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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=KWXhrImQ; 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 4DFE7606A31C for ; Wed, 11 Nov 2026 09:22:28 +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=KWXhrImQ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 944C71F00A0A; Wed, 11 Nov 2026 09:22:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794388948; bh=Qtfi79sNkt5m/VGYUDKWPyqCBF/QwcBOUK8ADQR5ugg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=KWXhrImQFuzpZKw/rzBUBYvNMKl1qxn7VBe2anF24Uyk9RZS1Nbr4w1h/RdmecNPZ TVcXiospwwgUvYCcCHCO0wY6pjT5qGDBpyEf30oX64aH2zamK+EQTsSgKROGj3VG2j PImW+0v70imhZhZtFxj7svIiwBrsbzUXtcNjs5slcjX+ovW4Ac6l7CpyvT4LwKb7Pz MzZANZtbMcc1vsZxVDJNYoNaSqn1cK1eppZhObGqluQeMfIImKrrLvIWpo2MOihMJx CmAeZOZlWXmsKZ9E3Symko1yg0hxb3Wco/zvnoFmZHkZZ8pmhQr9rsUzwSaLoqVeIO 7RI4baxf/39dg== Message-ID: Date: Wed, 11 Nov 2026 10:22:07 +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> <3e5ceb38-d04c-4cd4-a783-5d0edcedf5dd@kernel.org> <59c59696-2112-44e6-818c-50bd10789b70@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 09:43, Baolin Wang wrote: > > > 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)); If we're working on a folio, we better pass in a folio. That might require some cleanups, yes. diff --git a/include/linux/rmap.h b/include/linux/rmap.h index 52fff8dc74213..80fb7ef9bab92 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -884,6 +884,7 @@ struct page *make_device_exclusive(struct mm_struct *mm, unsigned long addr, #define PVMW_PGTABLE_CROSSED (1 << 16) struct page_vma_mapped_walk { + struct folio *folio; unsigned long pfn; unsigned long nr_pages; pgoff_t pgoff; /* Only meaningful if nr_pages > 1 and not a KSM walk */ @@ -898,6 +899,7 @@ struct page_vma_mapped_walk { #define DEFINE_FOLIO_VMA_WALK(name, _folio, _vma, _address, _flags) \ struct page_vma_mapped_walk name = { \ + .folio = folio, \ .pfn = folio_pfn(_folio), \ .nr_pages = folio_nr_pages(_folio), \ .pgoff = folio_pgoff(_folio), \ The question is really whether pfn_mkclean_range + mapping_wrprotect_range really need that sync as well. (we could identify by seeing !folio I guess) -- Cheers, David