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 12E87C3DA7A for ; Wed, 14 Oct 2026 18:44:28 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 1530E6B0095; Wed, 14 Oct 2026 14:44:27 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 12B406B0098; Wed, 14 Oct 2026 14:44:27 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0401D6B0099; Wed, 14 Oct 2026 14:44:26 -0400 (EDT) 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 D11E16B0095 for ; Wed, 14 Oct 2026 14:44:26 -0400 (EDT) Received: from smtpin01.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay01.hostedemail.com (Postfix) with ESMTP id 47EEA1C3E25 for ; Wed, 14 Oct 2026 18:44:26 +0000 (UTC) X-FDA: 85322107332.01.87FA5D7 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf20.hostedemail.com (Postfix) with ESMTP id 6F5B21C0003 for ; Wed, 14 Oct 2026 18:44:24 +0000 (UTC) Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=kjAPCYaW; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@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=1792003464; 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=g6JScfSsWgesg/1WtK9RTmCPMl1xA6BFFNt/w568P2w=; b=6bd41TvdJxLXzrtLm1Nn1RfUO2LVTcmn31SJvaT4T0U7AmfWPpbHQ4nBPrOJqEPXgSI+FF qqKypwfRhJlsVbf8rIS8uXFAuH8/nbkCqyya38LzGi5IzVVf1N2xqJFJX+TpMNOLQOKSoV LYrOWjRIrYgu+QW/KDr16sQr1pBz0RA= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=kjAPCYaW; spf=pass (imf20.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1792003464; b=d0pL0vgvAL97EOT8NuCoDE3a02w8iIp0V1CnsKSCa8YFqaq6LSaPaWdnI7/a14euZiaCbb GoJbnuxZQ8RjMrWHF3noWIT2jOF9r8D5/enUOTnQhgxx/uSko6q3sJRQolPV3uEt5/Sn8E flBFDpuPcLSSd/y8n4+1VJSd9GbD4t4= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 94B0E42BA3; Wed, 14 Oct 2026 18:44:23 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 699221F000C4; Wed, 14 Oct 2026 18:44:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792003463; bh=g6JScfSsWgesg/1WtK9RTmCPMl1xA6BFFNt/w568P2w=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kjAPCYaWrnOUgSCE1ZkOd4MPnsmhx/r5WxajxxuYFxUwucAaSttKAE/IXJCG+DuP2 i/SbALVGceqTj/O05V3UuqRooghqrq8qL+BxPEdjUQKfWV77/MLHM+3iZNY5Je4i6e 1TiNwuVqtJ+3R9EX8yfrLMrClw09i73W3EqwQRhBgPVNPYI0J0B2m/2nDvAPcfFy0u P+6h5mWmutCGtz1tL/lOJYTp5SP+vlc6SFLRrjtGyBnZhaTM2t4str2ceBkB2C+a+N DKe5k5FXge8ZMCjl6JOlheL3z1BqQXPRDCzup1q0YeajxmoC1X6unntHCovU3s4/m9 7xi8UByzUQgUw== Message-ID: <08a0907f-8e3a-4071-a2e4-fd1411c446f3@kernel.org> Date: Wed, 14 Oct 2026 20:44:16 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm: memory: fix truncation removing mapped folios To: Pedro Falcato Cc: Baolin Wang , akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, qi.zheng@linux.dev, jack@suse.cz, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com> <8245424a-f068-4be5-9e58-713cbff1f8ae@kernel.org> <405811f5-ee36-4b3c-80d0-7dc6827ab5d4@linux.alibaba.com> <6660d3bb-3485-4c34-8734-3ea99df920f1@kernel.org> 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 6F5B21C0003 X-Rspam-User: X-Stat-Signature: zye4utkfubuh96r4xiuc3khymjc3whx6 X-HE-Tag: 1792003464-886865 X-HE-Meta: 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 jFN4dSVl e2Kw7kEsRVsKD14DvRB0xQjG7wvv1piEBmsVHc4wlQs1zrhYFyTtdIRqXoWpaNrA+uPFt/so16h8OeO1wE2G14NMaHPmh29mXQhtRNLDrMse0RBbPsTo3I8DItIgFbDSsK5ccL4e6JPVdaxLAmbmPAAQ2TvcZXt33TW92TvU92TY/+K1xz3XrRfWuq4lQPlQfwrMnDDtawclcIRhT/1ljvuasseRZSRwG3IQiqtjSHOrp9/M6n6gtnRkkW02wcbc3qQq9 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/14/26 16:05, Pedro Falcato wrote: > On Wed, Oct 14, 2026 at 03:25:33PM +0200, David Hildenbrand (Arm) wrote: >> On 10/13/26 11:36, Baolin Wang wrote: >>> >>> >>> >>> I had considered this approach before, but it is not feasible. IIUC, the >>> sync_with_folio_pmd_zap() is used to address the race with zap_huge_pmd(), and >>> both the PMD clear and the folio mapcount decrement are performed under the PMD >>> lock in zap_huge_pmd(). >> >> Just to be clear, I am thinking about something like the following on the >> zap_pmd_range() side: >> >> diff --git a/mm/internal.h b/mm/internal.h >> index 38b1165212c94..a8820ad7e2f21 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -584,14 +584,15 @@ void free_pgtables(struct mmu_gather *tlb, struct >> unmap_desc *desc); >> void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte); >> >> /** >> - * sync_with_folio_pmd_zap - sync with concurrent zapping of a folio PMD >> + * 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(). >> * >> * 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 modified the folio to >> - * indicate that it's unmapped. Skipping the PMD without synchronization could >> - * make folio unmapping code assume that unmapping failed. >> + * zap_huge_pmd() or zap_empty_pte_table() may have cleared the PMD but not >> + * yet modified the mapped folios to indicate they are unmapped. Skipping the >> + * PMD without synchronization could make folio unmapping code assume that >> + * unmapping failed. >> * >> * Wait for concurrent zapping to complete by grabbing the PTL. >> */ >> diff --git a/mm/memory.c b/mm/memory.c >> index 8b0c2c735d3de..c585f5eb255c9 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -2139,9 +2139,7 @@ static inline unsigned long zap_pmd_range(struct >> mmu_gather *tlb, >> continue; >> } >> /* fall through */ >> - } else if (details && details->single_folio && >> - folio_test_pmd_mappable(details->single_folio) && >> - next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) { >> + } else if (details && details->single_folio && pmd_none(*pmd)) { >> sync_with_folio_pmd_zap(tlb->mm, pmd); >> } >> if (pmd_none(*pmd)) { >> >>> >>> However, the race here is different: tlb_flush_rmaps() is under the PTE lock, >>> while zap_empty_pte_table() is under the PMD lock, and reordering between the >>> two functions is also possible. That is, the folio mapcount operation could >>> execute after the acquire barrier (spin_lock()) of zap_empty_pte_table(), >>> allowing a reader on CPU 0 to incorrectly observe that the PMD is none while >>> mapcount > 0. >> >> I think we really just have to make sure to flush rmaps before zapping the PMD. >> Then it's really just the same thing we had before with sync_with_folio_pmd_zap(). > > Hmm, could we viably replace sync_with_folio_pmd_zap() with the memory barriers > instead? Seems lighter-weight and functionally equivalent? For the ordinary PMD zap case it's really: 1) Clear PMD 2) Drop refcount+mapcount So I don't think it's a direct replacement. If we had a proper way to invalidate + adjust + clear it could work. Like 1) Invalidate 2) Drop refcount+mapcount 3) Clear But in general I think: a) Likely all places where we have existing sync_with_folio_pmd_zap() have this problem (2). b) I don't want us to throw around memory barriers that way. This all belongs into a helper (like sync_with_folio_pmd_zap()) which we only call when required (e.g., details && details->single_folio) and that properly documents all the nasty details. c) Maybe there is some smart way to decide whether we need a memory barrier or a proper lock for now inside sync_with_folio_pmd_zap(). Based on the folio size should do (PMD vs. < PMD). But I assume grabbing the PMD lock briefly is not really a concern: if there would be something mapped we would have to lock it, and if it would be a PTE table we would lock that instead. [insert rant about our PMD table locking] -- Cheers, David