Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes

[Lance Yang <lance.yang@linux.dev>]

On Fri, Nov 06, 2026 at 09:57:47AM +0800, Baolin Wang wrote:
>
>
>On 11/5/26 9:22 PM, Lance Yang wrote:
>> 
>> On Thu, Nov 05, 2026 at 11:06:29AM +0100, David Hildenbrand (Arm) wrote:
>>> On 11/5/26 10:00, Baolin Wang wrote:
>>>> CC Barry, who added the TTU_SYNC in shrink_folio_list().
>>>>
>>>> On 11/5/26 1:48 PM, Lance Yang wrote:
>>>>>
>>>>>
>>>>> On 2026/11/5 12:54, Lance Yang wrote:
>>>>>>
>>>>>>
>>>>>> Good catch!
>>>>>>
>>>>>>
>>>>>> Right. Whether the folio can be PMD-mapped doesn't tell us whether its
>>>>>> PTE table can be reclaimed, so those extra checks can skip synchronization
>>>>>> that the caller explicitly asked for ;)
>>>>>
>>>>> Ouch ... one thing I missed ... what about order-0 reclaim?
>>>>>
>>>>> shrink_folio_list() only sets TTU_SYNC for large folio, so can't we still
>>>>> hit the same race there?
>>>>
>>>> Thanks Lance. But this matches the purpose of the TTU_SYNC flag, which is used
>>>> to avoid racy checks with PVMW_SYNC. If no TTU_SYNC flag is set, it means the
>>>> caller can tolerate some racy checks.
>>>>
>>>> For this case, I think we should set TTU_SYNC unconditionally in
>>>> shrink_folio_list(). Barry, what do you think?
>>>
>>> That makes sense. I hope this doesn't throw a big wrench into our reclaim
>>> performance numbers.
>>>
>>> Can someone benchmark this?
>>>
>>> But maybe we should really follow-up with the folio-size based optimization in
>>> sync_with_folio_pmd_zap(), where we only need a memory barrier (and sufficient
>>> documentation why something that cannot be PMD-mapped can get away cheaper).
>> 
>> Dug into the history a bit :D
>> 
>> TTU_SYNC goes back to Hugh's THP splitting fix [1]. A racing zap could
>> clear the PTE/PMD before updating mapcount, so the walker could skip the
>> lock and return before the accounting caught up.
>> 
>> Barry later enabled it for large-folio reclaim [2], to avoid skipping
>> temporarily cleared PTEs and ending up with a partially unmapped folio.
>> He mentioned that small folios had the same race, but with just one PTE,
>> they couldn't be partially unmapped. That's where the large-folio check
>> in shrink_folio_list() came from.
>> 
>> Then PTE table reclaim [3] added another way to end up with an empty PMD,
>> even for a folio that can't be PMD-mapped. The truncation fix [4] moved
>> the rmap flush before the PMD clear and extended synchronization in
>> zap_pmd_range() to cover those folios too.
>> 
>> Looking back at the truncation discussion [5], you also pointed out that
>> page_vma_mapped_walk() needed checking, and suggested a possible
>> folio-size based optimization in the helper.
>
>Thanks.

Cheers!

