Re: [RFC PATCH 4/5] hugetlb: don't permit pmd sharing if soft dirty in use

[Peter Xu]

On Wed, Feb 10, 2021 at 04:03:21PM -0800, Mike Kravetz wrote:
> If page modifications are being monitoried with the soft dirty mechanism,
> then this information is contained in page table entries.  Tracking is
> enabled on a 'per-process' basis.  hugetlb pmd sharing allows processes
> to share page table entries.  This makes pmd sharing incompatible with
> soft dirty monitoring.  So, pmd sharing must be disabled if soft dirty
> tracking is in use.
> 
> The VM_SOFTDIRTY flag is set in all vma's by default.  When soft dirty
> monitoring is started, the flag is cleared.  We can therefore allow pmd
> sharing in vmas with the VM_SOFTDIRTY flag set.  Check this when deciding
> to perform pmd sharing.
> 
> A subsequent patch will add code to allow soft dirty monitoring for hugetlb
> vmas.  Any existing pmd sharing will be undone at that time.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index b561b6867ec1..cf4aa63be9b1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5285,6 +5285,13 @@ static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
>  	unsigned long base = addr & PUD_MASK;
>  	unsigned long end = base + PUD_SIZE;
>  
> +	/*
> +	 * Do not allow pmd sharing if soft dirty tracking is in use.
> +	 */
> +	if (IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) &&
> +				!(vma->vm_flags & VM_SOFTDIRTY))
> +		return false;
> +

Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>

The indent of using three tabs is a bit weird to me, though.. I do see this
happen somewhere else in the repo too, so maybe it's a way to indent that I
don't know.

Thanks,

-- 
Peter Xu