[PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation
Lance Yang
ioworker0 at gmail.com
Tue Jun 24 09:25:03 PDT 2025
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 at oppo.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
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