Re: [PATCH] arm64: hugetlb: Restore TLB invalidation for BBM on contiguous ptes

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 6/29/22 15:23, Will Deacon wrote:
> Commit fb396bb459c1 ("arm64/hugetlb: Drop TLB flush from get_clear_flush()")
> removed TLB invalidation from get_clear_flush() [now get_clear_contig()]
> on the basis that the core TLB invalidation code is aware of hugetlb
> mappings backed by contiguous page-table entries and will cover the
> correct virtual address range.
> 
> However, this change also resulted in the TLB invalidation being removed
> from the "break" step in the break-before-make (BBM) sequence used
> internally by huge_ptep_set_{access_flags,wrprotect}(), therefore
> making the BBM sequence unsafe irrespective of later invalidation.
> 
> Although the architecture is desperately unclear about how exactly
> contiguous ptes should be updated in a live page-table, restore TLB
> invalidation to our BBM sequence under the assumption that BBM is the
> right thing to be doing in the first place.
> 
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Steve Capper <steve.capper@arm.com>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---

There is a checkpatch warning for commit message.

WARNING: Possible unwrapped commit description (prefer a maximum 75 chars per line)
#7: 
Commit fb396bb459c1 ("arm64/hugetlb: Drop TLB flush from get_clear_flush()")

total: 0 errors, 1 warnings, 77 lines checked

NOTE: For some of the reported defects, checkpatch may be able to
      mechanically convert to the typical style using --fix or --fix-inplace.

Otherwise LGTM.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

> 
> Found by inspection.
> 
>  arch/arm64/mm/hugetlbpage.c | 30 +++++++++++++++++++++---------
>  1 file changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index e2a5ec9fdc0d..3618ef3f6d81 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -214,6 +214,19 @@ static pte_t get_clear_contig(struct mm_struct *mm,
>  	return orig_pte;
>  }
>  
> +static pte_t get_clear_contig_flush(struct mm_struct *mm,
> +				    unsigned long addr,
> +				    pte_t *ptep,
> +				    unsigned long pgsize,
> +				    unsigned long ncontig)
> +{
> +	pte_t orig_pte = get_clear_contig(mm, addr, ptep, pgsize, ncontig);
> +	struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
> +
> +	flush_tlb_range(&vma, addr, addr + (pgsize * ncontig));
> +	return orig_pte;
> +}
> +
>  /*
>   * Changing some bits of contiguous entries requires us to follow a
>   * Break-Before-Make approach, breaking the whole contiguous set
> @@ -447,19 +460,20 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>  	int ncontig, i;
>  	size_t pgsize = 0;
>  	unsigned long pfn = pte_pfn(pte), dpfn;
> +	struct mm_struct *mm = vma->vm_mm;
>  	pgprot_t hugeprot;
>  	pte_t orig_pte;
>  
>  	if (!pte_cont(pte))
>  		return ptep_set_access_flags(vma, addr, ptep, pte, dirty);
>  
> -	ncontig = find_num_contig(vma->vm_mm, addr, ptep, &pgsize);
> +	ncontig = find_num_contig(mm, addr, ptep, &pgsize);
>  	dpfn = pgsize >> PAGE_SHIFT;
>  
>  	if (!__cont_access_flags_changed(ptep, pte, ncontig))
>  		return 0;
>  
> -	orig_pte = get_clear_contig(vma->vm_mm, addr, ptep, pgsize, ncontig);
> +	orig_pte = get_clear_contig_flush(mm, addr, ptep, pgsize, ncontig);
>  
>  	/* Make sure we don't lose the dirty or young state */
>  	if (pte_dirty(orig_pte))
> @@ -470,7 +484,7 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
>  
>  	hugeprot = pte_pgprot(pte);
>  	for (i = 0; i < ncontig; i++, ptep++, addr += pgsize, pfn += dpfn)
> -		set_pte_at(vma->vm_mm, addr, ptep, pfn_pte(pfn, hugeprot));
> +		set_pte_at(mm, addr, ptep, pfn_pte(pfn, hugeprot));
>  
>  	return 1;
>  }
> @@ -492,7 +506,7 @@ void huge_ptep_set_wrprotect(struct mm_struct *mm,
>  	ncontig = find_num_contig(mm, addr, ptep, &pgsize);
>  	dpfn = pgsize >> PAGE_SHIFT;
>  
> -	pte = get_clear_contig(mm, addr, ptep, pgsize, ncontig);
> +	pte = get_clear_contig_flush(mm, addr, ptep, pgsize, ncontig);
>  	pte = pte_wrprotect(pte);
>  
>  	hugeprot = pte_pgprot(pte);
> @@ -505,17 +519,15 @@ void huge_ptep_set_wrprotect(struct mm_struct *mm,
>  pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
>  			    unsigned long addr, pte_t *ptep)
>  {
> +	struct mm_struct *mm = vma->vm_mm;
>  	size_t pgsize;
>  	int ncontig;
> -	pte_t orig_pte;
>  
>  	if (!pte_cont(READ_ONCE(*ptep)))
>  		return ptep_clear_flush(vma, addr, ptep);
>  
> -	ncontig = find_num_contig(vma->vm_mm, addr, ptep, &pgsize);
> -	orig_pte = get_clear_contig(vma->vm_mm, addr, ptep, pgsize, ncontig);
> -	flush_tlb_range(vma, addr, addr + pgsize * ncontig);
> -	return orig_pte;
> +	ncontig = find_num_contig(mm, addr, ptep, &pgsize);
> +	return get_clear_contig_flush(mm, addr, ptep, pgsize, ncontig);
>  }
>  
>  static int __init hugetlbpage_init(void)

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