Re: [PATCH v4 3/3] mm: Batch-zap large anonymous folio PTE mappings

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

On 03/08/2023 15:10, David Hildenbrand wrote:
>>>
>>> With this patch, you'll might suddenly have mapcount > refcount for a folio, or
>>> am I wrong?
>>
>> Yes you would. Does that break things?
>>
> 
> It is problematic whenever you want to check for additional page references that
> are not from mappings (i.e., GUP refs/pins or anything else)
> 
> One example lives in KSM code (!compound only):
> 
> page_mapcount(page) + 1 + swapped != page_count(page)
> 
> Another one in compaction code:
> 
> if (!mapping && (folio_ref_count(folio) - 1) > folio_mapcount(folio))
> 
> And another one in khugepaged (is_refcount_suitable)
> 
> ... and in THP split can_split_folio() (although that can deal with false
> positives and false negatives).
> 
> 
> We want to avoid detecting "no other references" if there *are* other
> references. Detecting "there are other references" although there are not is
> usually better.
> 
> 
> Assume you have mapcount > refcount for some time due to concurrent unmapping,
> AND some unrelated reference. You would suddenly pass these checks (mapcount ==
> refcount) and might not detect other references.

OK. I'll rework with the 2 loop approach, assuming I can calculate the number of
free slots in the mmu_gather ahead of time.


> 
>>>
>>>> +
>>>> +    for (i = 0; i < nr_pages;) {
>>>> +        ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>>>> +        tlb_remove_tlb_entry(tlb, pte, addr);
>>>> +        zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
>>>> +        full = __tlb_remove_page(tlb, page, 0);
>>>> +
>>>> +        if (unlikely(page_mapcount(page) < 1))
>>>> +            print_bad_pte(vma, addr, ptent, page);
>>>
>>> Can we avoid new users of page_mapcount() outside rmap code, please? :)
>>
>> Sure. This is just trying to replicate the same diagnstics that's done on the
>> non-batched path. I'm happy to remove it.
> 
> Spotted it afterwards in the existing code already, so you're effetively not
> adding new ones.
>