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Message-ID: <27d174e0-c209-4851-825a-0baeb56df86f@redhat.com>
Date: Wed, 25 Jun 2025 12:00:39 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: 21cnbao@...il.com, akpm@...ux-foundation.org,
baolin.wang@...ux.alibaba.com, chrisl@...nel.org, 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 24.06.25 18:25, Lance Yang wrote:
> 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;
May I suggest that we clean all that up as we fix it?
Maybe something like this:
diff --git a/mm/rmap.c b/mm/rmap.c
index 3b74bb19c11dd..11fbddc6ad8d6 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1845,23 +1845,38 @@ 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)
+static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
+ struct page_vma_mapped_walk *pvmw, enum ttu_flags flags,
+ pte_t pte)
{
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);
+ struct vm_area_struct *vma = pvmw->vma;
+ unsigned long end_addr, addr = pvmw->address;
+ unsigned int max_nr;
+
+ if (flags & TTU_HWPOISON)
+ return 1;
+ if (!folio_test_large(folio))
+ return 1;
+
+ /* We may only batch within a single VMA and a single page table. */
+ end_addr = min_t(unsigned long, ALIGN(addr + 1, PMD_SIZE), vma->vm_end);
+ max_nr = (end_addr - addr) >> PAGE_SHIFT;
+ /* We only support lazyfree batching for now ... */
if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
- return false;
+ return 1;
if (pte_unused(pte))
- return false;
- if (pte_pfn(pte) != folio_pfn(folio))
- return false;
+ return 1;
+ /* ... where we must be able to batch the whole folio. */
+ if (pte_pfn(pte) != folio_pfn(folio) || max_nr != folio_nr_pages(folio))
+ return 1;
+ max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, fpb_flags,
+ NULL, NULL, NULL);
- return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL,
- NULL, NULL) == max_nr;
+ if (max_nr != folio_nr_pages(folio))
+ return 1;
+ return max_nr;
}
/*
@@ -2024,9 +2039,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
if (pte_dirty(pteval))
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))
- nr_pages = folio_nr_pages(folio);
+ nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, pteval);
end_addr = address + nr_pages * PAGE_SIZE;
flush_cache_range(vma, address, end_addr);
Note that I don't quite understand why we have to batch the whole thing or fallback to
individual pages. Why can't we perform other batches that span only some PTEs? What's special
about 1 PTE vs. 2 PTEs vs. all PTEs?
Can someone enlighten me why that is required?
--
Cheers,
David / dhildenb
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