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

[Ryan Roberts <ryan.roberts@arm.com>]

On 23/01/2024 13:06, David Hildenbrand wrote:
> 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.

I think you could not bother with the enforce_uffd_wp - just always enforce
uffd-wp. So that's one simplification vs mine. Then you just need an any_dirty
flag following the same pattern as your any_writable. Then just set dirty on the
whole batch in the child if any were dirty in the parent.

Although now I'm wondering if there is a race here... What happens if a page in
the parent becomes dirty after you have checked it but before you write protect
it? Isn't that already a problem with the current non-batched version? Why do we
even to preserve dirty in the child for private mappings?