Re: [PATCH v3 5/7] mm: replace vma->vm_flags indirect modification in ksm_madvise

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Jan 25, 2023 at 03:35:52PM -0800, Suren Baghdasaryan wrote:
> Replace indirect modifications to vma->vm_flags with calls to modifier
> functions to be able to track flag changes and to keep vma locking
> correctness.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c | 5 ++++-
>  arch/s390/mm/gmap.c                | 5 ++++-
>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index 1d67baa5557a..325a7a47d348 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -393,6 +393,7 @@ static int kvmppc_memslot_page_merge(struct kvm *kvm,
>  {
>  	unsigned long gfn = memslot->base_gfn;
>  	unsigned long end, start = gfn_to_hva(kvm, gfn);
> +	unsigned long vm_flags;
>  	int ret = 0;
>  	struct vm_area_struct *vma;
>  	int merge_flag = (merge) ? MADV_MERGEABLE : MADV_UNMERGEABLE;
> @@ -409,12 +410,14 @@ static int kvmppc_memslot_page_merge(struct kvm *kvm,
>  			ret = H_STATE;
>  			break;
>  		}
> +		vm_flags = vma->vm_flags;
>  		ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> -			  merge_flag, &vma->vm_flags);
> +			  merge_flag, &vm_flags);
>  		if (ret) {
>  			ret = H_STATE;
>  			break;
>  		}
> +		reset_vm_flags(vma, vm_flags);
>  		start = vma->vm_end;
>  	} while (end > vma->vm_end);

Add a comment on why the vm_flags are copied in case someone "optimises"
this in the future? Something like

/* Copy vm_flags to avoid any partial modifications in ksm_madvise. */

>  
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 3a695b8a1e3c..d5eb47dcdacb 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2587,14 +2587,17 @@ int gmap_mark_unmergeable(void)
>  {
>  	struct mm_struct *mm = current->mm;
>  	struct vm_area_struct *vma;
> +	unsigned long vm_flags;
>  	int ret;
>  	VMA_ITERATOR(vmi, mm, 0);
>  
>  	for_each_vma(vmi, vma) {
> +		vm_flags = vma->vm_flags;
>  		ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
> -				  MADV_UNMERGEABLE, &vma->vm_flags);
> +				  MADV_UNMERGEABLE, &vm_flags);
>  		if (ret)
>  			return ret;
> +		reset_vm_flags(vma, vm_flags);

Same.

Not necessary as such as there are few users of ksm_madvise and I doubt
it'll introduce new surprises.

With or without the comment;

Acked-by: Mel Gorman <mgorman@techsingularity.net>

-- 
Mel Gorman
SUSE Labs