Re: [PATCH] mm: use memory barriers for non PMD-sized folios to sync

["David Hildenbrand (Arm)" <david@kernel.org>]

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 <baolin.wang@linux.alibaba.com>
> ---
>  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