Re: [PATCH 09/12] hugetlb_vmemmap: Optimistically set Optimized flag

[Muchun Song]

On 2023/8/26 03:04, Mike Kravetz wrote:
> At the beginning of hugetlb_vmemmap_optimize, optimistically set
> the HPageVmemmapOptimized flag in the head page.  Clear the flag
> if the operation fails.
>
> No change in behavior.  However, this will become important in
> subsequent patches where we batch delay TLB flushing.  We need to
> make sure the content in the old and new vmemmap pages are the same.

Sorry, I didn't get the point here. Could you elaborate it?

> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>   mm/hugetlb_vmemmap.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index e390170c0887..500a118915ff 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -566,7 +566,9 @@ static void __hugetlb_vmemmap_optimize(const struct hstate *h,
>   	if (!vmemmap_should_optimize(h, head))
>   		return;
>   
> +	/* Optimistically assume success */
>   	static_branch_inc(&hugetlb_optimize_vmemmap_key);
> +	SetHPageVmemmapOptimized(head);
>   
>   	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
>   	vmemmap_reuse	= vmemmap_start;
> @@ -577,10 +579,10 @@ static void __hugetlb_vmemmap_optimize(const struct hstate *h,
>   	 * to the page which @vmemmap_reuse is mapped to, then free the pages
>   	 * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
>   	 */
> -	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, bulk_pages))
> +	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, bulk_pages)) {
>   		static_branch_dec(&hugetlb_optimize_vmemmap_key);
> -	else
> -		SetHPageVmemmapOptimized(head);
> +		ClearHPageVmemmapOptimized(head);
> +	}
>   }
>   
>   /**