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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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 <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.

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;
}