From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=115.124.30.99 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794386589; b=flA9TbbUc6jx330vA0iBPWFngqfVV2hdii/W0ElEm0OtykKd4bEatp0gpjNhpaJRaSbf tR+ERAwK9ylGkmE5UuTsWe3JAtaMtXUVoZ6XyuqelN0dbwMQb4FFbZTarDiRkpojksHAe 1h0F3gIESNLn2CzRuR2lSTRIlF20ZUzxzvoO3NsCTQ+UPcOcIRdI4tnP8lZTYMZYGzMhu tz1+5EOceeOdUgEGapTW58i7qpEclYeWqJsTJgAyOPhTVtrctqSJpdNRyO59e7ZDyFATJ YA7mnJK7IezfEho7KpRSWCKCrI6HKrbkf7ovXnjAhMPzWvCeLe49z46PMySocXecykw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794386589; h=DMARC-Filter:DKIM-Signature:X-Alimail-AntiSpam:Received:Message-ID: Date:MIME-Version:User-Agent:Subject:To:Cc:References:From: In-Reply-To:Content-Type:Content-Transfer-Encoding; bh=je3HtlkLcRLrQhsQj5hoLe+iA4mHnRWsfHzVv+rTg0M=; b=XUER7Qq/atFeO2Qhjy9qh2fMZGrvkiTlxTVJtGWfGUb9UKLiO1CCwMgGK3k59ekLf9OL o4LUJjsrt3T8xmrJIOyG+JeUTtxn3aT/xXMOO4S7fbvRy+ptLmYvqv1Bbu232HfSkrO23 Lg9FQOhdyRhvrBn7nYnnzs+UrB2PG/IFcqTiMUDKlwMjD/xzoNe09UVrkljaMSfltUm4p lgzDv0t48Jb4fli+xw8Mp0J/QFe8xfD+cZMD662WdSj21MrrhDRJ2kcHHS5bZL0WvailI MLPZnzxy2Ylp3+h6xOGPzBTVRIVSBqqgtgi0iL+UNh/jvLgc1yHk6VJfmL3wRr8H1Hw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass header.d=linux.alibaba.com header.i=@linux.alibaba.com header.a=rsa-sha256 header.s=default header.b=GmHCuvlc; arc=none smtp.remote-ip=115.124.30.99 Received: from out30-99.freemail.mail.aliyun.com (out30-99.freemail.mail.aliyun.com [115.124.30.99]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8FB46606A31C for ; Wed, 11 Nov 2026 08:43:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 8FB46606A31C Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.a=rsa-sha256 header.s=default header.b=GmHCuvlc DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1794386585; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=je3HtlkLcRLrQhsQj5hoLe+iA4mHnRWsfHzVv+rTg0M=; b=GmHCuvlcmXefncC19tym59Zclf7TYgsfpXqqlGb/ckZRrQ2BsW35/gpCuOitoVVHs1aIR+6/C5dmQuiXBLc+U/k6zfUQ/thuB7Y1mbAep8DTWVfNv4qmiP/iefi0bCAEKtnUPJSo75cbOYH517OZqPHPx6kWGmriVXDF7zeQtSc= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R141e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033032089153;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=16;SR=0;TI=SMTPD_---0XEYJi.Z_1794386582; Received: from 30.74.144.118(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XEYJi.Z_1794386582 cluster:ay36) by smtp.aliyun-inc.com; Wed, 11 Nov 2026 16:43:03 +0800 Message-ID: <59c59696-2112-44e6-818c-50bd10789b70@linux.alibaba.com> Date: Wed, 11 Nov 2026 16:43:01 +0800 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: use memory barriers for non PMD-sized folios to sync To: "David Hildenbrand (Arm)" , 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> <3e5ceb38-d04c-4cd4-a783-5d0edcedf5dd@kernel.org> From: Baolin Wang In-Reply-To: <3e5ceb38-d04c-4cd4-a783-5d0edcedf5dd@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/11/26 3:48 PM, David Hildenbrand (Arm) wrote: > 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. Since there are only two cases without a folio: mapping_wrprotect_range() and pfn_mkclean_range(), I'm hesitant to modify all page_vma_mapped_walk() callers to pass in a 'folio' parameter for these 2 cases. Or should we add a new flag for the pfn walk? / Walk a PFN range which need not belong to a folio / #define PVMW_PFN (1 << 2) Then the changes would be: } else if (!pmd_present(pmde)) { if ((pvmw->flags & (PVMW_SYNC | PVMW_PFN)) == PVMW_SYNC) sync_with_folio_pmd_zap(mm, pvmw->pmd, pfn_folio(pvmw->pfn)); goto next_pmd; }