Re: [PATCH 1/3] mm: add functions folio_in_range() and folio_within_vma()

Yin, Fengwei
2023-08-02T13:12:25Z

On 8/2/2023 7:35 PM, Ryan Roberts wrote:
> On 02/08/2023 12:14, Ryan Roberts wrote:
>> On 28/07/2023 08:09, Yin Fengwei wrote:
>>> It will be used to check whether the folio is mapped to specific
>>> VMA and whether the mapping address of folio is in the range.
>>>
>>> Also a helper function folio_within_vma() to check whether folio
>>> is in the range of vma based on folio_in_range().
>>>
>>> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
>>> ---
>>> mm/internal.h | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 69 insertions(+)
>>>
>>> diff --git a/mm/internal.h b/mm/internal.h
>>> index 5a03bc4782a2..63de32154a48 100644
>>> --- a/mm/internal.h
>>> +++ b/mm/internal.h
>>> @@ -585,6 +585,75 @@ extern long faultin_vma_page_range(struct vm_area_struct *vma,
>>> bool write, int *locked);
>>> extern bool mlock_future_ok(struct mm_struct *mm, unsigned long flags,
>>> unsigned long bytes);
>>> +
>>> +/*
>>> + * Check whether the folio is in specific range
>>> + *
>>> + * First, check whether the folio is in the range of vma.
>>> + * Then, check whether the folio is mapped to the range of [start, end].
>>> + * In the end, check whether the folio is fully mapped to the range.
>>> + *
>>> + * @pte page table pointer will be checked whether the large folio
>>> + * is fully mapped to. Currently, if mremap in the middle of
>>> + * large folio, the large folio could be mapped to to different
>>> + * VMA and address check can't identify this situation.
>>> + */
>>> +static inline bool
>>> +folio_in_range(struct folio *folio, struct vm_area_struct *vma,
>>> + unsigned long start, unsigned long end, pte_t *pte)
>>
>> This api seems a bit redundant to me. Wouldn't it be better to remove the vma
>> parameter and instead fix up the start/end addresses in folio_within_vma()?
>
> I have created a function as part of my "pte batch-zap" patch set [1], which
> counts the number of contiguously mapped pages of a folio
> (folio_nr_pages_cont_mapped()). I wonder if actually this should be the
> primitive, which can be shared for more cases. Then your folio_within_vma()
> function could just compare the nr_pages to folio_nr_pages() to decide if the
> folio is fully and contiguously mapped in the VMA.
That means we need to unify the parameters. But I don't care about the page and
you don't care about the VMA. :). Maybe we can share the PTE check part?

>
> I also wonder if you should change the name of folio_within_vma() to something
> like folio_test_cont_in_vma() to disambiguate from the case where the folio may
> be fully mapped with a discontiguity (although perhaps that's not possible
> because a mremap would result in distinct vmas... would a new mmap in the hole
> cause a merge of all 3?).
I don't think it's possible as mremap reuse original pgoff of VMA to new VMA. I suppose
it will prevent VMA merging. But I didn't check detail.

I hate to add the PTE check as it makes folio_within_vma() much heavy and can only
be called with page table holding. But MREMAP_DONTUNMAP could create the VMA which
just has part of folio mapped. And the first version folio_within_vma() can't identify
it.

Regards
Yin, Fengwei

>
> [1] https://lore.kernel.org/linux-mm/20230727141837.3386072-4-ryan.roberts@arm.com/
>
>>
>>> +{
>>> + pte_t ptent;
>>> + unsigned long i, nr = folio_nr_pages(folio);
>>> + pgoff_t pgoff, addr;
>>> + unsigned long vma_pglen = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
>>> +
>>> + VM_WARN_ON_FOLIO(folio_test_ksm(folio), folio);
>>> +
>>> + if (start < vma->vm_start)
>>> + start = vma->vm_start;
>>> + if (end > vma->vm_end)
>>> + end = vma->vm_end;
>>> +
>>> + pgoff = folio_pgoff(folio);
>>> + /* if folio start address is not in vma range */
>>> + if (pgoff < vma->vm_pgoff || pgoff > vma->vm_pgoff + vma_pglen)
>>> + return false;
>>> +
>>> + addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
>>> + if (addr < start || end - addr < folio_size(folio))
>>> + return false;
>>> +
>>> + /* not necessary to check pte for none large folio */
>>> + if (!folio_test_large(folio))
>>> + return true;
>>> +
>>> + if (!pte)
>>> + return false;
>>> +
>>> + /* check whether parameter pte is associated with folio */
>>> + ptent = ptep_get(pte);
>>> + if (pte_none(ptent) || !pte_present(ptent) ||
>>> + pte_pfn(ptent) - folio_pfn(folio) >= nr)
>>> + return false;
>>> +
>>> + pte -= pte_pfn(ptent) - folio_pfn(folio);
>>> + for (i = 0; i < nr; i++, pte++) {
>>> + ptent = ptep_get(pte);
>>> +
>>> + if (pte_none(ptent) || !pte_present(ptent) ||
>>> + pte_pfn(ptent) - folio_pfn(folio) >= nr)
>>> + return false;
>>> + }
>>
>> I don't think I see anything to ensure you don't wander off the end (or start)
>> of the pgtable? If the folio is mremapped so that it straddles multiple tables
>> (or is bigger than a single table?) then I think pte can become invalid? Perhaps
>> you intended start/end to always be within the same pgtable, but that is not
>> guarranteed in the case that folio_within_vma() is making the call.
>>
>> Also I want to check that this function is definitely always called under the
>> PTL for the table that pte belongs to?
>>
>>> +
>>> + return true;
>>> +}
>>> +
>>> +static inline bool
>>> +folio_within_vma(struct folio *folio, struct vm_area_struct *vma, pte_t *pte)
>>> +{
>>> + return folio_in_range(folio, vma, vma->vm_start, vma->vm_end, pte);
>>> +}
>>> +
>>> /*
>>> * mlock_vma_folio() and munlock_vma_folio():
>>> * should be called with vma's mmap_lock held for read or write,
>>
>