Re: [PATCH 1/1] powerpc: kvm: Reduce calls to get current->mm by storing the value locally

[Leonardo Bras]

Hello Paul, 
I sent this patch, but I have a question:

On Thu, 2019-09-19 at 19:27 -0300, Leonardo Bras wrote:
> Reduces the number of calls to get_current() in order to get the value of
> current->mm by doing it once and storing the value, since it is not
> supposed to change inside the same process).
> 
> Signed-off-by: Leonardo Bras <leonardo@xxxxxxxxxxxxx>
> ---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 9a75f0e1933b..f2b9aea43216 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -508,6 +508,7 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  	struct vm_area_struct *vma;
>  	unsigned long rcbits;
>  	long mmio_update;
> +	struct mm_struct *mm;
>  
>  	if (kvm_is_radix(kvm))
>  		return kvmppc_book3s_radix_page_fault(run, vcpu, ea, dsisr);
> @@ -584,6 +585,7 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  	is_ci = false;
>  	pfn = 0;
>  	page = NULL;
> +	mm = current->mm;

Here, current->mm is not always the same as kvm->mm? 

>  	pte_size = PAGE_SIZE;
>  	writing = (dsisr & DSISR_ISSTORE) != 0;
>  	/* If writing != 0, then the HPTE must allow writing, if we get here */
> @@ -592,8 +594,8 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  	npages = get_user_pages_fast(hva, 1, writing ? FOLL_WRITE : 0, pages);
>  	if (npages < 1) {
>  		/* Check if it's an I/O mapping */
> -		down_read(&current->mm->mmap_sem);
> -		vma = find_vma(current->mm, hva);
> +		down_read(&mm->mmap_sem);
> +		vma = find_vma(mm, hva);
>  		if (vma && vma->vm_start <= hva && hva + psize <= vma->vm_end &&
>  		    (vma->vm_flags & VM_PFNMAP)) {
>  			pfn = vma->vm_pgoff +
> @@ -602,7 +604,7 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  			is_ci = pte_ci(__pte((pgprot_val(vma->vm_page_prot))));
>  			write_ok = vma->vm_flags & VM_WRITE;
>  		}
> -		up_read(&current->mm->mmap_sem);
> +		up_read(&mm->mmap_sem);
>  		if (!pfn)
>  			goto out_put;
>  	} else {
> @@ -621,8 +623,7 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  			 * hugepage split and collapse.
>  			 */
>  			local_irq_save(flags);
> -			ptep = find_current_mm_pte(current->mm->pgd,
> -						   hva, NULL, NULL);
> +			ptep = find_current_mm_pte(mm->pgd, hva, NULL, NULL);
>  			if (ptep) {
>  				pte = kvmppc_read_update_linux_pte(ptep, 1);
>  				if (__pte_write(pte))

Thanks !

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