>> [1] https://lore.kernel.org/all/c1e95853-8bcd-d8fd-55fa-e7f2488e78f@google.com/
>> [2] https://lore.kernel.org/all/20240306095219.71086-1-21cnbao@gmail.com/
>> [3] https://lore.kernel.org/all/92aba2b319a734913f18ba41e7d86a265f0b84e2.1733305182.git.zhengqi.arch@bytedance.com/
>> [4] https://lore.kernel.org/all/14ebbd556cc4941f0ebc43da59fa6a2fdca25dc8.1793148609.git.baolin.wang@linux.alibaba.com/
>> [5] https://lore.kernel.org/all/08a0907f-8e3a-4071-a2e4-fd1411c446f3@kernel.org/
>> 
>> So, how about starting with order-0 folios?
>> 
>> ---8<---
>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>> index 74cca0e3c726..d01fe8282417 100644
>> --- a/include/linux/rmap.h
>> +++ b/include/linux/rmap.h
>> @@ -872,6 +872,8 @@ struct page_vma_mapped_walk {
>>   	spinlock_t *ptl;
>>   	unsigned int flags;
>>   	bool pgoff_is_anon : 1;
>> +	/* Set from the folio, not the number of pages being walked. */
>> +	bool folio_is_order0 : 1;
>>   };
>> 
>>   #define DEFINE_FOLIO_VMA_WALK(name, _folio, _vma, _address, _flags)	\
>> @@ -883,6 +885,7 @@ struct page_vma_mapped_walk {
>>   		.address = _address,					\
>>   		.flags = _flags,					\
>>   		.pgoff_is_anon = folio_test_anon(_folio),		\
>> +		.folio_is_order0 = !folio_test_large(_folio),	\
>>   	}
>> 
>>   static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 9b8864a8540a..e70869b38292 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -592,21 +592,29 @@ 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().
>> + * @order0: True if the target is known to be an order-0 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
>> - * mapcount, and zap_empty_pte_table() clears the PMD only after the rmaps
>> - * for the table have been removed, but nothing orders the pmd_none() read
>> - * against later mapcount reads. Skipping the PMD without synchronization
>> - * could make folio unmapping code assume that unmapping failed.
>> + * An empty PMD does not guarantee that a later mapcount read sees the
>> + * completed rmap updates. For PMD-mapped folios, zap_huge_pmd() clears
>> + * the PMD before updating mapcount, so we must wait for the PMD lock.
>>    *
>> - * Wait for concurrent zapping to complete by grabbing the PTL.
>> + * Order-0 folios cannot be PMD-mapped. Their fully ordered
>> + * atomic_add_negative() updates mapcount before PTE table reclaim clears
>> + * the PMD. The PTE lock preserves this ordering if another CPU reclaims
>> + * the table. A read barrier therefore suffices for order-0 folios.
>> + * Keep taking the PMD lock for all other folios.
>>    */
>> -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,
>> +					   bool order0)
>>   {
>> -	spinlock_t *ptl = pmd_lock(mm, pmdp);
>> +	if (order0) {
>> +		/* Order the PMD read before subsequent mapcount reads. */
>> +		smp_rmb();
>> +	} else {
>> +		spinlock_t *ptl = pmd_lock(mm, pmdp);
>> 
>> -	spin_unlock(ptl);
>> +		spin_unlock(ptl);
>> +	}
>>   }
>> 
>>   struct zap_details;
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 44376321180b..8955d1cd31b0 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2112,7 +2112,8 @@ 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_test_large(details->single_folio));
>>   			addr = next;
>>   			continue;
>>   		}
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index e6b6170c2f4f..36881f8fd45e 100644
>> --- a/mm/page_vma_mapped.c
>> +++ b/mm/page_vma_mapped.c
>> @@ -293,11 +293,9 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>   			spin_unlock(pvmw->ptl);
>>   			pvmw->ptl = NULL;
>>   		} else if (!pmd_present(pmde)) {
>> -			if ((pvmw->flags & PVMW_SYNC) &&
>> -			    thp_vma_suitable_order(vma, pvmw->address,
>> -						   PMD_ORDER) &&
>> -			    (pvmw->nr_pages >= HPAGE_PMD_NR))
>> -				sync_with_folio_pmd_zap(mm, pvmw->pmd);
>> +			if (pvmw->flags & PVMW_SYNC)
>> +				sync_with_folio_pmd_zap(mm, pvmw->pmd,
>> +							pvmw->folio_is_order0);
>> 
>>   			goto next_pmd;
>>   		}
>> @@ -379,6 +377,7 @@ unsigned long page_mapped_in_vma(const struct page *page,
>>   		.nr_pages = 1,
>>   		.vma = vma,
>>   		.flags = PVMW_SYNC,
>> +		.folio_is_order0 = !folio_test_large(folio),
>>   	};
>> 
>>   	if (folio_test_anon(folio))
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7db6be53f3a9..79cfa9bc475b 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1434,26 +1434,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>>   		 * processes. Try to unmap it here.
>>   		 */
>>   		if (folio_mapped(folio)) {
>> -			enum ttu_flags flags = TTU_BATCH_FLUSH;
>> +			enum ttu_flags flags = TTU_BATCH_FLUSH | TTU_SYNC;
>>   			bool was_swapbacked = folio_test_swapbacked(folio);
>> 
>>   			if (folio_test_pmd_mappable(folio))
>>   				flags |= TTU_SPLIT_HUGE_PMD;
>> +
>>   			/*
>> -			 * Without TTU_SYNC, try_to_unmap will only begin to
>> -			 * hold PTL from the first present PTE within a large
>> -			 * folio. Some initial PTEs might be skipped due to
>> -			 * races with parallel PTE writes in which PTEs can be
>> -			 * cleared temporarily before being written new present
>> -			 * values. This will lead to a large folio is still
>> -			 * mapped while some subpages have been partially
>> -			 * unmapped after try_to_unmap; TTU_SYNC helps
>> -			 * try_to_unmap acquire PTL from the first PTE,
>> -			 * eliminating the influence of temporary PTE values.
>> +			 * Synchronize with concurrent rmap updates before checking
>> +			 * folio_mapped(). Also avoid skipping temporarily cleared
>> +			 * PTEs, which could leave a large folio partially mapped.
>>   			 */
>> -			if (folio_test_large(folio))
>> -				flags |= TTU_SYNC;
>> -
>>   			try_to_unmap(folio, flags);
>>   			if (folio_mapped(folio)) {
>>   				stat->nr_unmap_fail += nr_pages;
>> --
>> 
>> atomic_add_negative() provides full ordering for mapcount updates. With
>> 243cf1f67b36, rmap removal completes before clearing PMD, including when
>> another CPU reclaims an empty table under PTE lock. So smp_rmb() is enough
>> here, without an extra writer-side barrier.
>
>I'm afraid the memory barrier here is subtle. __folio_remove_rmap() is 
>complex, and in the CONFIG_NO_PAGE_MAPCOUNT branch, 
>folio_sub_return_large_mapcount() doesn't provide full ordering, so we 
>may still need smp_wmb().

Yeah, agreed for large folios. In the diff above, only order-0 folios
use smp_rmb().

Note that order-0 folios take the fully ordered atomic_add_negative()
branch and skip the CONFIG_NO_PAGE_MAPCOUNT branch entirely.

static __always_inline void __folio_remove_rmap(struct folio *folio,
		struct page *page, int nr_pages, struct vm_area_struct *vma,
		enum pgtable_level level)
{
	...
	switch (level) {
	case PGTABLE_LEVEL_PTE:
		if (!folio_test_large(folio)) {
			nr = atomic_add_negative(-1, &folio->_mapcount);
			break;
		}

		if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
			nr = folio_sub_return_large_mapcount(folio, nr_pages, vma);
			...
		}
		...
	}
	...
}

And large folios still take the PMD lock :D

>Anyway, as David said, let's fix this race first. Memory barrier
>optimization requires careful analysis and clear documentation.
>

Yep, fine by me :D The fix goes first, of course. Thanks!

Cheers, Lance