Re: [PATCH v1 10/11] mm/memory: ignore dirty/accessed/soft-dirty bits in folio_pte_batch()

David Hildenbrand
2024-01-23T13:06:40Z

On 23.01.24 13:25, Ryan Roberts wrote:
> On 22/01/2024 19:41, David Hildenbrand wrote:
>> Let's ignore these bits: they are irrelevant for fork, and will likely
>> be irrelevant for upcoming users such as page unmapping.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> mm/memory.c | 10 ++++++++--
>> 1 file changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index f563aec85b2a8..341b2be845b6e 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -953,24 +953,30 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
>> set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
>> }
>>
>> +static inline pte_t __pte_batch_clear_ignored(pte_t pte)
>> +{
>> + return pte_clear_soft_dirty(pte_mkclean(pte_mkold(pte)));
>> +}
>> +
>> /*
>> * Detect a PTE batch: consecutive (present) PTEs that map consecutive
>> * pages of the same folio.
>> *
>> * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN.
>
> nit: last char should be a comma (,) not a full stop (.)
>
>> + * the accessed bit, dirty bit and soft-dirty bit.
>> */
>> static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>> pte_t *start_ptep, pte_t pte, int max_nr)
>> {
>> unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
>> const pte_t *end_ptep = start_ptep + max_nr;
>> - pte_t expected_pte = pte_next_pfn(pte);
>> + pte_t expected_pte = __pte_batch_clear_ignored(pte_next_pfn(pte));
>> pte_t *ptep = start_ptep + 1;
>>
>> VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>>
>> while (ptep != end_ptep) {
>> - pte = ptep_get(ptep);
>> + pte = __pte_batch_clear_ignored(ptep_get(ptep));
>>
>> if (!pte_same(pte, expected_pte))
>> break;
>
> I think you'll lose dirty information in the child for private mappings? If the
> first pte in a batch is clean, but a subsequent page is dirty, you will end up
> setting all the pages in the batch as clean in the child. Previous behavior
> would preserve dirty bit for private mappings.
>
> In my version (v3) that did arbitrary batching, I had some fun and games
> tracking dirty, write and uffd_wp:
> https://lore.kernel.org/linux-arm-kernel/20231204105440.61448-2-ryan.roberts@arm.com/
>
> Also, I think you will currently either set soft dirty on all or none of the
> pages in the batch, depending on the value of the first. I previously convinced
> myself that the state was unimportant so always cleared it in the child to
> provide consistency.

Good points regarding dirty and soft-dirty. I wanted to avoid passing
flags to folio_pte_batch(), but maybe that's just what we need to not
change behavior.

--
Cheers,

David / dhildenb