Re: [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

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

On 2025/9/30 14:33, Dev Jain wrote:
> 
> On 30/09/25 11:35 am, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> When splitting an mTHP and replacing a zero-filled subpage with the 
>> shared
>> zeropage, try_to_map_unused_to_zeropage() currently drops several 
>> important
>> PTE bits.
>>
>> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
>> incremental snapshots, losing the soft-dirty bit means modified pages are
>> missed, leading to inconsistent memory state after restore.
>>
>> As pointed out by David, the more critical uffd-wp bit is also dropped.
>> This breaks the userfaultfd write-protection mechanism, causing writes
>> to be silently missed by monitoring applications, which can lead to data
>> corruption.
>>
>> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
>> creating the new zeropage mapping to ensure they are correctly tracked.
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage 
>> when splitting isolated thp")
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Suggested-by: Dev Jain <dev.jain@arm.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>> v2 -> v3:
>>   - ptep_get() gets called only once per iteration (per Dev)
>>   - https://lore.kernel.org/linux-mm/20250930043351.34927-1- 
>> lance.yang@linux.dev/
>>
>> v1 -> v2:
>>   - Avoid calling ptep_get() multiple times (per Dev)
>>   - Double-check the uffd-wp bit (per David)
>>   - Collect Acked-by from David - thanks!
>>   - https://lore.kernel.org/linux-mm/20250928044855.76359-1- 
>> lance.yang@linux.dev/
>>
>>   mm/migrate.c | 14 ++++++++++----
>>   1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index ce83c2c3c287..bafd8cb3bebe 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -297,6 +297,7 @@ bool isolate_folio_to_list(struct folio *folio, 
>> struct list_head *list)
>>   static bool try_to_map_unused_to_zeropage(struct 
>> page_vma_mapped_walk *pvmw,
>>                         struct folio *folio,
>> +                      pte_t old_pte,
>>                         unsigned long idx)
> 
> Could have just added this in the same line as folio?

Sure ;p

> 
>>   {
>>       struct page *page = folio_page(folio, idx);
>> @@ -306,7 +307,7 @@ static bool try_to_map_unused_to_zeropage(struct 
>> page_vma_mapped_walk *pvmw,
>>           return false;
>>       VM_BUG_ON_PAGE(!PageAnon(page), page);
>>       VM_BUG_ON_PAGE(!PageLocked(page), page);
>> -    VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
>> +    VM_BUG_ON_PAGE(pte_present(old_pte), page);
>>       if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & 
>> VM_LOCKED) ||
>>           mm_forbids_zeropage(pvmw->vma->vm_mm))
>> @@ -322,6 +323,12 @@ static bool try_to_map_unused_to_zeropage(struct 
>> page_vma_mapped_walk *pvmw,
>>       newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
>>                       pvmw->vma->vm_page_prot));
>> +
>> +    if (pte_swp_soft_dirty(old_pte))
>> +        newpte = pte_mksoft_dirty(newpte);
>> +    if (pte_swp_uffd_wp(old_pte))
>> +        newpte = pte_mkuffd_wp(newpte);
>> +
>>       set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
>>       dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
>> @@ -344,7 +351,7 @@ static bool remove_migration_pte(struct folio *folio,
>>       while (page_vma_mapped_walk(&pvmw)) {
>>           rmap_t rmap_flags = RMAP_NONE;
>> -        pte_t old_pte;
>> +        pte_t old_pte = ptep_get(pvmw.pte);
>>           pte_t pte;
>>           swp_entry_t entry;
>>           struct page *new;
>> @@ -365,12 +372,11 @@ static bool remove_migration_pte(struct folio 
>> *folio,
>>           }
>>   #endif
>>           if (rmap_walk_arg->map_unused_to_zeropage &&
>> -            try_to_map_unused_to_zeropage(&pvmw, folio, idx))
>> +            try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
>>               continue;
>>           folio_get(folio);
>>           pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
>> -        old_pte = ptep_get(pvmw.pte);
>>           entry = pte_to_swp_entry(old_pte);
>>           if (!is_migration_entry_young(entry))
> 
> Looks good, the special bit does not overlay on any arch with the soft- 
> dirty bit.
> It shouldn't overlay with uffd-wp as well since split_huge_zero_page_pmd 
> does the
> same bit preservation.

Yeah. Thanks for double-checking the bit overlaps!

Good to know we're on solid ground here ;)

> 
> Reviewed-by: Dev Jain <dev.jain@arm.com>

Cheers!