD implies observing the rmap removals. > + * > + * Pairs with smp_wmb() in zap_pte_range(). > + */ > + smp_rmb(); > } > > struct zap_details; > diff --git a/mm/memory.c b/mm/memory.c > index fe72e66c9c2e..1ee286cdd3af 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2054,8 +2054,15 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb, > * Clear the PMD after the rmaps for the table have been removed, in case > * an incorrect mapcount is observed after unmapping the folio. > */ > - if (can_reclaim_pt && direct_reclaim && addr == end) > + if (can_reclaim_pt && direct_reclaim && addr == end) { > + /* > + * rmap changes need to be observed before page tables get zapped. > + * > + * Pairs with smp_rmb() in sync_with_folio_pmd_zap(). > + */ > + smp_wmb(); > direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval); > + } > pte_unmap_unlock(start_pte, ptl); > > /* > @@ -2110,7 +2117,8 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb, > /* fall through */ > } else if (pmd_none(*pmd)) { > if (details && details->single_folio) > - sync_with_folio_pmd_zap(tlb->mm, pmd); > + sync_with_folio_pmd_zap(tlb->mm, pmd, > + folio_nr_pages(details->single_folio)); > addr = next; > continue; > } > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index a931243ca42b..d793ad0c5364 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -295,7 +295,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > pvmw->ptl = NULL; > } else if (!pmd_present(pmde)) { > if (pvmw->flags & PVMW_SYNC) > - sync_with_folio_pmd_zap(mm, pvmw->pmd); > + sync_with_folio_pmd_zap(mm, pvmw->pmd, pvmw->nr_pages); > If there is no folio, why do we care about the sync? Should there be a folio here? Anyhow, I think sync_with_folio_pmd_zap() really should be consuming a folio. -- Cheers, David