Date:   Thu, 3 Aug 2023 08:24:01 +0800
From:   Yin Fengwei <fengwei.yin@...el.com>
To:     Ryan Roberts <ryan.roberts@....com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <yuzhao@...gle.com>, <willy@...radead.org>, <david@...hat.com>,
        <shy828301@...il.com>, <hughd@...gle.com>
Subject: Re: [PATCH 1/3] mm: add functions folio_in_range() and
 folio_within_vma()



On 8/2/23 22:59, Ryan Roberts wrote:
> On 02/08/2023 15:14, Yin, Fengwei wrote:
>>
>>
>> On 8/2/2023 10:08 PM, Ryan Roberts wrote:
>>> On 02/08/2023 14:46, Yin, Fengwei wrote:
>>>>
>>>>
>>>> On 8/2/2023 9:09 PM, Ryan Roberts wrote:
>>>>> On 02/08/2023 13:50, Yin, Fengwei wrote:
>>>>>>
>>>>>>
>>>>>> On 8/2/2023 7:14 PM, 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@...el.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()?
>>>>>> My understanding is it's necessary. As for madvise, we need to check whether
>>>>>> the folio is both in the range of VMA and also in the range of [start, end).
>>>>>
>>>>> But in folio_within_vma() you pass start as vma->vm_start and end as
>>>>> vma->vm_end. And in this function, you narrow start/end to be completely
>>>>> contained in vma. So surely there is only really one start/end you are
>>>>> interested in? Just seems a bit odd to me.
>>>> madvise() will call filio_in_range() with VMA and real range [start, end) passed
>>>> from user space.
>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>>> +{
>>>>>>>> +	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.
>>>>>> If pte is invalid for any reason (pass wrong parameter, not fully mapped etc), this
>>>>>> function just return false in page table entry check phase.
>>>>>
>>>>> Sorry I don't think this covers the issue I'm describing. If you have a
>>>>> pte-mapped THP that gets mremapped to straddle 2 pte tables, don't you have a
>>>>> problem?
>>>>>
>>>>> example for 4K base page set up:
>>>>>
>>>>> folio_nr_pages = 512
>>>>> first page of folio mapped at vaddr = 2M - 4K = 0x1FF000
>>>>>
>>>>> If you then call this function with the pte pointer for the second page in the
>>>>> folio, which is mapped at address 0x200000, that pte is pointing to the first
>>>>> pte entry in the table pointed to by the second pmd entry. The pte pointer can
>>>>> be legitimately manipulated to point to any entry within that table,
>>>>> corrsponding to vaddrs [0x200000, 0x400000). But you will end up subtracting 1
>>>>> from the pointer, intending that it now points to the pte entry that represents
>>>>> vaddr 0x1FF000. But actually it has fallen off the front of the table into some
>>>>> other arbitrary memory in the linear map. 0x1FF000 is represented in a different
>>>>> table, pointed to by the first pmd entry.
>>>> Yes. This can be an issue as hold the second page table lock can't prevent the first
>>>> part unmapped. Let me add another check vaddr align to folio_size in next version. 
>>>
>>> Locking is a problem but its not the only problem. The 2 tables are almost
>>> certainly not contiguous in virtual memory. So once you have moved the pointer
>>> to before the start of the second table, then you are pointing to arbitrary memory.
>> If vaddr is aligned to folio_size, suppose we are OK here (I have assumption that
>> large folio will not be larger than PMD size. Or it's possible on ARM platform?).
> 
> I *think* your assumption that a folio will never be bigger than PMD size is ok.
> (I'm guessing page cache never allocates bigger folios than that?).> 
> But its a bad assumption to assume folios are always mapped in a naturally
> aligned manner. mremapping a thp will cause non-natural alignment. User space
> requesting a file (that is in a large folio in pagecache) to be mapped to
> arbitrary (page-aligned) address will do that.
The none-aligned THP is what I want to filter out here. But I totally forget the
page cache which could cause same thing as mremap. I will use another way to check it.
Thanks.


Regards
Yin, Fengwei

> 
>>
>>
>> Regards
>> Yin, Fengwei
>>
>>>
>>>>
>>>> Regards
>>>> Yin, Fengwei
>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>> Also I want to check that this function is definitely always called under the
>>>>>>> PTL for the table that pte belongs to?
>>>>>> Yes. I should spell it out. Thanks.
>>>>>>
>>>>>>
>>>>>> Regards
>>>>>> Yin, Fengwei
>>>>>>
>>>>>>>
>>>>>>>> +
>>>>>>>> +	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,
>>>>>>>
>>>>>
>>>
> 

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