linux-kernel - Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

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Message-ID: <20250624162503.78957-1-ioworker0@gmail.com>
Date: Wed, 25 Jun 2025 00:25:03 +0800
From: Lance Yang <ioworker0@...il.com>
To: david@...hat.com
Cc: 21cnbao@...il.com,
	akpm@...ux-foundation.org,
	baolin.wang@...ux.alibaba.com,
	chrisl@...nel.org,
	ioworker0@...il.com,
	kasong@...cent.com,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-riscv@...ts.infradead.org,
	lorenzo.stoakes@...cle.com,
	ryan.roberts@....com,
	v-songbaohua@...o.com,
	x86@...nel.org,
	ying.huang@...el.com,
	zhengtangquan@...o.com
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

On 2025/6/24 23:34, David Hildenbrand wrote:
> On 24.06.25 17:26, Lance Yang wrote:
>> On 2025/6/24 20:55, David Hildenbrand wrote:
>>> On 14.02.25 10:30, Barry Song wrote:
>>>> From: Barry Song <v-songbaohua@...o.com>
>> [...]
>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>> index 89e51a7a9509..8786704bd466 100644
>>>> --- a/mm/rmap.c
>>>> +++ b/mm/rmap.c
>>>> @@ -1781,6 +1781,25 @@ void folio_remove_rmap_pud(struct folio *folio,
>>>> struct page *page,
>>>>    #endif
>>>>    }
>>>> +/* We support batch unmapping of PTEs for lazyfree large folios */
>>>> +static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
>>>> +            struct folio *folio, pte_t *ptep)
>>>> +{
>>>> +    const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>>> +    int max_nr = folio_nr_pages(folio);
>>>
>>> Let's assume we have the first page of a folio mapped at the last page
>>> table entry in our page table.
>>
>> Good point. I'm curious if it is something we've seen in practice ;)
> 
> I challenge you to write a reproducer :P I assume it might be doable 
> through simple mremap().
> 
>>
>>>
>>> What prevents folio_pte_batch() from reading outside the page table?
>>
>> Assuming such a scenario is possible, to prevent any chance of an
>> out-of-bounds read, how about this change:
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index fb63d9256f09..9aeae811a38b 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1852,6 +1852,25 @@ static inline bool 
>> can_batch_unmap_folio_ptes(unsigned long addr,
>>       const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>       int max_nr = folio_nr_pages(folio);
>>       pte_t pte = ptep_get(ptep);
>> +    unsigned long end_addr;
>> +
>> +    /*
>> +     * To batch unmap, the entire folio's PTEs must be contiguous
>> +     * and mapped within the same PTE page table, which corresponds to
>> +     * a single PMD entry. Before calling folio_pte_batch(), which does
>> +     * not perform boundary checks itself, we must verify that the
>> +     * address range covered by the folio does not cross a PMD boundary.
>> +     */
>> +    end_addr = addr + (max_nr * PAGE_SIZE) - 1;
>> +
>> +    /*
>> +     * A fast way to check for a PMD boundary cross is to align both
>> +     * the start and end addresses to the PMD boundary and see if they
>> +     * are different. If they are, the range spans across at least two
>> +     * different PMD-managed regions.
>> +     */
>> +    if ((addr & PMD_MASK) != (end_addr & PMD_MASK))
>> +        return false;
> 
> You should not be messing with max_nr = folio_nr_pages(folio) here at 
> all. folio_pte_batch() takes care of that.
> 
> Also, way too many comments ;)
> 
> You may only batch within a single VMA and within a single page table.
> 
> So simply align the addr up to the next PMD, and make sure it does not 
> exceed the vma end.
> 
> ALIGN and friends can help avoiding excessive comments.

Thanks! How about this updated version based on your suggestion:

diff --git a/mm/rmap.c b/mm/rmap.c
index fb63d9256f09..241d55a92a47 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1847,12 +1847,25 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page,
 
 /* We support batch unmapping of PTEs for lazyfree large folios */
 static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
-			struct folio *folio, pte_t *ptep)
+					      struct folio *folio, pte_t *ptep,
+					      struct vm_area_struct *vma)
 {
 	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
+	unsigned long next_pmd, vma_end, end_addr;
 	int max_nr = folio_nr_pages(folio);
 	pte_t pte = ptep_get(ptep);
 
+	/*
+	 * Limit the batch scan within a single VMA and within a single
+	 * page table.
+	 */
+	vma_end = vma->vm_end;
+	next_pmd = ALIGN(addr + 1, PMD_SIZE);
+	end_addr = addr + (unsigned long)max_nr * PAGE_SIZE;
+
+	if (end_addr > min(next_pmd, vma_end))
+		return false;
+
 	if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
 		return false;
 	if (pte_unused(pte))
@@ -2025,7 +2038,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				folio_mark_dirty(folio);
 		} else if (likely(pte_present(pteval))) {
 			if (folio_test_large(folio) && !(flags & TTU_HWPOISON) &&
-			    can_batch_unmap_folio_ptes(address, folio, pvmw.pte))
+			    can_batch_unmap_folio_ptes(address, folio, pvmw.pte, pvmw.vma))
 				nr_pages = folio_nr_pages(folio);
 			end_addr = address + nr_pages * PAGE_SIZE;
 			flush_cache_range(vma, address, end_addr);
--
Thanks,
Lance