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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=XpB76X8F; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 59788606A31C for ; Wed, 11 Nov 2026 07:48:54 +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 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-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: 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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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